the structure & stability of atoms. early atomic history there have been many different...

The Structure & The Structure & Stability of Stability of

AtomsAtoms

Early Atomic HistoryEarly Atomic History

There have been many different There have been many different theories, reflecting different times theories, reflecting different times and cultures, to explain the and cultures, to explain the composition of matter.composition of matter.

In addition, chemical reactions, In addition, chemical reactions, refinements of ores, purification of refinements of ores, purification of salt, etc. have been carried out for salt, etc. have been carried out for thousands of years.thousands of years.

Early Atomic HistoryEarly Atomic History

The ancient Greek philosophers The ancient Greek philosophers theorized that matter is discrete, rather theorized that matter is discrete, rather than continuous. than continuous.

Some, notably Demokritos, suggested Some, notably Demokritos, suggested that there is some small unit of matter that there is some small unit of matter that still retains the properties of the that still retains the properties of the larger sample. It was thought that these larger sample. It was thought that these smaller pieces of matter were indivisible, smaller pieces of matter were indivisible, and were given the name and were given the name atomosatomos from from which we get our modern word which we get our modern word atoms.atoms.

Early Atomic TheoryEarly Atomic Theory

During the next 2000 years, a lot During the next 2000 years, a lot was learned about matter. Several was learned about matter. Several elements were discovered, metals elements were discovered, metals were refined, acids prepared, etc.were refined, acids prepared, etc.

In the mid-1600s, the scientific In the mid-1600s, the scientific (rather than the philosophical or (rather than the philosophical or applied) study of the nature matter applied) study of the nature matter began to take shape. began to take shape.

Early Atomic TheoryEarly Atomic Theory Since most laboratories contained Since most laboratories contained

rudimentary equipment- burners and rudimentary equipment- burners and scales, many experiments involved the scales, many experiments involved the measurement of changes in volumes measurement of changes in volumes (for gases) and masses during chemical (for gases) and masses during chemical reactions.reactions.

Based on measurements and Based on measurements and observations, several scientific laws observations, several scientific laws were developed. These laws form the were developed. These laws form the basis for our understanding of the basis for our understanding of the composition of matter.composition of matter.

The Law of Conservation The Law of Conservation of Massof Mass

Antoine Lavoisier (1743-1794) Antoine Lavoisier (1743-1794) measured the masses of reactants measured the masses of reactants and products for a variety of and products for a variety of chemical reactions. He determined chemical reactions. He determined that matter is neither created nor that matter is neither created nor destroyed during a chemical destroyed during a chemical reaction. This is known as the reaction. This is known as the law of law of conservation of massconservation of mass..

The Law of Definite The Law of Definite ProportionProportion

Joseph Proust (1754-1826) determined the Joseph Proust (1754-1826) determined the chemical composition of many compounds. chemical composition of many compounds. He found that a given compound always He found that a given compound always contains the exact same proportion of contains the exact same proportion of elements by mass. This is known as the elements by mass. This is known as the law of definite proportionlaw of definite proportion..

For example, all samples of water contain For example, all samples of water contain 88.8% oxygen by mass, and 11.2% 88.8% oxygen by mass, and 11.2% hydrogen by mass.hydrogen by mass.

The Law of Multiple The Law of Multiple ProportionsProportions

This chemical law applies when two (or This chemical law applies when two (or more) elements can combine to form more) elements can combine to form different compounds.different compounds.

Common examples are carbon monoxide and Common examples are carbon monoxide and carbon dioxide, or water and hydrogen carbon dioxide, or water and hydrogen peroxide.peroxide.

John Dalton (1766-1844) conducted John Dalton (1766-1844) conducted experiments on these types of compounds, experiments on these types of compounds, and determined that there is a simple and determined that there is a simple relationship between the masses of one relationship between the masses of one element relative to the others.element relative to the others.


When two elements form a series of When two elements form a series of compounds, the ratios of the masses compounds, the ratios of the masses of one element that combine with a of one element that combine with a fixed mass of the other element are fixed mass of the other element are always always in a ratio of small whole in a ratio of small whole numbersnumbers..

The meaning of this law is difficult to The meaning of this law is difficult to understand unless it is illustrated understand unless it is illustrated using a specific series of compounds.using a specific series of compounds.


Consider the compounds of water and Consider the compounds of water and hydrogen peroxide. At this point in hydrogen peroxide. At this point in history, chemists knew the compounds history, chemists knew the compounds were different, and that they both were different, and that they both contain (or can be broken down into) contain (or can be broken down into) the elements hydrogen and oxygen. the elements hydrogen and oxygen. They did not yet know the formulas for They did not yet know the formulas for either compound, nor was the concept either compound, nor was the concept of atoms fully developed.of atoms fully developed.


Analysis of 100 grams of the compounds Analysis of 100 grams of the compounds produced the following data:produced the following data:

CompoundCompound Mass of Mass of oxygen/100oxygen/100

g of g of compoundcompound

Mass of Mass of hydrogen/1hydrogen/1

00g of 00g of compoundcompound

Grams of Grams of oxygen/graoxygen/gra

m of m of hydrogenhydrogen

waterwater 88.8 grams 88.8 grams OO

11.2 grams 11.2 grams HH

7.93 gO/gH7.93 gO/gH

Hydrogen Hydrogen peroxideperoxide

94.06 94.06 grams Ograms O

5.94 grams 5.94 grams HH


The Law of Multiple The Law of Multiple ProportionsProportions The The Law of Multiple ProportionsLaw of Multiple Proportions is illustrated when is illustrated when

the numbers in the last column are compared.the numbers in the last column are compared.

CompoundCompound Grams of Grams of oxygen/graoxygen/gra

m of m of hydrogenhydrogen

waterwater 7.93 gO/gH7.93 gO/gH

Hydrogen Hydrogen peroxideperoxide


15.8/7.93 = 2/1

The small whole number ratio suggests that there is twice as much oxygen in hydrogen peroxide as there is in water.

The Law of Multiple The Law of Multiple ProportionsProportions The key feature is that small The key feature is that small wholewhole numbers are numbers are

generated. The results support the hypothesis that generated. The results support the hypothesis that molecules consist of various combinations of atoms, molecules consist of various combinations of atoms, and that atoms are the smallest unit of matter. The and that atoms are the smallest unit of matter. The ratio doesn’t produce fractions, since there is no such ratio doesn’t produce fractions, since there is no such thing as a fraction of an atom.thing as a fraction of an atom.

For the example cited, we would propose that For the example cited, we would propose that hydrogen peroxide contains twice as many oxygen hydrogen peroxide contains twice as many oxygen atoms/hydrogen atoms than does water. We cannot, atoms/hydrogen atoms than does water. We cannot, however, determine the actual formula of either however, determine the actual formula of either compound.compound.

Dalton’s Atomic Theory Dalton’s Atomic Theory (1808)(1808)

1. Each element consists of tiny particles called 1. Each element consists of tiny particles called atoms.atoms.

2. The atoms of a given element are identical, 2. The atoms of a given element are identical, and differ from the atoms of other elements.and differ from the atoms of other elements.

3. 3. Compounds are formed when atoms of Compounds are formed when atoms of different elements combine chemically. A different elements combine chemically. A specific compound always has the same specific compound always has the same relative number and types of atoms.relative number and types of atoms.

4.4. Chemical reactions involve the reorganization Chemical reactions involve the reorganization of atoms, or changes in the way they are of atoms, or changes in the way they are bound together.bound together.

Sub-Atomic ParticlesSub-Atomic Particles

The period from approximately 1900-The period from approximately 1900-1915 involved the study of the nature 1915 involved the study of the nature of the atom, using two relatively new of the atom, using two relatively new tools: electricity and radioactivity.tools: electricity and radioactivity.

Scientists knew that atoms of Scientists knew that atoms of different elements had different different elements had different relative atomic masses and different relative atomic masses and different properties, and they wanted to find properties, and they wanted to find out the reasons for the differences.out the reasons for the differences.


J.J. Thomson (1856-1940) studied the J.J. Thomson (1856-1940) studied the properties of properties of cathode rayscathode rays. The rays are . The rays are produced in partially evacuated tubes produced in partially evacuated tubes containing electrodes at either end. containing electrodes at either end.

The rays are invisible, unless a The rays are invisible, unless a phosphorescent screen is used.phosphorescent screen is used.


(Cathode)

(Anode)

Cathode Rays


Thomson made the following Thomson made the following observations:observations:

1. The cathode rays had the same 1. The cathode rays had the same properties regardless of the metal used properties regardless of the metal used for the cathode.for the cathode.

2. The rays traveled from the cathode (- 2. The rays traveled from the cathode (- charged) to the anode (+ charged).charged) to the anode (+ charged).

3. The rays were attracted to the positive 3. The rays were attracted to the positive plate of an external electrical field, and plate of an external electrical field, and repelled by the negative plate.repelled by the negative plate.


Thomson concluded:Thomson concluded:

1. The cathode rays are a stream of 1. The cathode rays are a stream of negatively charged particles called negatively charged particles called electronselectrons..

2. All atoms contain electrons, and the 2. All atoms contain electrons, and the electrons from all elements are electrons from all elements are identical.identical.

3. The atom must also contain matter 3. The atom must also contain matter with a positive charge, as atoms are with a positive charge, as atoms are neutral in charge.neutral in charge.


Thomson also carried out deflection Thomson also carried out deflection measurements, in which he applied a measurements, in which he applied a magnetic field to deflect the beam along magnetic field to deflect the beam along with an external electrical field to with an external electrical field to straighten out the bent beam.straighten out the bent beam.

From his measurements, he was able From his measurements, he was able to calculate the charge/mass ratio of the to calculate the charge/mass ratio of the electron:electron:

e/m = -1.76x10e/m = -1.76x1088 coulombs/gram coulombs/gram


Robert Millikan (1868-1963) published Robert Millikan (1868-1963) published the results of his the results of his Oil Drop ExperimentOil Drop Experiment in in 1909. He designed an apparatus that 1909. He designed an apparatus that could be used to determine the charge could be used to determine the charge on an electron.on an electron.

The device used a fine mist of oil drops The device used a fine mist of oil drops that had been exposed to ionizing that had been exposed to ionizing radiation. The radiation caused some of radiation. The radiation caused some of the oil drops to take on one or more the oil drops to take on one or more electrons.electrons.

The Charge of the The Charge of the ElectronElectron

Sub-Atomic ParticlesSub-Atomic Particles Millikin determined that the charge on Millikin determined that the charge on

the electron is -1.60 x 10the electron is -1.60 x 10-19-19coulombs.coulombs. Using Thomson’s value for the charge to Using Thomson’s value for the charge to

mass ratio of the electron, the mass of mass ratio of the electron, the mass of the electron could be calculated.the electron could be calculated.mass of emass of e-- = = (-1.60 x 10 (-1.60 x 10-19 -19 coulombs) coulombs)

(-1.76 x 10(-1.76 x 108 8 coulombs/gram)coulombs/gram)= 9.11 x 10= 9.11 x 10-28-28 grams grams

= 9.11 x 10= 9.11 x 10-31-31 kilograms kilograms

Early Atomic ModelsEarly Atomic Models

J. J. Thomson had shown that all atoms J. J. Thomson had shown that all atoms contain negatively charged particles contain negatively charged particles called electrons. Combined with the called electrons. Combined with the work of Millikan, they discovered that the work of Millikan, they discovered that the electron has very little mass. electron has very little mass.

Thomson proposed that the bulk of the Thomson proposed that the bulk of the atom is a positively charged gel or cloud, atom is a positively charged gel or cloud, with most of the atomic mass and all of with most of the atomic mass and all of the positive charge uniformly distributed the positive charge uniformly distributed throughout the gel. throughout the gel.


The electrons were viewed as discrete, The electrons were viewed as discrete, very small particles that were stuck into very small particles that were stuck into the positively charged gel or cloud “the positively charged gel or cloud “like like raisins in a pudding.raisins in a pudding.” This model is ” This model is often called the plum or raisin pudding often called the plum or raisin pudding model of the atom.model of the atom.

The electrons could be knocked out of The electrons could be knocked out of the gel if enough energy is applied, and the gel if enough energy is applied, and this is the source of the cathode rays.this is the source of the cathode rays.


One of the key features of Thomson’s atomic model is that most of the atomic mass and all of the positive charge is uniformly distributed throughout the atom.

Early Atomic ModelsEarly Atomic Models Thomson had a graduate student, Thomson had a graduate student,

Ernest Rutherford, working for him. In Ernest Rutherford, working for him. In 1911, Rutherford, Geiger and Marsden 1911, Rutherford, Geiger and Marsden performed an experiment to confirm performed an experiment to confirm Thomson’s atomic model.Thomson’s atomic model.

They bombarded a thin gold foil with They bombarded a thin gold foil with alpha (alpha (αα) particles. The ) particles. The αα particles particles have twice the charge of an electron have twice the charge of an electron and are positive in charge, with a mass and are positive in charge, with a mass that is 7300 times greater than the that is 7300 times greater than the mass of an electron. mass of an electron.

Early Atomic ModelsEarly Atomic Models The The αα particles can best be thought of as particles can best be thought of as

a positively charged, fast traveling atomic a positively charged, fast traveling atomic sized bullet. They created a thin beam of sized bullet. They created a thin beam of αα particles and directed the beam at a particles and directed the beam at a very thin gold foil.very thin gold foil.


If Thomson’s model is correct, most of the α particles should pass right through the gold atoms. Some slight deflection might occur if the positively charged α particle travels near an electron.

Gold Foil ExperimentGold Foil Experiment

Early Atomic ModelsEarly Atomic Models The film that lined the apparatus showed The film that lined the apparatus showed

that most that most αα particles went through the foil particles went through the foil with little or no deflection. However, with little or no deflection. However, some of the particles were deflected at some of the particles were deflected at great angles.great angles.


The deflection of the The deflection of the αα particles was particles was consistent with a consistent with a large concentration large concentration of positive charge of positive charge and atomic mass. and atomic mass. This very small This very small extremely dense extremely dense positively charged positively charged area is called the area is called the nucleusnucleus.

Early Atomic ModelsEarly Atomic Models The atom is mostly The atom is mostly

empty space, with empty space, with the electrons found the electrons found outside of the outside of the nucleus. If the nucleus. If the nucleus was the size nucleus was the size of a pea, it would of a pea, it would have a mass of 250 have a mass of 250 million tons, and the million tons, and the electrons would electrons would occupy a volume occupy a volume approximately the approximately the size of a stadium.size of a stadium.

Atomic NucleusAtomic Nucleus


We now know that the positive charge We now know that the positive charge of an atom, contained in the nucleus, of an atom, contained in the nucleus, is due to particles called is due to particles called protonsprotons. .

Protons have a charge equal in Protons have a charge equal in magnitude to that of an electron, but magnitude to that of an electron, but positive in charge.positive in charge.

The mass of a proton is roughly 1800 The mass of a proton is roughly 1800 times greater than the mass of an times greater than the mass of an electron.electron.


The nuclei of atoms also can contain The nuclei of atoms also can contain neutronsneutrons. Neutrons are . Neutrons are neutral in charge, with a mass similar to that of a proton. neutral in charge, with a mass similar to that of a proton.

Neutrons are found in the nucleus of atoms, along with Neutrons are found in the nucleus of atoms, along with protons.protons.


During chemical reactions, atoms may During chemical reactions, atoms may lose or gain electrons to form charged lose or gain electrons to form charged particles called particles called ionsions..

Atoms of a given element may have Atoms of a given element may have differing numbers of neutrons. These differing numbers of neutrons. These forms of the same element are called forms of the same element are called isotopesisotopes..

It is the number of protons or the It is the number of protons or the atomic atomic numbernumber that defines the identity of the that defines the identity of the atom.atom.

Atomic SymbolsAtomic Symbols

The periodic table lists the elements The periodic table lists the elements in order of in order of increasing atomic increasing atomic numbernumber (the number of protons). (the number of protons).

The atomic number, represented by The atomic number, represented by the letter Z, is linked with the atomic the letter Z, is linked with the atomic symbol. For example, oxygen is symbol. For example, oxygen is atomic number 8, and any atom atomic number 8, and any atom containing 8 protons, regardless of containing 8 protons, regardless of the number of neutrons or electrons, the number of neutrons or electrons, is represented by the symbol O.is represented by the symbol O.


To indicate a specific isotope, the atomic To indicate a specific isotope, the atomic symbol must also contain the symbol must also contain the mass mass numbernumber..

The The mass numbermass number is the number of is the number of neutrons plus protons for a particular neutrons plus protons for a particular isotope. The mass number is isotope. The mass number is nevernever found found on the periodic tableon the periodic table..

Since the mass number is the number of Since the mass number is the number of particles (neutrons + protons) in the particles (neutrons + protons) in the nucleus, it is nucleus, it is always an integeralways an integer..

Isotopes of SodiumIsotopes of Sodium

Mass numberAtomic number

Atomic SymbolsAtomic Symbols For example, there are three isotopes of For example, there are three isotopes of

carbon:carbon:1212C, C, 1313C and C and 1414CC

The The mass numbermass number, if specified, appears , if specified, appears in the upper left corner of an atomic in the upper left corner of an atomic symbol. Since all carbon atoms have 6 symbol. Since all carbon atoms have 6 protons (carbon is atomic number 6 on protons (carbon is atomic number 6 on the periodic table), atoms of carbon may the periodic table), atoms of carbon may have 6, 7 or 8 neutrons in the nucleus.have 6, 7 or 8 neutrons in the nucleus.The isotopes are called The isotopes are called carbon-12, carbon-12, carbon-13 carbon-13 andand carbon-14 carbon-14..


If the atom has lost or gained electrons, If the atom has lost or gained electrons, the charge is written in the upper right the charge is written in the upper right corner of the atomic symbol.corner of the atomic symbol.

The atomic number, though optional, The atomic number, though optional, may be written in the lower left corner may be written in the lower left corner of the symbol.of the symbol.

3737ClCl1-1-

This ion of chlorine contains 17 This ion of chlorine contains 17 protons, 20 neutrons, and 18 electrons.protons, 20 neutrons, and 18 electrons.

Relative Atomic MassesRelative Atomic Masses

Once the formulas of simple gases Once the formulas of simple gases and compounds could be determined, and compounds could be determined, scientists could also determine the scientists could also determine the relative masses of the elements.relative masses of the elements.

For example, since equal volumes of For example, since equal volumes of gases contain equal numbers of gases contain equal numbers of particles (at the same T and P), the particles (at the same T and P), the masses of gases could be compared masses of gases could be compared to hydrogen, the lightest gas.to hydrogen, the lightest gas.

StoichiometryStoichiometry

Stoichiometry is a Greek word that Stoichiometry is a Greek word that means using chemical reactions to means using chemical reactions to calculate the amount of reactants calculate the amount of reactants needed and the amount of products needed and the amount of products formed.formed.

Amounts are typically calculated in Amounts are typically calculated in grams (or kg), but there are other grams (or kg), but there are other ways to specify the quantities of ways to specify the quantities of matter involved in a reaction.matter involved in a reaction.


As the early chemists explored the As the early chemists explored the nature of matter, they discovered that nature of matter, they discovered that atoms of the elements had different atoms of the elements had different masses.masses.

Avogadro’s Hypothesis which states Avogadro’s Hypothesis which states that under the constant temperature that under the constant temperature and pressure and pressure equal volumes of gases equal volumes of gases contain an equal number of particles contain an equal number of particles could be used to determine could be used to determine relative relative atomic massesatomic masses for gaseous elements. for gaseous elements.


Equal volumes of gases contain an Equal volumes of gases contain an equal number of particles.equal number of particles.

Although the number of particles Although the number of particles (atoms or molecules) in a liter of gas (atoms or molecules) in a liter of gas (at a specific T and P) wasn’t known, (at a specific T and P) wasn’t known, Avogadro’s Hypothesis said that a Avogadro’s Hypothesis said that a liter of any other gas under the same liter of any other gas under the same conditions would contain the conditions would contain the same same numbernumber of particles. of particles.


Equal volumes of gases contain an Equal volumes of gases contain an equal number of particles.equal number of particles.

Since the masses of the gaseous Since the masses of the gaseous samples could be determined, a samples could be determined, a comparative or comparative or relativerelative scale of scale of atomic and molecular masses could atomic and molecular masses could be derived.be derived.

Relative Atomic MassesRelative Atomic Masses Equal volumes of gases contain an equal Equal volumes of gases contain an equal

number of particles.number of particles. For example, if the masses of a liter of oxygen For example, if the masses of a liter of oxygen (O(O22), chlorine (Cl), chlorine (Cl22) and hydrogen (H) and hydrogen (H22) were ) were compared under identical conditions, the compared under identical conditions, the hydrogen sample has the smallest mass, and hydrogen sample has the smallest mass, and the chlorine sample has the largest mass.the chlorine sample has the largest mass.

The ratio of the masses of the 1 liter samples The ratio of the masses of the 1 liter samples is:is:

35.5/16.0/1.0035.5/16.0/1.00

ClCl22// OO22/ H/ H22

Relative Atomic MassesRelative Atomic Masses Equal volumes of gases contain an equal Equal volumes of gases contain an equal

number of particles.number of particles.

The ratio of the masses of the 1 liter samples The ratio of the masses of the 1 liter samples is:is:

35.5/16.0/1.0035.5/16.0/1.00

ClCl22// OO22/ H/ H22

Since all three gases are diatomic, we can say Since all three gases are diatomic, we can say that an oxygen atom is 16.0 times heavier than that an oxygen atom is 16.0 times heavier than a hydrogen atom, and that a chorine atom is a hydrogen atom, and that a chorine atom is 35.5 times heavier than a hydrogen atom.35.5 times heavier than a hydrogen atom.


A scale of A scale of relative atomic massrelative atomic mass was was devised. Individual atoms are much devised. Individual atoms are much too small to weigh, but the masses of too small to weigh, but the masses of large collections of atoms could large collections of atoms could easily be compared.easily be compared.

The relative masses of the atoms The relative masses of the atoms are listed on the periodic table. An are listed on the periodic table. An arbitrary unit, the arbitrary unit, the atomic mass unitatomic mass unit (amu) is used for relative masses.(amu) is used for relative masses.

Relative Atomic MassesRelative Atomic Masses Eventually, the carbon-12 isotope (Eventually, the carbon-12 isotope (1212C) C)

was assigned an atomic mass of exactly was assigned an atomic mass of exactly 12 atomic mass units, and all other 12 atomic mass units, and all other atomic masses are expressed relative atomic masses are expressed relative to this assignment.to this assignment.

The atomic mass for carbon, found on The atomic mass for carbon, found on the periodic table, is 12.01 amu, and the periodic table, is 12.01 amu, and notnot 12.000 amu. This is because the 12.000 amu. This is because the periodic table lists the periodic table lists the averageaverage relative relative atomic mass for all isotopes of the atomic mass for all isotopes of the element.element.

Relative Atomic MassesRelative Atomic Masses Carbon exists as three isotopes:Carbon exists as three isotopes:

1212C has a relative mass of exactly 12 amuC has a relative mass of exactly 12 amu1313C has a relative atomic mass of 13.003 C has a relative atomic mass of 13.003

amuamu1414C has a relative atomic mass of 14.0 amuC has a relative atomic mass of 14.0 amu

The value found on the periodic table, The value found on the periodic table, 12.01 amu, reflects the relative abundance 12.01 amu, reflects the relative abundance of the isotopes. The majority of carbon of the isotopes. The majority of carbon (98.89%) is (98.89%) is 1212C, with 1.11% C, with 1.11% 1313C, and a trace of C, and a trace of 1414C.C.


Chorine exists as two isotopes:Chorine exists as two isotopes:3535Cl, with a relative atomic mass of Cl, with a relative atomic mass of 35.0 amu35.0 amu

and and 3737Cl, with a relative atomic mass Cl, with a relative atomic mass of 37.0 amu.of 37.0 amu.

What does the atomic mass of What does the atomic mass of chlorine on the periodic table tell you chlorine on the periodic table tell you about the relative abundance of the about the relative abundance of the two isotopes?two isotopes?

MolesMoles Many chemical reactions are carried out Many chemical reactions are carried out

using a few grams of each reactant. using a few grams of each reactant. Such quantities contain huge numbers Such quantities contain huge numbers (on the order of 10(on the order of 102323) of atoms or ) of atoms or molecules.molecules.

A unit of quantity of matter, the A unit of quantity of matter, the molemole, , was established. A mole is defined as was established. A mole is defined as the number of carbon atoms in exactly the number of carbon atoms in exactly 12 grams of 12 grams of 1212C.C.

Avogadro determined the number of Avogadro determined the number of particles (atoms or molecules) in a mole.particles (atoms or molecules) in a mole.

MolesMoles Avogadro’s number = 6.022 x 10Avogadro’s number = 6.022 x 102323 particles/mole particles/mole

Atoms are so small, that a mole of most Atoms are so small, that a mole of most substances can be easily held in ones hand.substances can be easily held in ones hand.

CuAl

Hg Fe

I2

S

MolesMoles

If we consider objects we can see, a If we consider objects we can see, a mole of sand would cover the entire mole of sand would cover the entire planet and be several miles deep! planet and be several miles deep! However, the collection of atoms, However, the collection of atoms, called a mole, is very convenient in called a mole, is very convenient in the laboratory (just like dozens are the laboratory (just like dozens are useful in buying eggs or pencils).useful in buying eggs or pencils).

MolesMoles

A mole of any atom has a mass equal A mole of any atom has a mass equal to the element’s atomic mass to the element’s atomic mass expressed expressed in gramsin grams..

A mole of iron atoms has a mass of A mole of iron atoms has a mass of 55.85 grams; a mole of iodine 55.85 grams; a mole of iodine moleculesmolecules (I (I22) has a mass of (126.9) ) has a mass of (126.9) (2) = 253.8 grams.(2) = 253.8 grams.

MolesMoles The masses of each molar sample are The masses of each molar sample are

provided below.provided below.Cu = 63.55g Al=26.9

8g

Hg = 200.6 g

Fe=55.85 g

I2=253.8 g

S=32.07g

Molar MassMolar Mass For compounds, once the formula is For compounds, once the formula is

known, the mass of a mole of the known, the mass of a mole of the substance can be calculated by summing substance can be calculated by summing up the masses of all the atoms in the up the masses of all the atoms in the compound. compound. For example, hydrogen peroxide has the For example, hydrogen peroxide has the formula Hformula H22OO22::2H +2O = 2(1.008g) + 2(16.00g) = 34.02 2H +2O = 2(1.008g) + 2(16.00g) = 34.02 g/molg/mol A mole of hydrogen peroxide has a mass A mole of hydrogen peroxide has a mass of 34.02 grams.of 34.02 grams.

Nuclear ChemistryNuclear Chemistry

Many nuclei are unstable, and Many nuclei are unstable, and may emit particles and/or energy until may emit particles and/or energy until they reach a more stable combination they reach a more stable combination of protons and neutrons.of protons and neutrons.

A nuclear reaction differs from a A nuclear reaction differs from a chemical reaction, and results in the chemical reaction, and results in the formation of new elements as an formation of new elements as an unstable nucleus decays.unstable nucleus decays.

Nuclear StabilityNuclear Stability

For the lighter elements in the For the lighter elements in the periodic table, most of the stable periodic table, most of the stable nuclei contain approximately an nuclei contain approximately an equal number of protons and equal number of protons and neutrons. For heavier elements, the neutrons. For heavier elements, the neutron to proton ratio generally neutron to proton ratio generally increases in the stable isotopes.increases in the stable isotopes.


The mass of a nucleus is less The mass of a nucleus is less than the sum of the masses of its than the sum of the masses of its neutrons and protons. A very small neutrons and protons. A very small amount of mass is converted into the amount of mass is converted into the nuclear binding energynuclear binding energy..

During nuclear reactions, some During nuclear reactions, some of this energy is released. A very of this energy is released. A very small loss of mass results in a huge small loss of mass results in a huge release of energy.release of energy.

Nuclear Binding EnergyNuclear Binding Energy


Smaller atoms will undergo Smaller atoms will undergo fusion, and combine to form heavier fusion, and combine to form heavier atoms. Larger atoms will undergo atoms. Larger atoms will undergo fission, and break into smaller nuclei fission, and break into smaller nuclei and particles.and particles.

Nuclear EquationsNuclear Equations

Nuclear equations represent Nuclear equations represent only the nuclei of the particles, only the nuclei of the particles, rather than the atom complete with rather than the atom complete with electrons.electrons.

Nuclear equations must be Nuclear equations must be balanced. This means that the sum balanced. This means that the sum of the mass numbers on the left side of the mass numbers on the left side equals the sum of the mass numbers equals the sum of the mass numbers on the right side.on the right side.

Nuclear EquationsNuclear Equations

Likewise, the sum of the atomic Likewise, the sum of the atomic numbers on the left must equal the numbers on the left must equal the sum of the atomic numbers on the sum of the atomic numbers on the right.right.

Note that the Note that the identityidentity of the of the elements will change during a elements will change during a nuclear reaction.nuclear reaction.

Nuclear ReactionsNuclear Reactions

Unstable nuclei can decay in a Unstable nuclei can decay in a variety of ways. If the nucleus variety of ways. If the nucleus contains too many protons/neutron, contains too many protons/neutron, βemission may occur. A β particle is βemission may occur. A β particle is an electron, but it results from a an electron, but it results from a neutron disintegrating into a proton neutron disintegrating into a proton and an electron. The proton remains and an electron. The proton remains in the nucleus of the newly formed in the nucleus of the newly formed atom, and the electron is ejected.atom, and the electron is ejected.

βEmissionβEmission

The net result of β emission is an increase in the number of protons in the nucleus.

βEmissionβEmission

Nuclear reactions must be balanced in terms of the total of the atomic numbers and the total of the mass numbers.

α Emissionα Emission

Some unstable nuclei emit an α Some unstable nuclei emit an α particle. α particles are the same as particle. α particles are the same as a helium nucleus. The alpha particle a helium nucleus. The alpha particle contain two protons and two contain two protons and two neutrons. The net result is a neutrons. The net result is a decrease in both atomic number (by decrease in both atomic number (by 2) and mass number (by 4).2) and mass number (by 4).


Alpha emission is common in

heavier elements. All elements above 209Bi are radioactive. They often emit alpha particles (along with other forms of radiation) until they attain a stable nucleus.

γ Emissionγ Emission

Gamma (γ) emission is emission Gamma (γ) emission is emission of a very high energy form of of a very high energy form of radiation. It accompanies both α radiation. It accompanies both α and β emission, and represents a and β emission, and represents a release of energy. Since it is not a release of energy. Since it is not a particle, it is often omitted from particle, it is often omitted from nuclear equations.nuclear equations.

Positron EmissionPositron Emission

Nuclei with a neutron to proton ratio Nuclei with a neutron to proton ratio which is too low, may undergo positron which is too low, may undergo positron emission. A positron, βemission. A positron, β++, has the same , has the same mass as an electron, but it is positive in mass as an electron, but it is positive in charge. charge.

The positron results from a proton The positron results from a proton decaying into a neutron and ejecting the decaying into a neutron and ejecting the positron. As a result, the number of positron. As a result, the number of neutrons increases, and the number of neutrons increases, and the number of protons decreases.protons decreases.

Positron EmissionPositron Emission

A positron has the same mass as an electron but an opposite charge. It can be thought of as a “positive electron.”

Positron, +

Electron CaptureElectron Capture

Nuclei with a neutron to proton Nuclei with a neutron to proton ratio which is too low, may undergo ratio which is too low, may undergo electron capture. In this process, a electron capture. In this process, a proton in the nucleus combines with proton in the nucleus combines with an electron outside the nucleus to an electron outside the nucleus to form a neutron.form a neutron.

The net result is an decrease in The net result is an decrease in atomic number and an increase in atomic number and an increase in the mass number.the mass number.

Electron CaptureElectron Capture

the structure & stability of atoms. early atomic history there have been many different...

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