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We wrote about the Periodic table here and you can check that article out before you continue with this one. The physicochemical properties of an element depend on the electron configuration of its atom. According to the periodic law, the outer electron configuration of atoms recurs periodically hence any property of the atom that varies with the electron configuration exhibits periodicity.

The following physical properties of the atoms exhibit periodicity: atomic size, atomic volume, ionic radius, ionization energy, electron affinity, and electronegativity. We are going to define all these terms individually below.

Atomic Radius

The atomic radius of an element represents half of the intermolecular distance of two adjacent atoms. The values are determined by the X-ray or electron diffraction method. The atomic size is determined by two things namely:

  • Nuclear Charge: This is the number of protons in the nucleus of an atom. There is an attractive force between the nucleus and the electrons. Hence, nuclear charge tens to decrease atomic size.
  • Screening or Shielding Effect: Effect due to the repulsive force between the electrons in the inner shells and those in the outer shells. Hence screening effect tends to increase atomic size.

Variation Of Atomic Radius Along The Period & Group

There is a decrease in the atomic radius across the period due to the steady increase in the number of protons in the nucleus, the outermost electrons are drawn closer to the positive nucleus. Down the group, there is a steady increase in atomic size due to an increase in the number of inner shells.

Ionic Radius(Size)

Ionic Radius is of two types: the radius of a cation and the radius of an anion. If you recall, a cation is formed when a neutral gaseous atom loses one or more electrons hence the size of a cation is smaller than that of the corresponding neutral atom. An anion is formed when a neutral gaseous atom gains one or more electrons.

The size of an anion is formed when a neutral gaseous atom gains one or more electrons. Before we proceed to the next subheading, it is important to define an Isoelectonic ion. An isoelectronic ion is ions(cations & anions) that have the same electron configurations(same number of electrons) but different atomic numbers(different number of protons).

Ionization Energy

The electrons in an atom are held to the nucleus by the electrostatic force of attraction between the nucleus and the electrons however when an atom absorbs sufficient energy, an electron can be completely removed from the atom. The energy required to remove the outermost electron in an atom in gaseous state is called the first ionization energy.

Some of the factors that determine ionization energy include atomic size and screening effect by inner electrons, nuclear charge, and electronic configuration of the atoms.

Electron Affinity

The word affinity means liking. When a neutral atom X gains an electron, an anion Xn- is formed. Electron Affinity is the energy released when an isolated gaseous atom in its ground state accepts electrons to form negative atoms. Some of the factors that determine electron affinity include nuclear charge, and atomic size & screening effect.


Electro negativity is the ability of an atom to attract shared pairs of electrons to itself in a molecule. There is an increase in electronegativity across the period and a decrease down the group.

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