Story of becoming Ion

Atoms react to become stable by having 8 e in the outermost shell.
For this reason they either lose or gain or share electron.
If electrons is lost , positive ion is formed.
If electron is gained, negative ion is formed.
How?

Na atom has 11 electron which are arranged in 2,8,1 configuration. To become stable as that of nearest noble gas Ne it need to have 8 electrons in its outermost shell. Hence it looses its outermost electron.

Initially it has 11 protons and 11 electrons. Now 1 electron is lost so remaining electrons are 10 but protons remain the same that is 11. Since protons are positive and electrons are negative in charge so positive charge is greater than negative charge.This difference  appear on symbol of element as charge. Hence Na becomes Na ion. 

Similarly Cl atom has 17 electrons which are arranged in 2,8,7 configuration. To become stable as that of nearest noble gas Ar it need to have 8 electrons in its outermost shell. Hence it gains 1 electron in  its outermost shell.

Initially it has 17 protons and 17 electrons. Now 1 electron is added so remaining electrons are 18 but protons remain the same that is 17. Since protons are positive and electrons are negative in charge so Negative charge is greater than positive charge.This difference  appear on symbol of element as charge. Hence Cl becomes Cl ion.