Atoms Unit 3Atoms Unit 3Introduction to the AtomIntroduction to the Atom

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Some of the First Discussions Some of the First Discussions about the Atom….about the Atom….

DemocritusDemocritus (400 BC) (400 BC) named the atomnamed the atom, which , which means indivisible, and said it was the means indivisible, and said it was the smallest particlesmallest particle and and could not be broken could not be broken downdown. .

He was pretty much correct in that if you did He was pretty much correct in that if you did break the atom down, it wouldn’t be that break the atom down, it wouldn’t be that “element” any more. But he wasn’t aware “element” any more. But he wasn’t aware that the atom was made up of particles.that the atom was made up of particles.

Greeks Philosophers Continue the Greeks Philosophers Continue the Discussion and DisagreeDiscussion and Disagree

Aristotle (350 BC) believed Aristotle (350 BC) believed matter matter could always be broken down into could always be broken down into smaller and smaller parts. smaller and smaller parts.

And this is true to a certain extent… And this is true to a certain extent… but eventually you come to the small but eventually you come to the small basic particles that can no longer be basic particles that can no longer be broken apart.broken apart.

Dalton's Atomic TheoryDalton's Atomic Theory

In 1808, John Dalton proposedIn 1808, John Dalton proposed that that elements were composed of atoms & elements were composed of atoms & that only whole numbers of atoms that only whole numbers of atoms can combine to form compoundscan combine to form compounds

His ideas are now called the Atomic His ideas are now called the Atomic Theory of MatterTheory of Matter

Dalton’s Atomic Theory Dalton’s Atomic Theory (1766 - 1844 AD)(1766 - 1844 AD) was was widely accepted but not totally correctwidely accepted but not totally correct

1.1. All matter is composed of All matter is composed of extremely small particles extremely small particles called atomscalled atoms

2.2. Atoms of a given Atoms of a given element are identical in element are identical in size, mass, and other size, mass, and other properties; atoms of properties; atoms of different elements differ different elements differ in size, mass, & other in size, mass, & other propertiesproperties

ELEMENT2

ELEMENT2

ELEMENT 3

ELEMENT 3

ELEMENT 1

ELEMENT 1

ELEMENT4

ELEMENT4

Dalton’s Atomic TheoryDalton’s Atomic Theory

3.3. Atoms cannot be Atoms cannot be subdivided, subdivided, created, or created, or destroyeddestroyed

4.4. atoms of different atoms of different elements combine elements combine in simple whole # in simple whole # ratios to form ratios to form chemical chemical compoundscompounds

5.5. in chemical in chemical reactions, atoms reactions, atoms are combined, are combined, separated, or separated, or rearrangedrearranged

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Not all of Dalton’s claims held up to the Not all of Dalton’s claims held up to the scrutiny of experimentationscrutiny of experimentation

Atoms CAN be divided into even smaller Atoms CAN be divided into even smaller particlesparticles

Not every atom of an element has an Not every atom of an element has an identical mass (isotopes are atoms of the identical mass (isotopes are atoms of the same element, but with different mass same element, but with different mass numbers)numbers)

Atomic TheoryAtomic Theory

Dalton’s Atomic Theory of Matter has been Dalton’s Atomic Theory of Matter has been modified. modified.

What remains is…What remains is…1.1. All matter is composed of atomsAll matter is composed of atoms2.2. Atoms of any one element differ in Atoms of any one element differ in

properties from atoms of another elementproperties from atoms of another element

One of the disputed statements of Dalton was One of the disputed statements of Dalton was that atoms are indivisible (we now know that atoms are indivisible (we now know about protons, electrons, and neutrons)about protons, electrons, and neutrons)

Atomic TheoryAtomic Theory

In the 1800’s it was determined that atoms are In the 1800’s it was determined that atoms are actually composed of several basic types of actually composed of several basic types of smaller particles smaller particles

It’s the number and arrangement of these It’s the number and arrangement of these particles that determine the atom’s chemical particles that determine the atom’s chemical properties.properties.

A new definition of an atom is the one use A new definition of an atom is the one use today: the smallest particle of an element today: the smallest particle of an element that that retains the chemical propertiesretains the chemical properties of that of that original element.original element.

Atomic TheoryAtomic Theory

Plum Pudding ModelPlum Pudding Model

JJ Thomson’s cathode ray tube JJ Thomson’s cathode ray tube experiment in the late 1800s showed experiment in the late 1800s showed that atoms had smaller parts, called that atoms had smaller parts, called negative corpuscles; he developed the negative corpuscles; he developed the “plum pudding model.”“plum pudding model.”

The plum pudding model showed The plum pudding model showed electrons (the plums) mixed in together electrons (the plums) mixed in together with protons (the cake batter)with protons (the cake batter)

Negative particles

embedded in a sphere of

positive plasma-like matter.

THINK… Chocolate Chip

Cookie

Negative particles

embedded in a sphere of

positive plasma-like matter.

THINK… Chocolate Chip

Cookie

Atomic StructureAtomic Structure

Scientists still didn’t really understand Scientists still didn’t really understand how the particles were put together in how the particles were put together in an atom.an atom.

This was a difficult question to This was a difficult question to resolve, given how tiny atoms are.resolve, given how tiny atoms are.

Most thought it likely that the atom Most thought it likely that the atom resembled Thomson’s modelresembled Thomson’s model

Rutherford’s gold foil experimentRutherford’s gold foil experiment

In 1911, Ernest Rutherford provided a more In 1911, Ernest Rutherford provided a more detailed picture of the internal structure of the detailed picture of the internal structure of the atomatom

He showed that atoms had a nucleus where most He showed that atoms had a nucleus where most of the mass resided. of the mass resided.

His model had a nucleus with a hard dense His model had a nucleus with a hard dense positively charged mass, negatively charged positively charged mass, negatively charged electrons outside, and that the atom was actually electrons outside, and that the atom was actually mostly empty spacemostly empty space..

Rutherford ModelRutherford Model

Bohr Model of the AtomBohr Model of the Atom

Neils Bohr put Neils Bohr put electrons into electrons into different energy different energy levels or shells.levels or shells.

(This model is not (This model is not correct either… correct either… electrons electrons do notdo not travel in orbits travel in orbits or paths like the or paths like the model suggests)model suggests)

Modern Day TheoryModern Day Theory(Electron Cloud Theory)(Electron Cloud Theory)

The Modern The Modern Theory Theory suggests that suggests that electrons are electrons are located located somewhere in somewhere in a cloud.a cloud.

Basic and important fact to remember: All Basic and important fact to remember: All atoms contain the same basic parts, but atoms contain the same basic parts, but atoms of different elements have atoms of different elements have different numbers of protons.different numbers of protons.

The Periodic Table lists atoms in The Periodic Table lists atoms in consecutive order by their Atomic consecutive order by their Atomic Number Number

The atomic number is directly related to The atomic number is directly related to the number of protons in the nucleus of the number of protons in the nucleus of each atom of that elementeach atom of that element

Atoms have:

• A nucleus• small, dense part of the atom •consists of protons and neutrons

•An electron cloud• large part of the atom that is empty space except for the electrons that are moving very fast and very randomly around the nucleus Nucleus Electron

Cloud

The total number of protons & neutrons The total number of protons & neutrons determines the mass of the atomdetermines the mass of the atom

Called the “Mass Number” Called the “Mass Number” (atomic mass is the (atomic mass is the averaged mass of the isotopes and is given on the averaged mass of the isotopes and is given on the periodic table. Simply round the atomic mass to get periodic table. Simply round the atomic mass to get the mass number)the mass number)

A Carbon atom, has 6 protons and 6 A Carbon atom, has 6 protons and 6 neutrons, so its mass number is 12neutrons, so its mass number is 12

If you know the atomic number & mass If you know the atomic number & mass number of an atom of any element, you can number of an atom of any element, you can determine the atom’s composition and the determine the atom’s composition and the number of neutrons.number of neutrons.

The ProtonThe Proton

The protons are what give the atom its charge (+)The protons are what give the atom its charge (+)

They add mass to the atom as well. Each proton is They add mass to the atom as well. Each proton is equal to one AMU (atomic mass unit)equal to one AMU (atomic mass unit)

The number of protons in an atom determines what The number of protons in an atom determines what element it is. element it is.

The atomic number signifies the number of protonsThe atomic number signifies the number of protons

Protons are held together in the nucleus by the Protons are held together in the nucleus by the “Strong Force” otherwise they would repel each “Strong Force” otherwise they would repel each other.other.

The NeutronThe Neutron

The neutron adds mass (1 amu) to an atom The neutron adds mass (1 amu) to an atom but has but has NONO charge charge

Atoms of the same elements are identical Atoms of the same elements are identical due to the number of protons, but there can due to the number of protons, but there can be different numbers of neutrons (we call be different numbers of neutrons (we call those isotopes)those isotopes)

To find the number of neutrons, subtract To find the number of neutrons, subtract the atomic number from the mass number.the atomic number from the mass number.

ElectronsElectrons Electrons have almost no mass, and we DO NOT Electrons have almost no mass, and we DO NOT

count their mass.count their mass.

Located outside the nucleus in the electron cloud Located outside the nucleus in the electron cloud (aka: shells, orbitals, energy levels) moving at (aka: shells, orbitals, energy levels) moving at incredibly high speeds.incredibly high speeds.

Electrons have a negative (-) charge.Electrons have a negative (-) charge.

Electrons found in the outermost shells of the Electrons found in the outermost shells of the atom are responsible for chemical reactions. atom are responsible for chemical reactions. Electrons have different amounts of energy Electrons have different amounts of energy depending what energy level they are at.depending what energy level they are at.

Electrons can be removed and added to atoms Electrons can be removed and added to atoms quite easily, unlike protons.quite easily, unlike protons.

Subatomic ParticlesSubatomic Particles

ParticleParticle Symbol Symbol Charge Charge RelativeRelativeMassMass

ElectronElectron e e-- 1-1- 0 0

ProtonProton p p++ + + 1 1

NeutronNeutron n n 0 0 1 1

Location of Subatomic ParticlesLocation of Subatomic Particles

1010-13 -13 cmcm

electronselectrons

protonsprotons

neutronsneutrons

1010-8 -8 cmcm

nucleus

Atomic NumberAtomic Number

Counts the number Counts the number

of protonsof protons

in an atom and determines in an atom and determines what element it iswhat element it is

Atomic Number Atomic Number on the Periodic Tableon the Periodic Table

1111

NaNa

Atomic Number

Symbol

The symbol represents the element. RULE: The first letter is always capitalized, and IF

there is a second letter, it is lower case.

All atoms of an element have All atoms of an element have the same number of protonsthe same number of protons

1111

NaNa11 protons

Sodium

Atomic Mass on the Periodic TableAtomic Mass on the Periodic Table

1111

NaNa

22.9922.99

Atomic Number

Symbol

Atomic Mass

Atomic mass is the weighted average mass of all the atomic masses

of the isotopes of that atom. That is why there is a decimal.

Mass NumberMass Number

Counts the number Counts the number

of of

protons and neutronsprotons and neutrons

in an atomin an atom

(note: Atomic Mass is different from Mass Number. (note: Atomic Mass is different from Mass Number. On your periodic table of elements, the atomic mass On your periodic table of elements, the atomic mass

is usually given and you need to round it to the is usually given and you need to round it to the nearest whole number to use to figure Neutrons)nearest whole number to use to figure Neutrons)

Atomic NotationAtomic Notation

Show the Show the mass number and atomic mass number and atomic

numbernumber

Give the symbol of the elementGive the symbol of the element

mass numbermass number

23 23 NaNa sodium-sodium-2323

atomic numberatomic number 1111

Number of ElectronsNumber of Electrons

An atom is neutral when no charge is An atom is neutral when no charge is indicated.indicated.

The net charge is zeroThe net charge is zero

Remember: Atomic number = Number of Remember: Atomic number = Number of protonsprotons

and therefore….and therefore….

Number of protons = Number of electrons Number of protons = Number of electrons when the atom is neutral.when the atom is neutral.

Subatomic Particles Subatomic Particles Showing the P E NShowing the P E N

1616 3131 6565 OO P P Zn Zn

88 1515 3030

8 p8 p++ 15 p15 p++ 30 p30 p++

8 e8 e-- 1515 ee-- 30 e30 e--

8 n8 n 16 n16 n 35 n35 n

IsotopesIsotopes

Atoms with the same number of Atoms with the same number of

protons, but different numbers of protons, but different numbers of

neutrons. neutrons. Atoms of the same element (same Atoms of the same element (same

atomic number) with different mass atomic number) with different mass

numbersnumbers

Isotopes of chlorineIsotopes of chlorine

3535ClCl 3737ClCl1717 1717

chlorine - 35 chlorine - 37chlorine - 35 chlorine - 37

Learning Check Learning Check

Naturally occurring carbon consists of Naturally occurring carbon consists of three isotopes, three isotopes, 1212C, C, 1313C, and C, and 1414C. State the C. State the number of protons, neutrons, and number of protons, neutrons, and electrons in each of these carbon atomselectrons in each of these carbon atoms. .

1212CC 1313CC 1414CC 66 6 6 6 6

#p _______ _______ _______ #p _______ _______ _______

#e _______ _______ _______#e _______ _______ _______

#n _______ _______ _______ #n _______ _______ _______

SolutionSolution

1212CC 1313CC 1414CC 66 6 6 6 6

#p #p 6 6 6 6 6 6

#e #e 6 6 6 6 6 6

#n #n 6 6 7 7 8 8