Ionic and Covalent Radius

Ionic Radius

Ionic radius is the distance from an ion’s nucleus to which it exerts effect on its electron cloud.

When an atom loses or acquires electrons, ions form. When an atom loses an electron, it becomes a cation; when an atom acquires an electron, it becomes an anion. The ionic radius is defined as the distance between an ion’s nucleus and its outermost shell.

A cation’s atomic size will be smaller than that of the parent atom. An anion is slightly larger than its parent atom. This is due to the fact that as an atom gains electrons, the total number of electrons grows, causing more repulsion between electrons and therefore overshadowing the net effective nuclear charge.

Ionic Radius Periodic Table

As an example,

Potassium radius = 243pm.

The radius of the potassium ion is 138pm.

Isoelectronic entities are atoms and ions that contain an identical number of electrons. For example, both O2- and Mg2+ have ten electrons, yet they do not have the same ionic radius since their effective nuclear charge is different.

The radius of a cation is lower than that of an anion because a cation has a stronger positive charge (i.e. more protons) and hence attracts the electrons in the outermost orbital with greater force, resulting in the smaller size.

Trends in Ionic Radius along  Groups

Moving down a group in a periodic table, atoms add extra shells (number of electrons), causing the ionic radius of elements to grow.

Trends in Ionic Radius Over Period

With an example, let us examine the patterns in the ionic radius of elements throughout time. Period 3 shows that the atomic radius first falls, then unexpectedly increases, and then steadily decreases again. This is because the elements at the beginning of a period tend to produce cations, whereas the ones at the end of a period tend to form anions.

Covalent Radius 

The size of an atom that forms part of a single covalent bond is referred to as the covalent radius. Picometers (pm) or angstroms (A0) are used to measure covalent radius. The sum of two covalent radii should equal the length of a covalent connection between two atoms in theory, but in fact, the bond length is determined by the chemical environment. Covalent radius charts for double and triple covalent chemical bonds are also available.

Covalent radius trend along the group 

As  the n level (orbital size) grows as we proceed down a group, the covalent radius increases. Because the effective nuclear charge experienced by the electrons increases as we move left to right across a period, the electrons are drawn in closer to the nucleus and the covalent radius lowers.

Covalent radius trend along the period 

 The covalent radius reduces in a period as we move from left to left to right. The atomic number or nuclear charge increases as we proceed from left to right. The nucleus’s influence on the electron cloud grows stronger, the electron cloud shrinks, and the covalent radius lowers.

Covalent radius influencing factors

An atom’s radius expands in direct proportion to its atomic number. As you move down a given column of the periodic table, the radius of each consecutive adjacent atom increases. The number of complete electron shells rises as you move down the periodic table, resulting in a greater size

Conclusion 

The ionic radius of an atom in a crystal lattice is calculated by measuring its radius. When electrons are removed, an ion smaller than the source element is created. When electrons combine, they produce an ion that is larger than the source element.The covalent radius of an element is equal to one-half of the covalent bond distance of a molecule in which the atoms involved are singly bonded. In other terms, it is one-half of the distance between the nuclei of two identical atoms connected by a single covalent connection.