As you move from the light elements to the
heavy elements you keep periodically coming
across the same properties, which is why it's
called the Periodic Table. The recurring properties
are organised so that you can easily see similarities
between elements.
The Periodic table is arranged in PERIODS
and GROUPS, going from the light elements
at the top, to the heavy elements at the bottom.
The rows going ACROSS from left to right are
PERIODS.
Elements in the same period all share something
in common. They have the same number of energy
shells. Each new period row represents a new
shell. Elements in the first period have one
shell, and as we go down, the shells increase.
Hydrogen is in the first period. It has one
shell. Potassium is in the 4th period, and
has four energy shells, as do all the other
elements in this period.
The columns going down from top to bottom,
are the GROUPS. Elements in the same group
also have something in common. Elements in
the same group have the same number of electrons
in their outermost shell. The electrons in
the outershell are called the valence electrons.
This just means that these are the electrons
available for reactions and bond formation.
The number of electrons in the outer-shell
governs elements reactivity, which is why
elements in the same group have similar properties.
The group number can tell you how many electrons
are in this shell. For example let's look
at Group 7, Fluorine, Chlorine, Iodine, Bromine,
and Astatine. They all have 7 electrons in
their outmost shell, and all exhibit similar
chemical characteristics.
The properties show a gradual change going
down the group , as we go from period to period.
So if we look at Group 7 again, we can see they
are each in a different Period in the periodic
table. Telling us that each element in this
group, has its outer electrons on a different
shell.
So Chlorine is in group 7 period 3, therefore
it has 3 energy levels, with 7 electrons in
its outermost shell.
