Atomic Theory

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Chapter 9

• Atomic Theory
• Quantum Model of the Atom

2
Physics Review

• J.J. Thompson determined all mater contains
  electrons with cathode ray tube experiments.
• Rutherford demonstrated that atoms contain mostly
  empty space by bombarding metals with alpha
  particles (helium nuclei)
• Previously we talked about the plumb pudding
  model of an atom and Rutherfords planetary
  model.
• Then along came Bohr who applied Plancks the
  ideas of energy quantization to Rutherfords
  planetary model of an atom.

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Evidence for Quantization of Energy
The circular orbits of the Bohr model are
characterized by the principle quantum number, n,
which has positive integer values. e.g.1, 2, 3.
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About Energy
Energy is anything that has the capacity to do
work. Energy Examples
Food how much energy for a heart beat? Gasoline
how many goes to heat and how much to
work? Dynamite what can it move? Electricity
what can it move? Apple on a tree what can it
move?
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The Bohr Model of an Atom
Rutherford proposed the electrons were located in
orbits around the nucleus similar to planets
around a star. This is sometimes called the
planetary model, which is not the modern day
model of an atom.
Li atom 3p, 3n, 3e-
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The Bohr Model of an Atom
Neils Bohr developed a mathematical model based
on Rutherford's proposal using Plancks quantized
energy levels. In Bohrs model electrons could
only have fixed Potential energy levels.
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Classification of Electromagnetic Radiation
White light is the collection of all of the
colors in the rainbow. When white light is
passed through a glass prism the waves are bent
by the glass. The shortest (most energetic) are
bent the most.
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Potential and Kinetic Energy
Potential energy energy due to position above
the ground. Kinetic energy energy due to matter
in motion. Electromagnetic energy energy
possessing both particle and wave properties
traveling at the speed of light. Units of energy,
joule, or calorie.
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Properties of Waves
Wavelength (m) Amplitude (m) Speed 3.0X108
m/s
Energy (j)
Frequency (1/s, Hz)
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Electromagnetic Radiation
100
10-2
102
10-4
10-8
10-7
10-10
10-12
Micro Waves (Radar)
Infrared rays
Radio and Television waves
X- rays
Uv-rays
Visible rays
Gamma rays
Increasing wave length in meters
Increasing energy
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Visible Radiation Passing Through a Prism
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Continuous Spectrum
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Line Spectrum
Evidence for Quantization of Energy
The red-orange light from hydrogen gas passes
through a prism to form a line spectra. Each
different colored light has its own unique
energy. This is called an emission spectra
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Additional Emission Line Spectra
Note Each element has its own unique emission
spectra (Finger print)
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Emission vs. Absorption Spectra

• Early on physicists theorized that light emitted
  by the sun should be a continuous spectrum. They
  were troubled by black lines when observing the
  suns spectrum through a glass prism. Bunsen
  (Bunsen burner) and Kirchoff studied emission
  spectra from emission tubes containing various
  gaseous elements. Interesting the wavelengths of
  the black lines match know wave lengths of
  different elements. This lead physics to
  conclude that gases surround the sun that absorb
  the emitted wave lengths of the sun.

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Absorption Spectra
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What is Quantization
Quantization means no in between
Energy levels in atoms are quantized.
Anything that comes in units such as stairs
television channels, gears, and bookshelves are
quantized.
A turtle on stairs may only be at specific
heights. Its potential energy is quantized.
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Quantization
Are you quantized right now?
Coming to class you increase your potential
energy one quanta (step) at a time.
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Evidence for Quantization of Energy
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Evidence for Quantization of Energy
The circular orbits of the Bohr model are
characterized by the principle quantum number, n,
which has positive integer values. e.g.1, 2, 3.
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Evidence for Quantization of Energy
In terms of the Bohr model absorption and
emission looks like this.
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Evidence for Quantization of Energy
Electrons move between energy levels by absorbing
and emitting energy in the form of light.
We call the lowest energy level the ground state.
The higher energy level is called the excited
state.
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Evidence for Quantization of Energy
The Bohr model works well for the hydrogen atom
which has only one electron but performs poorly
for more complex atoms. This led to the
development of the current quantum mechanical
model describing the arrangement of electrons in
atoms. Unfortunately for us this model is more
complex than that developed by Neils Bohr.
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Quantum Numbers

• The location of an electron in an atom can be
  described in order of precision by its
• shell (a positive integer given the symbol n
  1,2,3,.)
• subshell (Orbital) (designated by letters s, p,
  d or f)
• orientation (orbital symmetrical on x, y, or z
  axis)
• Spin (clockwise or counter clockwise)

1/2
Like an electrons Social Security number 4px
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Quantum Numbers
As the value for n of a shell increases its
energy and distance from the nucleus increases.
This is similar to the Bohr model. Each shell
has a number of subshells equal to its value for
n (up to a maximum of 4). e.g. A shell with n
1 will have one subshell (s) A shell with n 2
will have two subshells (s,p) A shell with n 3
will have three subshells (s,p,d) A shell with n
4 will have four subshells (s,p,d,f) A shell
with n 5 will have four subshells (s,p,d,f,g)
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Quantum Numbers
Each subshell is designated with the letter s, p,
d or f. The subshells are named by putting the
value of n in front of the symbol for the
subshell. e.g. a p subshell in the second shell
is named 2p The subshells vary in order of energy
s lt p lt d lt f. The difference in energy between
subshells is much smaller than the difference in
energy between shells. Just like the space
between buildings is smaller than the space
between streets.
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Quantum Numbers

• As mentioned earlier each shell has a number of
  subshells equal to its value for n.
• Therefore for
• n 1 there will be one subshell 1s
• n 2 there will be two subshells 2s and 2p
• n 3 there will be three subshells 3s, 3p and
  3d
• n 4 there will be four subshells 4s, 4p, 4d
  and 4f
• n 5 there will be four subshells 5s, 5p, 5d
  and 5f

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Quantum Numbers
Each subshell is made up of one or more orbitals.
An orbital is a volume of space where an
electron is likely to be found. What is the
orbital for books called? Fish? Cars? It is
important not to confuse an orbit (a circular
path on which an electron moves in the Bohr
model) with an orbital. They are two very
different things.
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Quantum Numbers
An s subshell has one orbital which is
spherically shaped.
If you were to measure where the electron was
within an s subshell many, many times and plot
the results on a graph you would get something
like this.
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Quantum Numbers
The p-orbital is next in energy after the
s-orbital. The p-orbital has a dumbbell shape
with electrons located either side of the nucleus
in tear drop shaped lobes.
There are three types of p-orbitals all having
identical shape and energy directed along the x,
y and z axis.
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Quantum Numbers
Next in energy are the d-orbitals of which there
are five all with the same energy. The different
d-orbitals do have different shapes as well
orientation. These are followed by the seven
f-orbitals.
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Quantum Numbers
The way in which electrons are organized into
shells, subshells and orbitals in an atom is
called the electronic configuration. The
electronic configuration of an atom can be
determined using the Aufbau rule also known as
the building up principle.
Aufbau comes from the German meaning construction
although it was the Danish physicist Neils Bohr
who came up with the idea !!
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Quantum Numbers
The Aufbau Principle states that The orbitals
of lower energy are filled in first with the
electrons and only then the orbitals of high
energy are filled. What is the lowest energy
orbital of an atom?
1s orbital
What is the third lowest energy orbital of an
atom?
2p orbital
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Quantum Numbers

• As we have seen previously for p, d and f
  subshells there are multiple orbitals with the
  same energy, called degenerate orbitals.
• In particular
• p subshells have three orbitals with the same
  energy
• d subshells have five orbitals with the same
  energy
• f subshells have seven orbitals with the same
  energy
• Each of these orbitals may accommodate a maximum
  of two electrons.

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Quantum Numbers
If there are multiple orbitals with the same
energy how do we decide which orbital to put an
electron?
We use Hunds rule which states Electrons fill
degenerate orbitals one at a time before doubling
up in the same orbital
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Quantum Numbers
Using Hunds rule how would we put three
electrons in a p subshell ?
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Quantum Numbers
When we do put two electrons in one orbital then
they obey the Pauli exclusion principle.
only electrons with opposite spin can occupy the
same orbital
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Quantum Numbers
How would we use our rules to build up the
electron configuration of a Li atom?
Li has Z 3 so has 3 e-.
We can write this in shorthand as 1s22s1
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Quantum Numbers
How would we use our rules to build up the
electron configuration of a N atom?
N has Z 7 so has 7 e-.
We can write this in shorthand as 1s22s22p3
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Quantum Numbers
Beyond the 3p subshell the orbitals dont fill in
an obvious way.
For example the 4s level lies lower in energy
than the 3d .
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Diagonal Rule
There is an easy way to remember the sequence of
the energies of the subshells.
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Quantum Numbers
So now we have everything we need to determine
the electronic configuration of any atom What is
the electronic configuration of a Na atom?
Start here
Z 11, Na has 11 e-
3s1
1s2
2s2
2p6
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Quantum Numbers
The electrons in the highest energy shell of an
atom are (those furthest from the nucleus) are
called the valence electrons. These electrons
are very important as when atoms interact with
each other it is through their valence electrons.
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Diagonal Rule Within the Periodic Chart
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Periodic Chart
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PERIODIC TRENDS

• Atomic size
• Increases from top to bottom (gaining a new outer
  shell)
• Decreases left to right in a period (Increase of
  protons attracts electrons stronger, thus
  contracting the element.
• Ease of ionization (relative ease of losing
  electrons)
• The larger the atom, the further electrons are
  from nucleus, and the less they are held by
  nucleus.
• Metals and nonmetals
• Metallic character (increases going away from
  metals)
• Nonmetallic character

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Periodic Trends
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Metals, Nonmetals, and Mettaloids
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Chapter 9Review
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How many electrons fill the following
sublevels? a. The 3rd level. b. The 4th
level. c. The 5th level
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How many electrons fill the following
sublevels? a. The 3rd level. 2(n)2
2(3)218 b. The 4th level. c. The 5th level
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How many electrons fill the following
sublevels? a. The 3rd level. 2(n)2
2(3)218 b. The 4th level. 2(n)2 2(4)232
c. The 5th level
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How many electrons fill the following
sublevels? a. The 3rd level. 2(n)2
2(3)218 b. The 4th level. 2(n)2 2(4)232
c. The 5th level 2(n)2 2(5)250
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How many orbitals are in an atom with n3?
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How many orbitals are in an atom with n3?
Answer
n 3 refers to the third energy level, which
contains the s, p, and d sublevels. There is
only one orbital in the s, three in the p, and
five in the d sublevels for a total of 9
orbitals.
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Identify the element with the following electron
configurations, also stating the group and period
the where the element where found. a.
1s22s2p63s23p64s23d104p4 b. Ar3s23d7
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Identify the element with the following electron
configurations, also stating the group and period
the where the element where found. a.
1s22s2p63s23p64s23d104p4 Se, Period 4, Group
VI (6) b. Ar3s23d7
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Identify the element with the following electron
configurations, also stating the group and period
the where the element where found. a.
1s22s2p63s23p64s23d104p4 Se, Period 4, Group
VI (6) b. Ar4s23d7 Co, Period 4, Group
9
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The End