Atomic Radius Trend

The article is discussing the variation of atomic radius trend or change of the atomic radius down the group as well as the period along from left to the right.

Atomic Radius Trend

There are certain physical properties of elements such as melting point, boiling point, density, enthalpy of fusion and vapourisation. Out of Multiple factors, one factor that effects such properties is the atomic radius.

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Atomic Radius Trend in a Period

Along a period, the atomic radius trend (or radii) of the elements generally decrease from left to right. The atomic radii of the elements present in the second period are given.

Element Li Be B C N O
Nuclear Charge +3 +4 +5 +6 +7 +8
Atomic Radius (pm) 152 111 88 77 75 74

From the values, it is clear that the alkali metal (Li) placed on the extreme left has the maximum atomic radius while the halogen (F) on the extreme right has the minimum value.

Explanation

In moving from left to the right in a period,

The nuclear charge gradually increases by one unit and at the same time, one electron is also being added to the electron shell. Due to increased nuclear charge from left to the right, the electrons are also getting attracted more and more towards the nucleus. Consequently, the atomic size is expected to decrease as shown in the case of the elements of the second period.

It may be noted that the atomic radius of the last element in each period which is a noble gas element is quite large. For example, in the second period, the atomic radius of neon (Ne) is 160 pm (not shown). Actually, it is van der Waals radius while the rest of the elements have covalent radii. Therefore, no comparison can be made. The van der Waals radii also decrease in a period from left to the right.

 Atomic Radius Trend in Group

The atomic radii of the elements in every group of the periodic table increase as we move downwards. Covalent Radii of the alkali metals present in group 1 are given. Since all of them are metallic in nature, their metallic radii have also been given for reference.

Element Li Na K Rb Cs Fr
Nuclear Charge +3 +11 +19 +37 +55 +87
Covalent Radius (pm) 123 157 202 216 235
Metallic Radius (pm) 125 186 231 244 262

From the values, it is quite clear that the atomic radius of lithium (Li) is the minimum while that of caesium (Cs) is the maximum. The value of the last element francium (Fr) is not known since being as radioactive in nature, it is unstable.

Explanation

On moving down a group, there is an increase in the principal quantum number and thus, an increase in the number of electron shells. Therefore, the atomic size is expected to increase. But at the same time, there is an increase in the atomic number or nuclear charge also. As a result. the atomic size must decrease. However, the effect of an increase in the electron shells is more pronounced than the effect of an increase in nuclear charge. Consequently, the atomic size or atomic radius increases down a group. This is well supported by alkali metals of group 1 the values given in Table for the alkali metals.

This is all about the basics of Atomic Radius Trend.