Electrons are always moving around
the nucleus and so possess potential
and kinetic
energy. But they can only possess certain values
of energy, or specific energy levels. (Credit should be given to Niels
Bohr for proposing this theory.) Click here
if you need an illustration of this concept.

According to Bohr's model of the atom, electrons orbit about the
nucleus much like the way planets orbit the sun. Different energy levels are
associated with the different orbits. The diagram below shows the Bohr model
for fluorine. The nucleus of fluorine has 9 protons. Surrounding the nucleus
of fluorine is 9 electrons. The electrons arrange themselves in 3 orbits:

In the first orbit, there are 2 electrons.

In the second orbit, there are 7 electrons.

In the third orbit, there are no electron.

Bohr deduced that:

electrons inside an atom possess different energies

electrons in the first orbit belong to
the first energy level

electrons in the second orbit belong to
the second energy level

electrons in the third orbit belong to
the third energy level...... etc ......

each energy level of an atom could only accomodate
a certain number of electrons. The maximum number of electrons that can populate
a certain energy level is given by the following formula.

where n = the specific energy level

For example:

The maximum number of electron in the first energy
level (n = 1) is 2 (1)^{2}
= 2 electrons

The maximum number of electron in the second
energy level (n = 2) is 2
(2)^{2} = 8
electrons

The maximum number of electron in the third energy
level (n = 3) is 2 (3)^{2}
= 18 electrons .... etc ...

See if you can figure out the maximum numbers of electrons in the 4th, 5th,
6th and 7th energy level.

Electrons fill the principal energy levels starting from n = 1 to
n = 7.

Section
4.6 Arrangement
of Electrons
in Principal
Energy
Levels ..p87