The arrangement of the chemical elements according to the atomic number is known as the periodic table. Thus, the periodic properties of the elements (chemical periodicity) are made clear by this type of arrangement. The horizontal rows are called periods, and the vertical columns are called rows. On the left side of the periodic table, metals are placed, and the non-metals are placed on the periodic table’s right side. The elements in groups (vertical columns) of the periodic table exhibit similar chemical behaviour. This is because, in a group, an equal number of electrons are distributed in their valence shells.

A proton is added to the nucleus and an electron to the valence shell as we move across the period. The number of electrons in the valence shell remains constant as we move down the group. Some properties like size (radius) of atoms and ions, ionisation energies, and electron affinities vary periodically as the elements’ electronic structure changes. Terms like isoelectronic species and isodiaphers are also explained in this article.

Variation in Covalent radius

Covalent radius is defined as one-half the distance between the nuclei of two identical atoms when joined by a covalent bond. On moving from left to right across a period, the covalent radii decrease. Down the group, the principal quantum number, n, increases by one for each element. Thus, the electrons are being added to a region of increasingly distant space from the nucleus.

Effective nuclear charge Zeff

This concept is nothing but a force exerted on a particular electron by the nucleus, taking into account any electron-electron repulsions. The pulling of the nucleus results in the shielding of outer electrons by the inner electrons.


Zeff = Z – Shielding. (In the case of hydrogen, the atomic number is one which implies that it has only one electron in the outermost shell and so the nuclear charge (Z) and the effective nuclear charge (Zeff) become equal)

Variation in Ionic radii

The concept of ionic radii explains the size of an ion. In cation, it resembles the parent atom by having the same number of protons and fewer electrons. For example, the covalent radius of an aluminium atom (1s22s22p63s23p1) is 118 pm, whereas the ionic radius of an Al3+ (1s22s22p6) is 68 pm. In the case of anion, it is formed by adding one or more electrons to an atom’s valence shell. Here the Zeff is decreased per electron as the electron-electron repulsion is greater. For example, a sulphur atom ([Ne]3s23p4) has a covalent radius of 104 pm, whereas the ionic radius of the sulphide anion ([Ne]3s23p6) is 170 pm.

Isoelectronic species

It refers to two atoms, ions, or molecules with the same electronic structure and the same number of valence electrons. Carbon monoxide (CO) and dinitrogen (N2) are isoelectronic. Both have ten valence electrons. Examples for isoelectronic series P3–, S2–, Cl, Ar, K+, Ca2+, and Sc3+ ([Ne]3s23p6).


They are a set of nuclides with different protons and neutrons, but there is an exact difference between the protons and neutrons—for example, Thorium-234 and Uranium-238.

Neutrons in Uranium = 146

Protons in Uranium = 92

Therefore, the difference between neutrons and protons is 146 – 92 = 54

Hence, Thorium-234 and Uranium-238 are considered Isodiaphers

Variation in Ionisation energy

To remove an electron from an isolated atom or molecule, a fixed amount of energy is required. This amount of energy is known to be ionisation energy. It decreases going down a group and increases from left to the right across the periodic table. There are first, second, third ionisation energies. An element’s first ionisation energy is the energy required to remove the outermost, or least bound, electron from a neutral atom of the element.


The concept of electronic configuration understands periodic trends. The one-half the distance between the nuclei of two identical atoms joined by a covalent bond is known as covalent radii. It decreases across a period because the effective nuclear charge experienced by the electrons increases.

  • Isoelectronic species have the same electronic structure and the same number of valence electrons.
  • Isodiaphers are a set of nuclides that have a different number of protons and neutrons.
  • The ionisation energy increases from left to right on the periodic table and decreases down the group.