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Atomic size. The atomic or ionic radius is the size of the atom or the ion of an element and is usually considered as the distance between the nucleus and the outermost shell of the electrons of the atom.
Trends in atomic sizes in the periodic table. The atomic size of the elements (atoms) decreases from left to right in a period because the nuclear charge increases with the increase in atomic number. Hence the electrons are attracted more and the force of attraction by the nucleus are more. Thus decrease in size.
Atomic radius. may be defined as the distance between the nuclei and the outer most electron present in valence shell. The atomic radii of the elements are generally studied in the form of three radii,
(1) Covalent radius (2) Metallic radius (3) Van der waals radius.
1 – Covalent radius. It may be given as half of the distance between the centers of nuclei of two similar atoms held together by pure covalent single bond. It may be noted that covalent radii are generally shorter than the atomic radii as the covalent bond is formed as a result of the orbital overlap of the bonding atoms and overlap region becomes common to the two atoms.
2 – Metallic radius. Half of the inter nuclear distance between two adjacent atoms in a metallic bond represent the crystal radius or metallic radius. The value of metallic radius is always more than that of the covalent radius as there is no overlap of the atomic orbitals.
3 – Van der waals radius. It may be given as half the inter nuclear distance between two neighboring molecules in the solid state of non-metals.
Ionic radius. The distance from the nucleus of an ion from the point upto which it is influenced by its electron cloud.
Cation is smaller than its parent atom. A cation or a positive ion is formed when an atom loses one or more electrons, the number of protons remaining same in the nucleus. This is due to the fact the ratio of the positive charge (or protons) in the nucleus to the number of electrons increases, hence the electrons are pulled in thereby reducing the size of the atom.
The size of an anion is larger than its parent atom. In the formation of anion (or negative ion), one or more electrons are added to an atom. Therefore the effective nuclear charge decreases. The pull, exercised by the nucleus on the electrons is less. Therefore the electrons move a little further away from the nucleus. Thus the size of anion is larger than its parent atom.