Question

Which of the following elements has an atomicity of ‘one’?
A. Helium
B. Hydrogen
C. Sulphur
D. Ozone

Answer 1

Hint: First think about the definition of atomicity and what property of an element it gives information about. Also consider how it varies in different elements. Atomicity is basically defined as the number of atoms present in one molecule.

Complete step by step solution:
-The atomicity of any element is defined as the number of atoms of that element present in the molecule of that element. This can change depending on the element since the valency of the element also has to be satisfied while forming the bonds in a molecule.
-Elements are not always found in their monatomic form in nature, most of the time, they bond with other atoms of the same element to satisfy their valencies and form a bond to give diatomic, triatomic, or polyatomic molecules.
-Depending on the number of electrons in the outermost shell of the atom, the valency and in turn the atomicity is decided. Although the atoms of the element have bonded to each other the overall substance is still considered to be pure since the only atoms present are of that particular element. The atoms adapt to a lower energy state as opposed to a non-bonded and high energy state.
-The bonds formed between the molecules are covalent since there isn’t any difference in the electronegativities and the electrons can be shared.
-We know that since inert gases have a complete octet, they will not require to bond with any other atom to achieve a stable state. Hence, they are mostly found in the monoatomic state with the atomicity as 1.
-Among the given options, helium is the only inert gas present and is a monatomic gas with an atomicity of 1. Hence, the correct answer to this question is ‘A. Helium’

Note: Remember that valency is not equal to atomicity, let us take the example of hydrogen. We know that its valency is 1 since it can donate or accept a maximum of one electron. But, its atomicity is 2 since it naturally forms a diatomic gas ${{H}_{2}}$ by the sharing of electrons.