The periodic table is an arrangement of the known elements based on their chemical properties.
The
elements are arranged in order of
increasing atomic number across the rows (periods).
The atomic number (Z) equals the number of protons in the nucleus of an atom.
This is the number in the box for each element that is an integer.
Usually atoms of higher atomic number also have higher relative atomic mass (Ar).
This number also appears in the box with the symbol for the element and is often NOT an integer.
The properties of elements are referred to as being
periodic because elements at the same position in different
periods (rows) have similar chemical properties. Thus elements in the same
column (Group) are in the same chemical family.
The elements of Group
are all nonreactive gases (He, Ne, Ar...).
The elements of Group 2 are all metals (Be, Ca, Mg...).
Note that different numbering systems for the columns may be used.