Question

How to find out the charges of polyatomic ions?

Accepted Answer

Polyatomic ions are the group of atoms that act as an ion. Each polyatomic ion carry a specific charge, which is determined by their numbers of valence electrons.

Ways to find out the charges of polyatomic ions are:

1. Calculation of oxidation number of each atom in the ion.

For example, in the hydroxide ion, which has an oxygen atom and a hydrogen atom. The oxidation number of oxygen is $- 2$ , and the oxidation number of hydrogen is $+ 1$ . On addition of the oxidation numbers of all the atoms in the polyatomic ion, $(- 2 + 1 = - 1)$ . Thus, the charge on the polyatomic ion is $- 1$ .

2. Draw the Lewis dot structure of the ion.

For example, consider the ammonium ion $({NH}_{4}^{+})$ , which has one nitrogen atom and four hydrogen atoms. The nitrogen atom is surrounded by valence electrons. Each hydrogen atom has a single dot next to each one to represent their valence electrons. Now, draw the atoms of the ion bonded together with covalent bonds. In the example, the four hydrogen atoms will bond to the nitrogen atom, and each hydrogen atom's lone electron will form a covalent bond with one of nitrogen's electrons. Replace each bonding pair of electrons with a line to symbolise a covalent bond. Here, the N would have four bonds, each attached to an H.

Thus, in the ammonium ion $({NH}_{4}^{+})$ , which contains $9$ electrons ( $5$ from $N$ and $1$ from each of the four $H$ atoms).

$9 - 1 = 8$ electrons

One electron is subtracted because the entire molecule has a $+ 1$ charge.

Species	Number of Electrons
$(i) {Be}^{2 +}$	$(a) 0$
$(ii) H^{+}$	$(b) 10$
$(iii) {Na}^{+}$	$(c) 2$
$(iv) {Mg}^{+}$	$(d) 11$
	$(e) 4$

How to find out the charges of polyatomic ions?

Important Questions on Chemical Bonding and Molecular Structure

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