Nomen ___________________________________________
Dies ___________________________________________
Atomic History
Scientist
Democritus
Dalton
J.J. Thompson
Rutherford
Theory
First person to
hypothesize that there
were atoms, small
indivisible particles of
matter
1. Matter is composed
of indivisible
particles (atoms)
2. All atoms of the
same element are
identical in size,
mass, and chemical
property.
3. Atoms of specific
elements are
structurally
different from
atoms of a different
element.
4. Atoms combine in
certain wholenumber ratios to
form compounds
5. In a chemical
reaction, atoms are
separated,
combined, or
rearranged.
Discovered the first
subatomic particle, the
electron.
Experiment
N/A
N/A
Thought the atom was like
a marble
Cathode Ray
Experiment: Gas is
excited and emits cathode
rays. These rays move in a
Thought the atom was
straight line. When these
positively charged with
rays are passed through
negative electrons stuck in an electromagnetic field
it, like raisin in pudding.
(i.e. a field with a + and an
The “plum pudding”
– side), the rays bend
model.
towards to positive side.
Since opposite sign
attract, they must be
negative. Electrons.
Atom is mostly empty
Gold Foil: Radon is used
space. It contains a small,
to shoot positive alpha
dense, positively charged
nucleus.
Bohr
Schrodinger
Heisenberg
Planetary model – dense,
positive nucleus at the
center with electrons in
orbits like planets around
it
Why don’t the negative
electrons crash into the
positive nucleus?
Electrons are in distinct
energy orbitals around the
nucleus. Each orbital has a
different amount of
energy. Electrons “jump”
from one orbital to the
other.
Orbitals are not circular
orbits where electrons
travel. They are defined as
the area that has the
highest probability where
the electron can reside.
“Clouds”. Electrons move
so fast have the properties
of waves. WaveMechanical Model.
Uncertainty Principle:
You can never know
exactly where an electron
is AND how fast it is
moving.
particles at thin gold foil.
Most particles go straight
through (atom mostly
empty space) but some
deflect and a few bounce
straight back (due to
dense positive nucleus)
N/A
Mathematical model only.
Equation that defines the
probable areas where
electrons can be found.
Atomic Structure
Location
Mass
Charge
Extra Info
Proton
Nucleus
Neutron
Nucleus
1 AMU
+1
Proton number is
the atomic number.
This defines the
element.
1 AMU
0
Neutrons
contribute to the
mass and the
stability of the
nucleus of an atom
Electron
In a cloud outside
the nucleus
1/1600 AMU
-1
Electrons account
for the chemical
reactivity of the
atom. Found in
orbitals outside the
nucleus.
Atomic Number = # of Protons
Mass Number = # of protons + # of neutrons
= Atomic Number + # of neutrons
AMU: 1 amu is defined as 1/12 the mass of a Carbon-12 atom.
***If you are given the atomic mass, you can subtract the atomic number to find the
number of neutrons!***
In a neutral atom, i.e. an atom that does not have a charge overall, the number of
protons equals the number of electrons!!
Isotopes are atoms of the same element (i.e. have the same atomic number, same number
of protons) that have different masses.
The mass number in the periodic table is the average mass number, i.e. the weighted
average of all naturally occurring isotopes of that element. When doing problems with
isotopes, always use the atomic mass given to you not the average mass.
Solving Average Atomic Mass Problems:
Atomic Mass (amu)
Percent Abundance
14
12
11
0.04
0.95
0.01
ADD
Total (Multiply AM by %)
(amu)
0.56
11.4
0.11
12.07
Ways to right isotopes:
14
C (mass written in the upper left hand side)
Carbon-14 (mass written after name)
Number of protons can always be found on the periodic table.
Nuclear Chemistry – Review
Radioisotope – an isotope of an element whose nucleus is unstable; emits radiation
to become more stable.
Two reason nuclei could be unstable:
(1) Ratio of neutrons to protons is outside of belt of stability
(2) Too large (> 83 protons)
For smaller nuclei, atoms tend to want a 1:1 ratio of neutrons to protons. For large
nuclei, atoms tend to want a ration of 1.5:1 neutrons to protons.
Penetration power – gamma > beta > alpha
Alpha
Beta
Gamma
Positron
Neutron
Symbol
Charge
4
He
2
+2

e



0 e
+1
Penetration
Power
Low, paper stops it
-1
Medium, clothes
stop it
0
High, only metal
stops it
+1
Medium, similar to
beta particles
0
(Used in artificial
transmutation)
1n
0
Balancing Nuclear Reactions
Type of
Decay
Alpha
Beta
Gamma
Positron
Electron
Capture
Particle
Emitted
Helium
nucleus
Beta or an
electron
X-Ray
Photon
Beta plus
or positron
Beta or an
electron
Change in
Mass
Decrease
by 4
No change
No change
No change
No change
Balancing Nuclear Equations:
(1) Atomic masses on both sides MUST balance
(2) Atomic numbers on both sides MUST balance
Emission: particle on the right (product)
Capture: particle on the left (reactant)
Half-Life
Change in
Atomic #
Decrease
by 2
Increase by
1
No change
Decrease
by 1
Decrease
by 1
Radioactive substances decay at constant rate
The rate of decay is measured in half-lives
Example: The half life of Strontium-90 is 29 years. How much is left in a 10g
sample after four half lives?
**Two ways to solve!**
Way 1: Set up chart
Example:
Number of Half Lives
0
1
2
3
4
Elapsed Time (years)
0
29
58
87
116
Amount of
Strontium-90 Left
(grams)
10
5
2.5
1.25
0.625
Steps:
1. Set up three columns – Number of Half Live, Elapsed Time, and Amount
Remaining
2. Place a zero (O) in the first row of # of half-lives and elapsed time. This row is
for your original amount!
3. In the next row, put 1 for #of half-lives and look up the half-life in Table N.
Put the time in “Time”. This is the amount of time that has passed after one
half-life.
4. In the “Amount Remaining” column, divide by 2 every time you move down a
row (i.e. another half-life has passed) and multiply by two as you move up (in
the case where you are finding the original amount).
5. In the “Time” column, you add the half-life amount as you go down (DO NOT
MULTIPLY!!!). I.e. if the half life is 2 years, your column should go 0yr, 2 yr,
4yr, 6yr, 8yr NOT 0yr, 2, 4, 8, 16, etc.
6. Write “X” in the row you are trying to solve for. Use the rules above to solve.
Way 2: Formula!
(t/t1/2)
N = N (1/2)
0
Where:
N = amount remaining
No = original amound
T = time
T1/2 = half-life
**Nota bene: To find original amount and time, you would need to be able to solve
exponential equations.
Transmutation: When atoms of one element gain or lose subatomic particles to
become atoms of another element, i.e. the atomic number changes!
Natural Transmutation: Unstable radioisotope decays. Ie. Alpha, beta, gamma,
electron capture, positron decay
Artificial Transmutation: High speed particle hits nucleus and causes decay. I.e.
fission, fusion, etc.
Fission: Heavy nuclei split into lighter nuclei with the release of energy. Some mass
converted into energy. Used in nuclear reactions
Fusion: Light nuclei slammed together to create heavier elements. Happens in sun
and the stars.
Applications
Electron Configurations
Bohr Model:
Each row on the periodic table represents a different energy shell in the Bohr model.
(1) The Bohr model shows that the electrons in atoms are in orbits of differing
energy around the nucleus.
(2) Bohr used the term energy levels (or shells) to describe these orbits of
differing energy. He said that the energy of an electron is quantized, meaning
electrons can have one energy level or another but nothing in between.
(3) The energy level an electron normally occupies is called its ground state. But
it can move to a higher-energy, less-stable level, or shell, by absorbing
energy. This higher-energy, less-stable state is called the electron’s excited
state.
(4) After it’s done being excited, the electron can return to its original ground
state by releasing the energy it has absorbed, as shown in the diagram below.
(5) Sometimes the energy released by electrons occupies the portion of the
electromagnetic spectrum (the range of wavelengths of energy) that humans
detect as visible light. Slight variations in the amount of the energy are seen
as light of different colors.
**Bohr’s model only really worked for hydrogen**
N = 1 - 2 electrons
N = 2 – 8 electrons
N = 3 - 18 electrons
Valence electrons – Electrons in the outermost energy shell. These are the
electrons involved in chemical reactions.
Principal Quantum
Number (n)
Angular Momentum
Quantum Number (l)
Magnetic Quantum
Number (m(l))
Spin Quantum Number
Distance away from the
nucleus (higher levels
equals further away, more
energy)
Defines the shape of the
orbital. Combined with
the n defines the shape
and size.
0 = s (sphere)
1 = p (dumbbell)
2 = d (double dumbbell)
3=f
4=g
Defines orientation in
space. Can tell you have
many orbitals for a given
shape.
Describes electron spin.
Can have one up and one
Can have positive, whole
number integers; 1, 2, 3, 4,
5, etc. Row on periodic
table
Can have value from 0 to
n-1.
Example: in the 5th energy
level can have 0, 1, 2, 3, 4
Range is –l to +l
Example: For p orbitals
can have -1, 0 +1 (i.e. three
orbitals oriented in
different directions)
I.e. in the p orbital, there
are three orbitals. For
down electron per orbital.
each of those you can have
two electrons (one up, one
down) which means 6
total.
Led to…the Quantum-Mechanical Model
The quantum mechanical model is based on quantum theory, which says matter also
has properties associated with waves. According to quantum theory, it’s impossible
to know the exact position and momentum of an electron at the same time. This is
known as the Uncertainty Principle.
The quantum mechanical model of the atom uses complex shapes of orbitals
(sometimes called electron clouds), volumes of space in which there is likely to be
an electron. So, this model is based on probability rather than certainty.
Four numbers, called quantum numbers, were introduced to describe the
characteristics of electrons and their orbitals:
Principal quantum number: n
Angular momentum quantum number: l
Magnetic quantum number: m(l)
Spin quantum number: m(s)