The basis of the modern form of periodic table in atomic number. The physical and chemical properties of the elements are periodic functions of their atomic numbers.

Some of the key features are:

• Arrangement of electrons in increasing order of their atomic numbers.
• There are 7 horizontal periods and 18 vertical groups in the modern periodic table.
• Metals are found at the the left end and the centre of the periodic table, non metals are found at the right end of the periodic table while metalloids are arranged in zig-zag pattern.
• Physical and chemical properties follow periodic trends.