Question

Explain the formation of (i) sodium ion, and (ii) chloride ion, from their respective atoms giving the number of protons and number of electrons in each one of them. What is the reason for positive charge on a sodium ion and a negative charge on a chloride ion ?

Answer 1

(i) Formation of Sodium ion:

A neutral sodium atom consists of 11 protons and 11 electrons. The protons and electrons are equal in number, thus keeping the overall charge of the electrically neutral sodium atom zero.
A sodium ion is formed when the neutral atom loses an electron.

	Sodium atom (Na)	Sodium ion (Na+)
Electrons	11 (−charge)	10 (−charge)
Protons	11 (+ charge)	11 (+charge)
Overall charge	Zero	+1

It is clear that a sodium ion has one electron less than the electrically neutral sodium atom. In a sodium ion, the number of protons is more than the number of electrons, which gives the ion an overall positive charge of one unit.

A proton has an overall positive charge and an electron has an overall negative charge. In a neutral atom, these charges cancel out each other keeping the neutral atom electrically uncharged. However, when a neutral atom loses an electron, the number of protons in the ion is more than the number of electrons. The extra proton gives the ion an overall positive charge (since a proton is positively charged).

(ii) Formation of chloride ion:

A neutral chlorine atom consists of 17 protons and 17 electrons. The positive charge of protons and the negative charge of electrons cancel  each other, thus keeping the overall charge on the electrically neutral chlorine atom zero.

A chloride ion is formed when the neutral chlorine atom accepts an electron.

	Chlorine atom (Cl)	Chloride ion (Cl−)
Electrons	17(−charge)	18(−charge)
Protons	17(+charge)	17(+charge)
Overall charge	Zero	−1

It is clear that a chloride ion has one electron more than the electrically neutral chlorine atom. In a chloride ion, the number of protons is less than the number of electrons, which gives the ion an overall negative charge of one unit.

A proton has an overall positive charge and an electron has an overall negative charge. In a neutral atom, these charges cancel each other keeping the neutral atom electrically uncharged. However, when a neutral atom accepts an electron, the number of protons in the ion becomes less than the number of electrons. The extra electron gives the ion an overall negative charge (since an electron is negatively charged).