Chapter 3 Atomic Structure

3-1 Early Models of the Atom3-2 Discovering Atomic Structure

3-3 Modern Atomic Theory3-4 Changes in the Nucleus

3-1 Early Models of the AtomWhat are atoms?What are the postulates of Dalton’s atomic theory?

Democritus

Ancient Greek – 450BCProposed that all matter is composed of

tiny, invisible particles called atomsNo one believed him during his lifetime

◦Including AristotleHis beliefs were not accepted until the 17th

and 18th centuries

Acceptance

Was not accepted until 2 discoveries were made◦Lavoisier’s law of conservation of matter◦Joseph Louis Proust’s law of constant

composition A compound will always contain the same

proportions by mass of elements◦Water will always have 88.9% oxygen (O) and 11.1%

hydrogen (H)

John Dalton (1766 – 1844)

English school teacherStudied past theories of atoms and laws of

matterFormed an atomic theory of matter

Dalton’s Atomic Theory of Matter

Ea element is composed of extremely small particles called atoms

All atoms of a given element are identical, but they differ from those of any other element

Atoms are neither created/destroyed in any chem rxn

A given compound always has the same relative #s and kinds of atoms

Atoms

The smallest particle of an element that retains the chemical identity of that element

There are 118 elements wh means there are 118 different kinds of atoms.

Atoms

Atoms are like the words in these slides.◦If we broke it all apart, separated and organized

the letters, you would find only 26 piles.◦But by taking letters from different piles we can

create millions of very different wordsJust like words can be separated into letters,

matter can be separated into atoms.◦These separated atoms are called elements

Think of all the words you could make with the letters A, D, and M….

Scanning Tunneling Microscope (STM)

Produces images of atomsCreated in 1981

◦Nickel Platinum

Chemistry In Action (p93)

Consumer Tip◦“100 Percent Natural”

Macroscopic vs Microscopic

Macroscopic – looking a the whole picture◦A tree

It is made of the leaves, branches, trunk, roots

Microscopic – the more detailed vision of an object and what makes it function◦A leaf off a tree and the little veins that carry

the nutrients through it

Macroscopic vs Microscopic

Chemists make their observations in the macroscopic world◦ It is the world in wh we all

live In order to understand

that world, the goal is to understand the atoms that the world is made of◦ Discoveries/Possibilities b/c

of the study of atoms Deciphering the genetic

code Designing plastics Understanding the hole in

the ozone Imprinting data on silicon

chips

3-2 Discovering Atomic StructureHow is atomic structure related to electricity?What did cathode rays indicate about atoms?What did Rutherford conclude from his alpha-scattering experiment?

Electric Charges

Scientists couldn’t figure out why atoms of one element acted differently than another element’s atoms

Michael Faraday (1791-1867) said that the structure of an atom was directly related to electricity

Electric Charges

Atoms contain particles that have electrical charges

Benjamin Franklin

An object will either have a positive or negative charge

2 like charges will repel◦ Positive w/ positive◦ Negative w/ negative

2 opposite charges will attract◦ Positive w/ negative

Franklin didn’t know where these charges came from

Cathode Rays and Electrons

Electric current - A moving stream of electrical charges◦Electricity from wall socket or battery

Studying electrical currents provide keys to understanding electrical charges

Mid-1800s, began studying electric currents in glass tubes w/ little air

Cathode Rays and Electrons

Tube attached on ea end to a battery◦Positive and negative

Negative = cathode Positive = anode

Radiation travels from cathode to anode◦b/c radiation came from cathode end, called

cathode ray and the tube a cathode ray tube

Cathode Rays and Electrons

Cathode Ray tube being effected by a magnet◦http://www.youtube.com/watch?v=7YHwMWcxe

X8&feature=related

Battery

- +

Electrons

Negative particles within the atom◦JJ Thompson (1856-1940)

Mass of 9.11 x 10 -28 gram◦0.000000000000000000000000000911 gram◦Robert Millikan (1868-1953)

Radioactivity

Henry Becquerel (1852-1908)◦ Placed uranium on photo

paper and an image appeared

◦ Uranium was emitting radiation

Radioactivity: spontaneous emission of radiation from an element

Marie Curie and husband Pierre discovered the elements of radium and polonium were also radioactive

The Nuclear Atom

Thompson said there were electrons in the atom (neg charge)

Why is the atom neutral then?Rutherford’s Gold Foil Experiment

◦http://www.youtube.com/watch?v=5pZj0u_XMbc◦Called this center the nucleus

Has a positive charge Very small

◦ If the atom was the size of a football stadium, the nucleus would be smaller than a dime sitting in the middle Electrons would be smaller than Franklin Roosevelt’s eye on the

dime

The Nuclear Atomhttp://www.ndt-ed.org/EducationResources/HighSchool/Magnetism/reviewatom.htm

How far are the electrons from the nucleus?

If the earth was the nucleus, the electrons would cover an area as large as the distance b/w the earth and nearest stars

3-3 Modern Atomic TheoryWhat are the names and properties of the 3 subatomic particles?How can you determine the # of protons, neutrons, and electrons in an atom/ion?What is an isotope? What is atomic mass?

Subatomic Particles

We know atoms are made from protons, neutrons, and electrons◦Recently scientists have found even smaller

particles Quarks, Gluons, Mesons, Muons, and others

◦They don’t seem to impact any Chemistry so chemists ignore Physicists study them

The Structure of the Atom

Nucleus◦Contains the protons and neutrons

Protons = positive – p+

◦Have the same but opposite charge as electrons Neutrons = neutral/no charge – n0

Electrons ◦Negatively charged – e-

◦Move in the space outside nucleus – e- cloud◦Very small compared to p+

2000 e- = 1 p+

Size of Subatomic Particles

Mass◦Too small for normal measurements◦Has own unit - atomic mass unit (amu)◦P+ and n0 = 1 amu, e- = 0 amu b/c so small

Length◦Diameter = 0.100 – 0.500 nanometer

Nanometer = nm = 10-9 meter◦If you drew a line across a penny (1.9 cm), you would

touch 810 million copper atoms◦If you lined up all 810 million nuclei, you would only

have a line 4 x 10-6 meter long 4 millionths of a meter

Atomic Numbers

Henry Moseley (1887-1915)◦Student of Rutherford◦Discovered atoms of ea element contained

differ positive chargesLead to the idea that an atom’s identity

comes from the # of p+ in nucleusCall this # atomic number

Atomic Number

The # of protonsEa element has a unique atomic #Can tell an element’s atomic # from

periodic table

Neutral Atom

The p+ are positiveThe e- are negativeThe atom is neutralThis means, the p+ must equal the e-

◦For N, atomic # = 7 Means p+ = 7

◦Means e- = 7

Examples

How many protons and electrons in:◦Oxygen (O)

8 p+ and e-

◦Magnesium (Mg) 12 p+ and e-

◦Silicon (Si) 14 p+ and e-

What element has 11 protons?◦Sodium

Ions

When an atom gains/loses e-, it will have a chargeWhen an atom has a charge, called ionCharge of ion = #p+ - #e-

◦ If a magnesium atom loses 2 e-, ionic form has a charge of: #p+ - #e- = 12 – 10 = +2

◦ It is important to add the plus (+) sign into the answer Also possible to have a negative (-)

◦ Some people write the charge with the +/- after the # (2+)After you have calculated the charge, to write it

with the element symbol, add it as a subscript◦ For our magnesium example: Mg+2

Examples

Write the chemical symbol for the ion w/:◦ 9 p+ and 10 e-

F-

◦ 13 p+ and 10 e-

Al+3

◦ 7 p+ and 10 e-

N-3

How many p+ and e- are present in:◦ S-2 ion

16 p+ and 18 e-

◦ Li+ ion 3 p+ and 2 e-

Write the chemical symbol for the ion w/:◦ 12 p+ and 10 e-

Mg+2

◦ 74 p+ and 68 e-

W+6

Isotopes

All atoms of the same element, have the same # of p+

They may not have the same # of n0

If atoms have the same # of p+ but different # of n0 , we call them isotopes

Most elements have at least 1 isotope◦1 usually more frequent than another

In nature, it is usually a mixtureTo tell isotopes apart, we use the mass #

Mass Number

Mass # = #p+ + #n0

◦An atom w/ 17p+ and 18n0 would have an mass # of 35 Mass # = 17 + 18 = 35

◦b/c 17 p+, tells us it is a chlorine atom Chlorine – 35

A way to write the element symbol w/ atomic and mass #s would be:

Cl3717 element symbolmass #

atomic #

Examples

How many protons, neutrons, and electrons are in the following ions?◦ Fe+2

26 p+, 24 e-, and 30 n0

◦ Al+3

13 p+, 10 e-, and 14 n0

◦ Se-2

34 p+, 36 e-, and 45 n0

Write the complete chemical symbol for the ion w/◦ 21 p+, 24 n0, and 18 e-

Sc+3

◦ 53 p+, 74 n0, and 54 e-

I-

2656

13

27

34

79

2145

53127

Atomic Mass

The average mass of all the isotopes of an element

Listed in the periodic table

Practice Problems# 1-30

3-4 Changes in the NucleusWhat changes accompany nuclear reactions?What is radioactivity?

Nuclear Reactions

Change the composition of an atom’s nucleus

Produces alpha, beta, or gamma radiation◦Alpha and beta radiation comes from radiation

emitted from the nucleus

Nuclear Stability

Almost all atoms have stable nuclei◦Not radioactive

Radioactivity could have harmful effects – good its rare to find in nature

Why are some more stable than others?◦# of p+ and n0 in the nucleus◦Some combinations cause instability

Nuclear Stability

In nucleus, p+ and n0 are packed together in a very small space

How do p+ stay together in the small space if like charges repel?◦Held there by strong nuclear force

Can only be found in this situation◦Neutrons act like a net to hold the p+ in along

with the strong nuclear force

Nuclear Stability

Pattern of stability◦Atomic # 1-20 – nuclei stable, = # of p+ and n0

◦Beyond 20 p+ - more n0 needed to keep stable◦Atomic # above 83 – radioactive nuclei

No # of n0 will make it stableAtoms unstable if too many or too few

neutrons◦Atoms w/ too many emit beta radiation

Types of Radioactive Decay

Alpha (α)◦Alpha particles have 2 p+ and 2 n0

◦Identical to Helium – 4 nucleus◦Travel only a few cm◦Easily stopped by paper or clothing◦Usually doesn’t pose a health threat unless

actually enters the body

He42

+2 He42 α4

2

Types of Radioactive Decay

Beta (β)◦High speed electrons (not the ones around the

nucleus)◦Comes from charges inside a nucleus◦A neutron changes into a p+ and e-

p+ stays in nucleus e- (beta particle) is propelled out of nucleus at high

speed◦100 times more penetrating than alpha

Able to penetrate 1-2 mm of solid material Able to pass through clothing and damage skin

e0-1

- e0-1 β0

-1

Types of Radioactive Decay

Gamma (γ)◦Very energetic form of light our eyes can’t see◦Doesn’t have any particles◦More penetrating than others◦Able to penetrate deep into solid material

Body tissue◦Stopped only by heavy shielding

Concrete or lead

γ00

When an atom emits radiation, it undergoes radioactive decay◦ Called decay b/c nucleus is decomposing to form a new

nucleusThe best way to understand the decay is w/ a

nuclear equationRa Rn α226

8822286

42+

Types of Radioactive Decay

Partner Activity

Look at Figure 3-30 on p115 and answer the questions◦Would this protective suit protect the worker

from alpha radiation?◦Why would a person working w/ alpha radiation

also need to be concerned w/ gamma radiation?

◦Would protective clothing such as this stop gamma radiation from penetrating the worker’s skin?

Beta decay equation

131

53 I Xe + β131

54

0

-1

Practice Problems

Alpha decay of 79 Au

Alpha decay of 92 U

Beta decay of 11 Na

Alpha decay of 94 Pu

Alpha decay of 91 Pa

Beta decay of 87 Fr

185

238

24

242

231

233

Chapter 3 Review

Multiple Choice◦all

True/False◦all

Concept Mastery◦(20-22, 25)

Critical Thinking and Problem Solving◦29, 31-33