Chapter 7.3, Problem 35SSC

Interpretation Introduction

Interpretation:

The difference between monoatomic and polyatomic ion with example

Concept introduction:

The atoms lose electrons or gain electrons to form ion. Based on the number of atoms present in the ions it is monatomic or polyatomic. Both of these carry charges and can be cation or anion. Both of these forms ionic compounds due to electrostatic forces of attraction between the oppositely charged ions.

Answer to Problem 35SSC

The difference between monoatomic and polyatomic is that monoatomic ions has single atoms while polyatomic ions have more than one atom.

Explanation of Solution

A monoatomic ion is formed when the atom gains or loses electrons. But it will display a net charge as the balance between the number of electrons and number of protons are not balanced. In other words, when comparing with neutral atom, there are extra electrons or less number of electrons resulting in negatively or positively charged ions.

Example: Chlorine in its neutral state has atomic number 17. This means that it has 17 protons and 17 electrons. The chlorine atom can gain extra electron and become a negatively charged ion, i.e chloride ion. It is depicted below as

$\begin{array}{l}\text{ Cl + }{e}^{-}\to {\text{ Cl}}^{-}\\ \text{17 electrons 18 electrons}\\ \text{17 protons 17 protons }\end{array}$

It can be seen that chlorine initially had 17 electrons and after gaining 1 electron will have 18 electrons though the number of protons are the same. But as Chloride ion has extra electron, it has net charge of -1.

A polyatomic ion is formed when group of covalently bonded atoms carries a net charge as there is no balance of charges between the number of protons and number of electrons.

The net charge is equal to the sum of the charges of individual ions present in the molecule.

Example: Hydroxide is polyatomic ion having two atoms, Hydrogen and Oxygen. It is bonded by covalent bond as there is sharing of electrons between oxygen and hydrogen. But, there are three lone pairs of electrons in oxygen. Considering the net charge for hydroxide ion, ${\text{OH}}^{-}$ it is -1.

Differences between Monatomic and Polyatomic ions

Monatomic	Polyatomic
It is composed of single atom per ion	It is composed of several atoms per ion
It is formed by the loss or gain of electrons present in outer shell	It is formed by covalent bonds between several atoms
It does no form any chemical bond	It is formed covalent bonds
Example: Chloride ion, Bromide ion, etc	Example: Hydroxide ion, Nitrate ion, etc

Conclusion

The difference between monoatomic and polyatomic is that monoatomic ions has single atoms while polyatomic ions have more than one atom.