chapter 3 atomic structure
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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
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