 The distance from the centre of the nucleus to the outermost shell holding the electrons is termed as Atomic Radius.

In simpler terms, it can be well-defined as something comparable to the radius of a circle, where the nucleus is the centre of the circle, and the outermost orbital of an electron is the outer edge of the circle.

However, it is hard to determine the atomic radius because of the uncertainty in the positions of electrons.

Hence, to measure the atomic radius of an atom precisely, Heisenberg’s Uncertainty Principle can be used. Heisenberg’s Uncertainty Principle says that the radius of an atom can be determined based on the distance between nuclei of two bonded atoms. There is no secure radius of an atom; depending upon the bond it has formed, it can have different radii. Ionic radius, Covalent Radius and Vander waals Radius are three generally used classifications of the atomic radius, based on categories of bonds formed.

## Here are the types of atomic radius, depending upon the types of bonds formed by the atoms, listed below:

### Important Questions on Atomic Radius

1) What is the meaning of atomic radius?

Atomic radius can typically be defined as the distance from the centre of the nucleus to the border of the last electron surrounding the nucleus. While the radius of the circle can be understood as the atomic radius, the nucleus can be understood as the centre of the circle, and the outermost orbital of an electron can be taken as the outer boundary of the circle.

2) Explain how the atomic radii are a periodic property.

Due to the below mentioned points, atomic radius is a period property:

• Variation in a group: As we move from top to bottom in a group, with the increase in atomic number, the atomic radius of elements increases. Due to the increase in shell number, the attraction of the nucleus for the electrons decreases. Therefore, along with the group, the atomic radius increases.
• Variation in periods: The atomic radius decreases when we move from left to right in a period. Again, moving left to right in a period, the nuclear charge rises by one unit in each following element. However, the number of shells remains the same, which is why the electrons are strongly attracted by the nucleus, and the atomic radius decreases along the period.

3) What do you mean by covalent radius?

Covalent radius is defined as the radius of an atom which is under a covalent bond with another atom. Measuring the bond lengths of covalently bonded atoms is another way of measuring covalent radius. Covalent radius is half of the bond length when two same kinds of atoms are bonded. In some molecules of chlorine and oxygen, covalent radius can be measured easily by measuring half of the bond length. Covalent can be naturally defined as half of the internuclear distance between two bonded atoms.

4) What is the ionic radius?

Ionic radius can be defined as the radius of the atom which forms ionic bonds. Electrons and nuclei are limited by atomic bonds, which result in no specific shapes of atoms and ions. The unit for measuring the ionic radius is Armstrong (A0) or picometer (pm). The typical radius of an ionic atom varies from 30 to 200 picometers.

5) What is meant by Bohr Radius?

The atomic radius of an atom when it is in its lowest energy state is defined as the Bohr radius. Bohr’s radius is fundamentally relevant to atoms and ions with a single electron, like Hydrogen.

6) Van der Waals radius is larger than the covalent radius, what is the reason behind this fact?

The Van der Waals forces of attraction are weak. Thus, in case of atoms held by Van der Waal forces, the internuclear distance is much larger than those between covalently bonded atoms. Since, in a covalent bond a part of an electron cloud becomes common because it is formed by the overlapping of two half-filled atomic orbitals. Therefore, covalent radii are always less than the van der Waal radius.

7) Is it possible to measure atomic radius directly?

Due to the uncertainty in the position of the outermost electron, the atomic radius cannot be measured directly.

8) Among S, Se, P and As, which element will have the largest atomic radius?

While moving down in a periodic table the atomic radius keeps on increasing and moving left to right will show a decreasing trend in atomic radius. Arsenic and Selenium are in the lowermost row but arsenic is more to the left. Therefore, As has the largest atomic radius.

9) Among P, Bi, N and As, which element will have the smallest atomic radius?

The above-mentioned elements are in the same groups in the periodic table and we know that the atomic radius keeps on increasing from top to bottom. Moving down the group results in the addition of another shell, which increases the atomic size. Hence, in this case Nitrogen has the smallest atomic size.

10) Does atomic number increase across a period table?

The size of an atom usually decreases as we move from left to right for a certain period. Hence, the atomic radius decreases across a period and increases down a group.

### Practice Questions

1. Name the element that has the largest atomic radius.
4. If the distance between the nuclei of two atoms in a metallic bond is 200 pm, what will be the atomic radius of one atom?

#### Define Van Der Waals Radius.

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