Question

The radius of an atom in a molecule is expressed as

	1.	Atomic radius
	2.	Covalent radius
	3.	van der Waals radius
	4.	Non bonding radius

Answer 1

The radius of an atom can be expressed in different ways , depending upon the conditions under which the atom exists. Many times an atom is bonded to another atom where the electron densities of the two atoms overlap so in that case the radius of a single atom is difficult to measure. This can be represented as :

The boundary of an atom is not well defined so is represented as dotted lines in the image . In the image , the electron densities of two atoms overlap which are bonded to each other by a covalent bond and r is the radius of the atom which is the distance from the centre of atom w(the nucleus resides) to the boundary of atom.

So atomic radius is not expressed as such but is expressed in different ways as :

1. Covalent radius : This is the radius of an atom which is covalently bonded to another atom. Covalent radius is defined as one half the distance between the centres of two atoms of the same element which are covalently bonded to each other to form a molecule. As the distance between the centres of nuclei of two atoms is the bond length of molecule so covalent radius is also defined as one half the bond length of a molecule.

For example : H_{​​​​​​2} exists as a molecule so the radius of a hydrogen atom (H) can be expressed as the covalent radius.

2. Van der Waals radius : It is defined as one half the distance between the centres of nuclei of two non-bonded atoms in close vicinity to each other.

Covalent and van der waals radius can be shown in one image as :

3. Metallic radius : It is defined as one half the distance between two metal ions lying close to each other in a metal lattice.

So,

The radius of an atom in a molecule is expressed as (2.) Covalent radius because a molecule is formed when two atoms combine or form a bond with each other which is called a covalent bond and in case two atoms covalently bond to each other to form a molecule , the radius of the atom can be expressed as covalent radius.

van der Waals radius is the radius between two non-bonding atoms , that is between atoms of two different molecules so radius of atom in a molecule cannot be expressed as van der Waals radius. So option 3 is incorrect.

Non bonding radius is van der Waals radius which is again not the correct answer. So option 4 is incorrect.

Option 1 is also incorrect because in a molecule we rather talk about covalent radius and not any other radius.

So option 2 is the correct answer.