Electrons are involved in bonding,
and therefore, their arrangement in an atom is very
important.
Subatomic particle
Atoms are composed of protons, neutrons and electrons. The atomic
number is equal to the number of protons. The charge of the atom or
ion is equal to the number of protons � the number of electrons.
The number of electrons can be determined knowing the atomic number
and charge.
Energy levels, subshells and orbitals
Electrons are placed in energy levels. These energy levels are
sub-divided into subshells (labeled s, p, d or f). The s subshell
is the lowest energy and begins in level 1. The p subshell is
higher energy and therefore doesn�t begin until level 2. The d is
higher energy and begins in level 3 and the f is even higher energy
and begins in level 4. The subshells are further sub-divided into
orbitals (s has 1 orbital, p has 3 orbitals, d has 5 orbitals and f
has 7 orbitals). Each orbital can hold 2 electrons.
Rules for writing electron configurations
The Aufbau principle states that energy levels must
be filled from the lowest to the highest and you may not move on to
the next level unless the previous level is full. Use the periodic
table as a guide (read left to right):
|
1s |
|
|
|
|
2s |
|
|
2p |
|
3s |
|
|
3p |
|
4s |
|
3d |
4p |
|
5s |
|
4d |
5p |
|
6s |
4f |
5d |
6p |
|
7s |
5f |
6d |
7p |
Hund�s Rule says that when
placing electrons in orbitals of equal energy, place one in each
orbital before doubling up in order to arrive at the lowest energy
configuration. The Pauli Exclusion Principle
states that when electrons do share an orbital, they must be of
different �spin.�
Writing electron configurations
The Boxes and Arrows method uses boxes to show
orbitals and arrows to signify electrons. An up arrow and a down
arrow have different �spins.� The spectroscopic method
uses superscripts to show the number of electrons in a subshell
(specific orbitals are not shown). The noble gas method
uses a noble gas (the far right column) to represent the
inner, or core, electrons and just shows the outer level of
electrons using the same method as spectroscopic.
Exceptions to the rules
There are a few exceptions to the rules listed above when filling
electron configurations. A half-full �s� orbital and a �d� subshell
with 5 or 10 is more stable than following the Aufbau Principle.
Cr, Mo, W: s1 d5 and Cu, Ag, Au:
s1 d10
