The elements in the Periodic Table are arranged in an increasing order of atomic number (Z) as we travel from the left to the right across the table.

The horizontal rows are called periods and the vertical columns, groups. As we travel across each period, there is progression from metals to non-metals. Elements of the same group possess the same electronic configuration; for example, lithium (Li) and sodium (Na). The electronic configuration for lithium is 2+1 and for sodium 2+8+1. As we can see, both of them have the same valency ie a shortage of 1 electron. This explains the +1 charge this group possesses.

There is a break between groups II and III where 10 groups of elements can be found. These groups are called the transition elements and are metals. The transition elements have variable valency.

The groups below the transition elements which are on different rows are called lanthanides and actinides. Lanthanides are elements with atomic numbers from 57 to 70. Lanthanides are also known as rare