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Chemistry - The modern periodic law

The modern periodic law

Mendeleev used increasing atomic weight (i.e. relative atomic mass), in
conjunction with similar chemical properties, as the basis for arranging
the then known elements, claiming that the properties of the elements
formed a periodic function of their atomic weights.

With better knowledge of the role of electron arrangements in determining
the behaviour of elements, we can claim that the are a of their

The atomic number, defined as the number of positive charges (or protons)
in the nucleus of an atom, also tells us the number of electrons and allows
us to determine the electron arrangement. When the elements are arranged in
order of increasing atomic number, similar chemical properties coincide
with similar outer-shell electron arrangements.

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