Ionic Radius

It can also be defined as the distance of the valence shell of electrons from the centre of the nucleus. The ionic radius of an ion is the effective distance between the nucleus and the outermost shell of an ion having an electron; to be precise it is the distance from the nucleus up to which it has influence on its electron cloud, as we go down the group, ionic radius increase and it decreases for metals forming cations, as the metals lose their outer electrons. The ionic radius increases for non metals as the effective nuclear charge decreases due to the number of electrons exceeding the number of protons, it decreases across a period. This is due to the fact that metal cations lose electrons, and which further decreases the overall radius, mainly because the number of protons in the nucleus increases across the period, which ends up causing attraction to bonding pairs of electrons more strongly.

Atomic Radius

The atomic radius is half the diameter of a neutral atom. In other words, it is half the diameter of an atom. The ionic radius is half the distance between two gas atoms that are just touching each other. In case of metals, the atomic radius is generally larger than the ionic radius of the same element. We know that atomic size is the distance between the centre of the nucleus of an atom and its outermost shell and the atomic radius is defined as the shortest distance between the atom’s nuclei and the outermost shell of that atom. The atomic radius is generally a measure of the size of its atoms, usually the distance from the centre of the nucleus to the boundary of the surrounding shells of electrons. The atomic radius of atoms generally decreases from left to right across a period; there are some exceptions, such as the oxygen radius being slightly greater than the nitrogen radius, as within a period, protons are added to the nucleus as electrons are being added to the same principal energy level and as we go down in the group, the atomic radius increases. The reason for this is the increase in the number of shells down the group due to which the force of attraction over the valence shell of an atom decreases and thus the ionisation energy also decreases.

Atomic radii are divided into three types:

Covalent radius: The covalent radius is a measure of the size of an atom. The covalent radius, rcov, is a measure of the size of an atom that forms part of one covalent bond.

Van der Waals radius: A Van der Waals radius is a measure for the size of an atom that is not chemically or ionically or covalently bonded. In general a van der Waals radius is defined as half the closest distance of two equal, non-covalently bound, atoms.

Metallic radius: Metallic radius is half the distance between centres of nuclei of two atoms of metal held together by metallic bond.

Ionic radius: The distance between the nucleus and the electron in an ion’s outermost shell is called the ionic radius.

Conclusion

The ionic radius increases on descending the group and decreases on going across a period and it is applicable only if the elements are the same type of ion, either cations or anions.

Conversely, as the ionic radius increases, charge density decreases and it results in weaker bonds during the formation of a lattice so the overall enthalpy change is less exothermic, also the lattice enthalpy increases with higher ionic charge and with smaller ionic radius (due to increased force of attraction), The smaller the ion the greater will be its charge density and the force of electrostatic attraction that it will exert. The ionic bonds tend to be stronger than covalent bonds due to the coulombic attraction between ions of opposite charges ionic radius is not a permanent trait, but changes depending on coordination number, spin state etc.