Atomic Size and Atomic Radius

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Atomic Radius Trend

While we learn about the behaviour of a compound or an atom, the size of the atoms plays an essential role. One of the ways in which we can express the size of an atom is with the help of atomic radius. It helps us in understanding why some of the molecules fit in together and the rest contain parts which get crowded under different conditions. The atomic size is defined by its orbital edge. However, these orbital boundaries are quite fuzzy and tend to vary in different conditions. For standardizing the measurement of the atomic radius, the distance between the nuclei of two similar atoms that are bonded together is measured. We can, therefore, define the atomic radius of elements as half of the distance between the nuclei of similar atoms which are bonded together. 

Today, we will learn about what is atomic radius periodic table, the atomic radius definition, what is the atomic radius trend, what is atomic size periodic table, and the atomic size trend.


Atomic Radius Definition

Let us now define the atomic radius.

The atomic radius of an element refers to the measure of the size of the element’s atoms, which is typically the mean distance from the nucleus centre to the boundary of its surrounding shells of the electrons. However, since the boundary is not well-defined, there are several non-equivalent definitions of the atomic radius. There are three types of atomic radii which are Van der Waals radius, covalent radius and ionic radius.

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Measurement of Atomic Radius

Let us learn how the atomic radius is measured and what is its unit.

Atomic radii are measured for elements. The units for measuring the atomic radii are picometers, which is equal to 10−12 meters. Consider, for, example, the internuclear distance between two hydrogen atoms in the H2 molecule is known to be 74 pm. Hence, the atomic radius of the hydrogen atom is 74/2 = 37  pm


Atomic Size Trend

Let us learn about what is the trend in atomic radii down a group.

When we move down the group or across the row or column in the periodic table, we would observe a lot many trends in the elements, both physical and chemical. Consider, for example, while moving down the group of the non-metals, the reactivity of the elements tends to decrease, whereas, it tends to increase when we move down the group of the representative metals.

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When we combine two atoms, we can then estimate their atomic size when we check the distance between the two atoms. The other method through which we can measure the atomic size of a given non-metallic element is by the formation of a single covalent bond between the two atoms and then check the distance between the two atoms. The radius which is found by this method is called the covalent radii of the element. However, in the case of a metal, it is known as a metallic radius. It is defined as one half of the distance between the nuclei of the two adjoining metal ions that are joined by a metallic bond.

The atomic radius of an atom is measured with the help of X-ray or several other spectroscopy methods. The atomic radii of the elements tend to vary in the periodic table but a fixed manner. We can explain this trend when we consider the nuclear charge and the energy level.

Typically, the atomic radius tends to decrease as we move from the left to right in a period and it tends to increase when we move downwards in a group. The reason being that in periods, the valence electrons tend to lie in the same outermost shell. The atomic number tends to increase within the same period when we move from the left towards the right which tends to increase the overall effective nuclear charge. The increase in the attractive forces, in turn, reduces the atomic radius of the elements.

You know that the force of attraction between the protons and electrons tend to play a very important role in the increasing or decreasing pattern of the atomic radius.

FAQ (Frequently Asked Questions)

1. What is Van der Waals Radius?

Ans: Van der Waals Radius refers to one half of the distance between the nuclei of two similar non-bonded isolated atoms or the two adjacent identical atoms which belong to two neighbouring molecules of a given element in its solid-state. The magnitude of the Van der Waals radius tends to depend on how the atoms are packed when the element is in its solid-state.

Consider, for example, the internuclear distance between the two adjacent atoms of chlorine of the two neighbouring molecules in the solid-state is known to be 360 pm. Hence, the Van der Waals radius of the chlorine atom would be 180 pm.

2. How do Atomic radii vary within a group?

Ans: The atomic radii vary within a group in the following manner. The atomic radii of elements tend to increase when there is an increase in the atomic number as we go from top to bottom in a group. As we move downwards in the group, the principal quantum number tends to increase. A new energy shell gets added at every succeeding element. The valence electrons tend to lie much farther and farther away from the central nucleus. As a result, the attraction of the nucleus towards the valence electron decreases. Therefore the atomic radius tends to increase.