Atomic TheoryAtomic Theory

DemocritusDemocritus The Greek philosopher The Greek philosopher DemocritusDemocritus (460 B.C. – 370 (460 B.C. – 370

B.C.) was among the first to suggest the existence B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”)of atoms (from the Greek word “atomos”) His atomic model was called the “solid sphere” His atomic model was called the “solid sphere”

modelmodel

He believed that atoms were He believed that atoms were indivisibleindivisible and and indestructibleindestructible

He named the smallest piece of matter “atomos,” He named the smallest piece of matter “atomos,” meaning “not to be cut.”meaning “not to be cut.”

DemocritusDemocritusBased on his theory: Based on his theory: Atoms were Atoms were infiniteinfinite in number, always moving in number, always moving and capable of joining togetherand capable of joining togetherAtoms were Atoms were smallsmall, hard particles that were all , hard particles that were all made of the same material but were made of the same material but were differentdifferent shapes and sizes.shapes and sizes.Problems with the theory: Problems with the theory:

did not explain chemical behaviordid not explain chemical behavior was was not based on the scientific methodnot based on the scientific method – but just – but just

philosophyphilosophy

John DaltonJohn Dalton 1766 - 18441766 - 1844• All matter is made of atoms.All matter is made of atoms.• Atoms of an element are identical.Atoms of an element are identical.• Each element has different atoms.Each element has different atoms.• Atoms of different elements combine in Atoms of different elements combine in

constant ratios to form compounds.constant ratios to form compounds.• Atoms are rearranged in reactions.Atoms are rearranged in reactions.• Was called the “billiard ball” or “solid sphere” Was called the “billiard ball” or “solid sphere”

modelmodel

John DaltonJohn Dalton

Improved on Democritus’s theory Improved on Democritus’s theory Dalton’s theory was based on scientific Dalton’s theory was based on scientific

experimentation experimentation His ideas account for the law of conservation His ideas account for the law of conservation

of mass - atoms are neither created nor of mass - atoms are neither created nor destroyeddestroyed

Agree with the law of constant composition - Agree with the law of constant composition - elements combine in fixed ratioselements combine in fixed ratios

John DaltonJohn Dalton

Problems with Dalton’s Theory:Problems with Dalton’s Theory: Discoveries were made that showed Discoveries were made that showed

that that atoms are divisibleatoms are divisible into smaller into smaller subatomic particles:subatomic particles:Electrons, protons, and neutronsElectrons, protons, and neutronsThere are many other particles There are many other particles

J.J. ThomsonJ.J. Thomson

Called the “plum pudding” modelCalled the “plum pudding” model

Provided the first hint that an atom is Provided the first hint that an atom is made of even made of even smallersmaller particles. particles.

Thomson ModelThomson Model

He proposed that atoms were made from He proposed that atoms were made from

a positively a positively chargedcharged substancesubstance with with

negatively charged electrons negatively charged electrons scatteredscattered

about, like raisins in a pudding.about, like raisins in a pudding.

Thomson ModelThomson Model

Thomson studied the passage of an electric current through a gas.

As the current passed through the gas, it gave off rays of negatively charged particles.

J. J. ThomsonJ. J. Thomson Improvements: Improvements:

Discovered particles smaller than the atom Discovered particles smaller than the atom exist, therefore, the atom was exist, therefore, the atom was divisibledivisible

He also concluded that there must be He also concluded that there must be positivelypositively and and negativelynegatively charged particles charged particles within the atom.within the atom.

Discovered the electronDiscovered the electron Problems:Problems:

Could not discover the protonCould not discover the proton

Ernest Rutherford’sErnest Rutherford’sGold Foil Experiment - 1911Gold Foil Experiment - 1911

• Positively charged alpha particle were fired at a thin sheet of gold foil.

• Some of the positively charged particles bounced away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.

Rutherford’s FindingsRutherford’s Findings

a) The nucleus is smallb) The nucleus is densec) The nucleus is positively chargedd) The electrons move around the

nucleus like planets orbit the sun.e) Called the nuclear model or the

planetary model

Problems: Based on classical physics, later abandoned because of the discoveries in quantum physics.

Conclusions/Improvements:

Bohr ModelBohr Model In 1913, the Danish scientist Niels Bohr In 1913, the Danish scientist Niels Bohr

proposed an improvement to Rutherford’s proposed an improvement to Rutherford’s model. His model is sometimes referred model. His model is sometimes referred to as the Rutherford-Bohr model . to as the Rutherford-Bohr model .

Bohr’s ModelBohr’s Model Electrons orbit the nucleus in orbits that have a Electrons orbit the nucleus in orbits that have a

set size and energy. set size and energy. The energy of the orbit is related to its size. The The energy of the orbit is related to its size. The

lowest energy is found in the smallest orbit. lowest energy is found in the smallest orbit. Radiation is absorbed or emitted when an Radiation is absorbed or emitted when an

electron moves from one orbit to another. electron moves from one orbit to another. Problems:Problems:

We cannot predict the exact location and orbit We cannot predict the exact location and orbit of electrons.of electrons.

Predictions fail to work on larger atoms.Predictions fail to work on larger atoms.

James ChadwickJames Chadwick 1932 – confirmed the existence of the neutron – a

particle with no charge, but a mass nearly equal to a proton

He was trying to identify the extra mass of an atomic nucleus by firing alpha particles into a beryllium target and allowing the resulting radiation to interact with paraffin wax.

The interactions between the radiation and the hydrogen in the wax led to the discovery of the neutron.

James ChadwickJames Chadwick

Improvements: Improvements: With the discovery of the neutron, the With the discovery of the neutron, the

atomic model seemed more complete than atomic model seemed more complete than ever. ever.

The overall charges remained the sameThe overall charges remained the same Now there no longer seemed to be a Now there no longer seemed to be a

discrepancy between the atomic mass and discrepancy between the atomic mass and the atomic number.the atomic number.

Electron CloudElectron Cloud A space in which A space in which

electrons are likely to electrons are likely to be found.be found.

Electrons Electrons whirlwhirl about about the nucleus billions of the nucleus billions of times in one secondtimes in one second

They are not moving They are not moving around in around in randomrandom patterns.patterns.

Location of electrons Location of electrons depends upon how depends upon how much much energyenergy the the electron has.electron has.

Electron CloudElectron Cloud

Depending on their energy they are locked Depending on their energy they are locked into a certain area in the cloud.into a certain area in the cloud.

Electrons with the Electrons with the lowestlowest energy are found energy are found in the energy level in the energy level closestclosest to the nucleus to the nucleus

Electrons with the Electrons with the highesthighest energy are energy are found in the found in the outermostoutermost energy levels, energy levels, farther from the nucleus.farther from the nucleus.

Atomic StructureAtomic Structure

An atom consists of:An atom consists of: nucleusnucleus – contains protons and neutrons – contains protons and neutrons electronselectrons in space around the nucleus. in space around the nucleus. The atom is mostly empty spaceThe atom is mostly empty space

The The AtomAtom

NucleusNucleus

Electron cloudElectron cloud

ATOMIC COMPOSITIONATOMIC COMPOSITION Protons (pProtons (p++))

+ electrical charge+ electrical charge mass = 1.672623 x 10mass = 1.672623 x 10-24-24 g g relative mass = 1.007 atomic mass units (amu) relative mass = 1.007 atomic mass units (amu)

but we can round to 1but we can round to 1 Electrons (eElectrons (e--))

negative electrical chargenegative electrical charge relative mass = 0.0005 amu relative mass = 0.0005 amu

but we can round to 0but we can round to 0 Neutrons (nNeutrons (noo))

no electrical chargeno electrical charge mass = 1.009 amu mass = 1.009 amu but we can round to 1but we can round to 1

Atomic Number, ZAtomic Number, ZAll atoms of the same element have All atoms of the same element have

the same number of protons in the the same number of protons in the nucleus, nucleus, ZZ

1313

AlAl

26.98126.981

Atomic numberAtomic number

Atom symbolAtom symbol

AVERAGE Atomic MassAVERAGE Atomic Mass

Atomic NumberAtomic Number

Atoms are composed of Atoms are composed of identicalidentical protons, protons, neutrons, and electronsneutrons, and electrons How then are atoms of one element different How then are atoms of one element different

from another element?from another element? Elements are different because they contain Elements are different because they contain

different numbers of different numbers of PROTONSPROTONS The “The “atomic numberatomic number” of an element is the ” of an element is the number number

of protonsof protons in the nucleus in the nucleus # protons in an atom = # electrons# protons in an atom = # electrons

Atomic NumberAtomic Number

ElementElement # of protons# of protons Atomic # (Z)Atomic # (Z)

CarbonCarbon 66 66

PhosphorusPhosphorus 1515 1515

GoldGold 7979 7979

Mass Number, AMass Number, A A carbon atom with 6 protons and 6 A carbon atom with 6 protons and 6

neutrons is the mass number = 12 neutrons is the mass number = 12 atomic mass unitsatomic mass units

Mass NumberMass Number (A) (A)= # protons + # neutrons= # protons + # neutrons

NOT on the periodic table…(Round the NOT on the periodic table…(Round the AVERAGE atomic mass on the table)AVERAGE atomic mass on the table)

A boron atom can have A boron atom can have A = 5 p + 5 n = 10 amuA = 5 p + 5 n = 10 amu

A

Z

10

5B

A

Z

10

5B

Mass NumberMass Number

Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0

NuclideNuclide pp++ nn00 ee-- Mass #Mass #

Oxygen Oxygen - - 1010

- - 3333 4242

- - 3131 1515

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

Complete Symbols/ Nuclear NotationComplete Symbols/ Nuclear Notation

Contain the symbol of the element, Contain the symbol of the element, the mass number and the atomic the mass number and the atomic number.number.

X Massnumber

Atomicnumber

Subscript →

Superscript →

SymbolsSymbols Find each of these: Find each of these:

a)a) number of protonsnumber of protons

b)b) number of number of neutronsneutrons

c)c) number of number of electronselectrons

d)d) Atomic numberAtomic number

e)e) Mass NumberMass Number

Br80 35

SymbolsSymbols If an element has an atomic If an element has an atomic

number of 34 and a mass number of 34 and a mass number of 78, what is the: number of 78, what is the:

a)a) number of protonsnumber of protons

b)b) number of neutronsnumber of neutrons

c)c) number of electronsnumber of electrons

d)d) complete symbolcomplete symbol

SymbolsSymbols If an element has 91 If an element has 91

protons and 140 neutrons protons and 140 neutrons what is the what is the

a)a) Atomic numberAtomic number

b)b) Mass numberMass number

c)c) number of electronsnumber of electrons

d)d) complete symbolcomplete symbol

SymbolsSymbols If an element has 78 If an element has 78

electrons and 117 neutrons electrons and 117 neutrons what is the what is the

a)a) Atomic numberAtomic number

b)b) Mass numberMass number

c)c) number of protonsnumber of protons

d)d) complete symbolcomplete symbol

IsotopesIsotopes

IsotopesIsotopes

Dalton was wrong about all Dalton was wrong about all elements of the same type elements of the same type being identicalbeing identical

Atoms of the same element Atoms of the same element cancan have different numbers of have different numbers of neutronsneutrons..

Thus, different mass numbers.Thus, different mass numbers.These are called These are called isotopesisotopes..

IsotopesIsotopes

Frederick SoddyFrederick Soddy (1877-1956) proposed the (1877-1956) proposed the idea of isotopes in 1912idea of isotopes in 1912

Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.

Soddy won the Nobel Prize in Chemistry in Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and 1921 for his work with isotopes and radioactive materials.radioactive materials.

Naming IsotopesNaming Isotopes

We can also put the mass We can also put the mass number number afterafter the name of the the name of the element:element:carbon-12carbon-12carbon-14carbon-14uranium-235uranium-235

Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.

IsotopeIsotope ProtonsProtons ElectronsElectrons NeutronsNeutrons NucleusNucleus

Hydrogen–1Hydrogen–1

(protium)(protium) 11 11 00

Hydrogen-2Hydrogen-2

(deuterium)(deuterium) 11 11 11

Hydrogen-3Hydrogen-3

(tritium)(tritium)

11 11 22

IsotopesIsotopesElements occur in nature as mixtures of isotopes.

Isotopes are atoms of the same element that differ in the number of neutrons.

Isotopes Isotopes & Their & Their

UsesUsesBone scans with Bone scans with radioactive radioactive technetium-99. technetium-99.

Isotopes & Their Isotopes & Their UsesUses

The The tritiumtritium content of ground water is content of ground water is used to discover the source of the used to discover the source of the water, for example, in municipal water water, for example, in municipal water or the source of the steam from a or the source of the steam from a volcano. volcano.

Atomic MassAtomic Mass How heavy is an atom of oxygen?How heavy is an atom of oxygen?

It depends, because there are different It depends, because there are different kindskinds of oxygen atoms. of oxygen atoms.

We are more concerned with the We are more concerned with the average average atomic mass.atomic mass.

This is based on the abundance This is based on the abundance (percentage) of each variety of that (percentage) of each variety of that element in nature.element in nature. We don’t use grams for this mass because We don’t use grams for this mass because

the numbers would be too small.the numbers would be too small.

Measuring Atomic MassMeasuring Atomic Mass Instead of grams, the unit we use is the Instead of grams, the unit we use is the Atomic Atomic

Mass UnitMass Unit (amu)(amu) It is defined as one-twelfth the mass of a carbon-12 It is defined as one-twelfth the mass of a carbon-12

atom.atom. Carbon-12 chosen because of its Carbon-12 chosen because of its isotope purityisotope purity. .

Each isotope has its own atomic mass, thus we Each isotope has its own atomic mass, thus we determine the average from percent abundance.determine the average from percent abundance.

To calculate the average:To calculate the average: Multiply the atomic mass of each isotope Multiply the atomic mass of each isotope

by it’s abundance (expressed as a by it’s abundance (expressed as a decimal), then add the results.decimal), then add the results.

If not told otherwise, the mass of the If not told otherwise, the mass of the isotope is expressed in isotope is expressed in atomic mass unitsatomic mass units (amu)(amu)

Atomic MassesAtomic Masses

IsotopeIsotope SymbolSymbol Composition of Composition of the nucleusthe nucleus

% in nature% in nature

Carbon-12Carbon-12 1212CC 6 protons6 protons

6 neutrons6 neutrons

98.89%98.89%

Carbon-13Carbon-13 1313CC 6 protons6 protons

7 neutrons7 neutrons

1.11%1.11%

Carbon-14Carbon-14 1414CC 6 protons6 protons

8 neutrons8 neutrons

<0.01%<0.01%

Atomic mass is the average of all the naturally occurring isotopes of that element.

Carbon = 12.011

Average Atomic MassAverage Atomic Mass

weighted average of all isotopesweighted average of all isotopes on the Periodic Tableon the Periodic Table round to 2 decimal placesround to 2 decimal places

100

(%)(mass(mass)(%) )

Avg.AtomicMass

Avg.AtomicMass

D. Average Atomic MassD. Average Atomic Mass

EXEX: Calculate the avg. atomic mass of oxygen if : Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% its abundance in nature is 99.76% 1616O, 0.04% O, 0.04% 1717O, and 0.20% O, and 0.20% 1818O.O.

100

(18)(0.20)(17)(0.04))(16)(99.76 16.00amu

Avg.AtomicMass

D. Average Atomic MassD. Average Atomic Mass

EXEX: Find chlorine’s average atomic mass if : Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.35 and 2 are chlorine-37.

10

(37)(2)(35)(8)35.40 amu