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Atomic radius is the distance measured from the centre of the nucleus to the external shell carrying electrons. It is also known as the distance measured from the centre of the nucleus to the point until the density of the electron cloud becomes maximum.
TYPES OF ATOMIC RADII
There are three types of atomic radii:-
Covalent Radius:-Covalent radius is a half of the distance between the nuclei of two covalently bonded atoms of the same element of a molecule. Therefore, m covalent is equal to ½ (internuclear distance between the two bonded atoms). The internuclear distance between two bonded atoms is known as the bond length. Therefore,m covalent = ½( bond length)
Van der waals Radius:-It is a half of the distance between the nuclei of two similar non-bonded lonely atoms or two nearest similar atoms belonging to two adjacent molecules of an element in the solid-state. The Van der Waals radius magnitude is dependent on the configuration of the atoms when the elements are in the solid-state.
Metallic Radius:- Metallic radius is explained as a half of the distance between the nuclei of two atoms in a crystal or between two nearest metal ions in the metallic network.
MEASUREMENT OF THE ATOMIC RADIUS
An atom doesn’t have a fixed radius. The radius of an atom can only be initiated by measuring the distance between the nuclei of two connecting atoms, and then by halving that distance.
Same atoms can be initiated to a different radius depending on what is surrounding it.
The atoms are pulled nearly together and so the measured radius is less than if they are just meeting. This is what you would get if you had metal atoms in a metallic structure, or atoms that are covalently bonded to each other. The type of atomic radius being measured here is known as the metallic radius or the covalent radius determined by the bonding.The units for measurement of atomic radii is picometer ,which is equal to 10−12 metres. As an example, the internuclear distance between the two hydrogen atoms in an H2 molecule is measured to be 74 pm.
The attractive forces between the atoms are much less, and the atoms are importantly “unsquashed”. This measure of atomic radius is known as the van der Waals radius after the weak attractions present in this situation.
Variation of Atomic radius
Within a Period
The atomic radius abruptly increases as we get from halogens to inert gases. And the reason is that inert gases have completely filled orbitals. Therefore the inter-electronic is maximum. We express atomic size in terms of Van der Waals radius since they do not form covalent bonds. And the Van der Waals radius is always higher than that of the covalent bonding radius. As a result, the atomic size of the inert gas in a period is much bigger than that of the prior halogen.
The nuclear charge increases by one unit in each successive element as we travel from left to right in a period, but the number of shells remains constant. And The increased nuclear charge attracts the electrons from all shells that are closer to the nucleus. As a result, each individual shell becomes smaller and smaller. Therefore, as we travel from left to right in a period, the atomic radius decreases.
The Covalent and Van der Waals radii decrease as the atomic number grows as we move from left to right in a period. The alkali metals on the extreme left of the periodic table are the biggest in a given period. The halogens on the far right side of the periodic table are the smallest. The atomic size of nitrogen is the smallest. Following nitrogen, the atomic size of oxygen increases and then lowers for fluorine. The atoms of inert gases are bigger than those of the preceding halogens.
Within a Group
Elements’ atomic radii grow as the atomic number in a group increases from top to bottom. As we proceed with the group, the primary quantum number increases. A new energy shell is added for each subsequent element. Valence electrons are ejecting from the nucleus. The nucleus’s attraction to the electron diminishes as a result. The atomic radius increases as a result.
CONCLUSION
The atomic radius is the size of the atom, commonly measured via distance from the nucleus of the atom to the electron clouds around the nucleus. While there is no physical presence of an orbital in atoms, it is difficult to measure the atomic radius. Thus sometimes different methods are used to measure the radius while they are bonded in a molecule.
