ELECTRON CONFIGURATIONS
Atoms with similar properties are arranged in the same columns on the periodic
table. The main reason that they have similar properties is because their outer
electrons arrangements are similar. In this tutorial we will look at how chemist
use notations to show the arrangements of electrons around the nucleus of an
atom. Atoms have layers or shells of electrons surrounding their nucleus, and
the number of shells depends on the number of electrons in the atom. A very
simple atom, like hydrogen, only has one electron and therefore only needs one
shell to hold its electron. Larger atoms, such as uranium, have many electrons
(92) and therefore need more shells to hold its electrons. These shells vary in
size depending on how close to the nucleus they are and the electrons within
them vary in their amount of energy. The shells closest to the nucleus are
smaller and cannot hold very many electrons and have electrons that are lower
in energy. As you move farther away from the nucleus, the shells increase in
size and in the energy of the electrons within them. Chemists call these
electron shells Principal Energy Levels and number them from the nucleus
out, with n being the letter used to signify an energy level.. A principal energy
level is actually a very broad region where electrons can be traveling. Within a
principal energy level there are sublevels of varying energies. Within the
sublevels there are atomic orbitals, which are more specific regions where
electrons can be found.
The principal energy level number (n) is equal to the number of sublevels
within that energy level. These sublevels are given letters to identify the type
of sublevel within an energy level. Each different sublevel contains a different
number of atomic orbitals and each orbital can hold a maximum of 2 electrons.
Below is a breakdown of the different types of sublevels and the number of
orbitals and electrons that can possibly occupy these sublevels.
sublevel
# orbitals
# electrons
s
1
2
p
3
6
d
5
10
f
7
14
An s sublevel contains electrons that are lower in energy than a p, d, or f
sublevel within the same principal energy level. The first principal energy
level (n = 1) would only contain an s sublevel, and could only hold a maximum
of 2 electrons. The second principal energy level (n = 2) would contain s and p
sublevels and could therefore hold a maximum of 8 electrons. The third
principal energy level (n = 3) would have s, p, and d sublevels and could hold
18 electrons. The fourth principal energy level (n = 4) would have s, p, d, and f
sublevels and could contain up to 32 electrons. As you move farther away
from the nucleus the energy levels increase in size and their number of
sublevels and they also start to get closer together. When you move beyond the
third principal energy level there is an overlapping of sublevels between the
principal energy levels. When electrons fill these orbitals they fill according
to the following set of rules:
1. Aufbau Principle: Electrons fill orbitals of lowest energy first
2. Pauli Exclusion Principle: There can be no more than 2 electrons in an
orbital. If 2 electrons are in the same orbital, these have opposite spins.
3. Hund’s Rule (Bus seat rule): Electrons enter empty orbitals of equal
energy first before doubling up.
Electron Configurations
Electron configurations are used by chemists to show how electrons are
arranged around the nucleus of an atom. Electron configurations show the
principal energy levels that an atom has and the sublevels within the
principal energy level and then shows how many electrons are in each
sublevel. Below is an example of an electron configuration for neon.
1s22s22p6
The numbers in front of the letters are the principal energy levels that neon
has electrons in. The letters are the types of sublevels that are within the
principal energy levels and the superscripts are the number of electrons in
each one of the sublevels. The superscripts should add up to the number of
electrons in the neutral atom. Below are examples of electron configurations
for other atoms.
Chlorine 17 electrons 1s22s22p63s23p5
Iron 26 electrons
1s22s22p63s23p64s23d6
Silver 47 electrons
1s22s22p63s23p64s23d104p65s24d9