Periodic Table

Horizontal rows in a periodic table are called periods.

Vertical columns in a periodic table are called groups.

Period number tells you about how many shells an element has, while group number depicts how many electrons should be in the last shell.

For example if you want to do an electronic configuration of sodium , Na

Spot Na in the periodic table, it belongs to group no 1 and period no 3.

That means, it has 3 shells and 1 electron in the last shell. We know that 1st shell can only hold 2 electrons, 2nd and 3rd shell can hold maximum 8.electrons.

So the electronic configuration of sodium would be 2, 8, 1