VARIATION OF ATOMIC RADIUS IN THE PERIODIC TABLE

                                 ATOMIC RADIUS

Atomic radius is simply the distance between the nucleus of an atom and it's outermost electron. It is also half the distance between the nucleus of two atoms. This idea is complicated because use not all atoms are normally bound together in the same way. Some are bound by covalent bonds in molecules, some are attracted to each other in ionic crystals and others are held in metallic crystals. A vast majority of elements form covalent molecules in two like atoms are held together by a single covalent bond. The covalent radii of these molecules are often refered to as atomic radii. Atomic radius is measured in picometer (pm), angstrom or nanometer (nm). The variation of atomic radius is observed throughout the periodic table.

  VARIATION OF ATOMIC RADIUS ON THE PERIODIC TABLE

ACROSS THE PERIOD: Atomic size decreases gradually from left to right across the period. This is due to the fact that, within a period or family of elements, all electrons are added to the same shell or orbital. However, at the same time, protons are being added to the  nucleus (that is increasing the nuclear charge), making it more positively charged. The effect of increasing proton number exceeds that of the increasing electron number; therefore, there is a greater force of nuclear attraction. This means that the nucleus attracts the electron more strongly, pulling the atoms shell closer to the nucleus. The valence electrons are held closer towards the nucleus of the atom. As a result, the atomic radius decreases.


DOWN THE GROUP: Down a group,  atomic radius increases. The valence electrons occupy higher levels due to the  increasing quantum number (n) (that is increase in number of shell). As a result the valance electrons are further away from the nucleus as the number of shells increases. Electron shielding prevents these outer electrons from being attracted to the nucleus; thus, they are loosely held, and the resulting atomic radius is large.

NOTE: The atomic radius of Na, mg and aluminium are: 0.156, 0.136, 0.125 and their respective ionic radius are: 0.125, 0.095 and 0.050.

From the above, it can be seen that,

1. A cation is smaller than the corresponding parent atom.

REASON:

• The whole outer shell of electrons has been loss

• The nuclear charge can pull the remaining electrons closer to the nucleus because there is now less electron-electron repulsion. As a result, the atom cation becomes smaller than the parent atom. Taking sodium atom as a case study, when sodium looses one electron, the nuclear charge of sodium increases and also the force of attraction between the nucleus and outer electron also increases thereby leading to a contraction in size.

     2. An anion is larger than the corresponding parent atom.

REASON:

• The anion has gained electrons, resulting in a more electron-electron repulsion. This repulsion will help increase the atomic size of the anion.

It is very important for students to know that the knowledge of atomic radius is needed for proper understanding of ionization energy, because ionization energy is inversely proportional to atomic radius (that is as atomic radius increases, ionization energy reduces and vise versa).

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