Download ATOMIC STRUCTURE

THE TRUE MYSTERY OF THE WORLD IS THE
VISIBLE, NOT THE INVISIBLE.
- Oscar Wilde -
ATOMIC STRUCTURE
"I think I can safely say that nobody understands
quantum mechanics."
- Richard Feynman-
QUANTUM OF ENERGY – THE ENERGY REQUIRED TO MOVE AN
ELECTRON FROM ONE ORBIT OR ENERGY LEVEL TO ANOTHER.
PHOTONS OF LIGHT REPRESENT DISCRETE PACKETS OF ENERGY
WITH THE WAVELENGTH OF LIGHT DETERMINING THE AMOUNT OF
ENERGY
E = hc/l
where h = constant, c = speed of light
l = wavelength of light
ERWIN SCHRODINGER USED THESE RELATIONSHIPS TO
DESCRIBE THE BEHAVIOR OF ELECTRONS IN ATOMS.
ATOMIC ORBITAL – A REGION IN SPACE WITH A HIGH
PROBABILITY OF FINDING AN ELECTRON.
WHERE DO OUR CURRENT IDEAS OF
ATOMIC STRUCTURE COME FROM?
de Broglie put forward that all objects in
motion have wave nature. The smaller an
object is, the greater the wave nature.
This means that the electron would
behave as much like a wave as a particle.
Schrodinger came up with a differential
wave equation that would describe the
motion of an electron about a nucleus in
three dimensions.
Chemists call these wave functions
“orbitals”.
The math involved is very complex.
However, the results do an excellent job
of describing why atoms of different
elements behave as they do, and they are
very useful in describing how atoms
interact to form molecules.
They even do a good job helping to
describe the shapes of molecules. This
is very important in biochemistry and
medicine.
From the quantum theory, there are four quantum
numbers that describe electron orbitals.
n = principal quantum number - what shell the
electron goes in (n = 1, 2, 3...)
l = angular momentum quantum number - what subshell
the electron goes in (0 < l < n-1)
ml = magnetic quantum number – how many orbitals
the subshell is broken into (-l < ml < l)
ms = spin quantum number (- 1/2 or + 1/2)
Principal Quantum
Number
(shell)
Number of Subshells
Type of Subshell
n=1
1
1s (1 orbital)
n=2
2
2s (1 orbital)
2p (3 orbitals)
n=3
3
3s (1 orbital)
3p (3 orbitals)
3d (5 orbitals)
n=4
4
4s (1 orbital)
4p (3 orbitals)
4d(5 orbitals)
4f (7 orbitals)
The quantum numbers, in a sense, tell us
how many electrons can go where.
There are some rules that tell us how
electrons fill shells.
ELECTRON FILLING RULES
Aufbau (build-up) Principal - Electrons
enter and fill lower energy orbitals before
higher energy orbitals.
Pauli Exclusion Principal - orbitals can
contain a maximum of two electrons which
must be of opposite spin.
Hund’s Rule - when there are orbitals of equal
energy, electrons will enter the orbitals oneat-a time until all of the orbitals are half filled
and only then will pairing occur.
So, let’s take a look at how electrons
go into orbitals as we move through
the periodic table.
Hydrogen has only one electron, so it
would go in the first shell. The first
shell has only one subshell, the 1s.
So, we’d write that as:
Hydrogen, H
1s1
Helium has an atomic number of 2, so it
has two electrons. This second electron
would also go in the 1s subshell, but it
would have opposite spin.
Hydrogen, H
Helium, He
1s1
2
1s
This fills up this shell and subshell.
Notice H and He are in the first row or first
period. Electrons are going into the first
shell.
Lithium has an atomic number of 3, so
this means that it has 3 electrons.
The first shell is filled, so the third
electron has to go in the second shell. It
will go in the 2s subshell, as this has the
lowest energy.
Hydrogen, H 1s1
2
Helium, He
1s
Lithium, Li
1s2 2s1
Beryllium has an atomic number of 4, so 4
electrons.
Two in the first shell, and two in the 2s
subshell.
1
1s
Hydrogen, H
Helium,
He 1s2
2
1
Lithium,
Li 1s 2s
Beryllium, Be 1s2 2s2
This fills the 2s subshell. The next
electrons will have to go in the 2p subshell.
Boron has an atomic number of 5, so it has 5
electrons.
B 1s2 2s2 2p1
There are actually 3 2p subshells, all of equal
energy, px, py, pz.
One electron will go into each of these. After
each has an electron, the the additional three
electrons will pair up.
Right now, don’t be too concerned. Just
remember that 6 electrons can go into the p
subshells.
So, for the rest of the second row (period),
we have:
Lithium, Li 1s2 2s1
2
2
Beryllium, Be 1s 2s
Boron, B
1s2 2s2 2p1
Carbon, C
1s2 2s2 2p2
Nitrogen, N 1s2 2s2 2p3
Oxygen, O 1s2 2s2 2p4
2
2
5
Fluorine, F
1s 2s 2p
Neon, Ne
1s2 2s2 2p6
This gives us a filled first shell and a filled
second shell.
The order in which the third shell would
fill would be:
Sodium, Na
1s2 2s2 2p6 3s1
2
2
6
2
Magnesium, Mg 1s 2s 2p 3s
Aluminum, Al
1s2 2s2 2p6 3s2 3p1
2
2
6
2
2
Silicon, Si
1s 2s 2p 3s 3p
Phosphorus, P 1s2 2s2 2p6 3s2 3p3
Sulfur, S
1s2 2s2 2p6 3s2 3p4
2
2
6
2
5
Chlorine, Cl
1s 2s 2p 3s 3p
Argon, Ar
1s2 2s2 2p6 3s2 3p6
There is one very important reason we are
going through this.
It is the number of electrons in the outer
shell (valence electrons) that determine the
properties of the element.
If you look at the periodic table, you will
see that all of the elements in a given
column have the same number of valence
electrons.
So, they will have similar properties. A
column is called a family.
A row is called a period. The period number
tells what shell is being filled.
The octet rule says that the maximum
number of electrons that can occur in any
outer shell is 8, except for the first shell,
which is 2. This represents a stable
configuration, and elements will react by
losing, gaining or sharing electrons to obtain
this configuration.