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The group
number of an element in the periodic table
is the same as the
number of electrons in its outer shell.
This is what gives the element its
chemical
properties.
The number of electron shells which an element has
shows the period
(row) in the periodic table in which
it is found.
The first period contains hydrogen and helium.
The second period is from lithium to neon.
The third period is from sodium
to argon and so on.
We can write the electron structure (also called the electronic structure,
the electron configuration or the
electronic configuration)
of an element according to the
number of electrons in its shells.
The inner shells are always full, and have the numbers 2, 8, 8, 18.
For example, potassium has the
electron structure 2, 8, 8, 1.
Potassium is an alkali metal. It is in
group 1 (1
electron in its outer
shell)
and period 4 (4
shells).
Fluorine has the electron
structure 2,7.
Fluorine is a halogen. It is in
group 7 (7 electrons in its
outer shell)
and period 2 (2
shells).
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