Electronegativity is the tendency of an atom to attract the shared pair of electrons in a chemical bond towards itself.. It is, thus, a measure of the ability of an atom in a molecule to attract electrons.
In general, electronegativity increases from left to right across any period and from bottom to top in any group of the periodic table. The most highly electronegative elements (Fluorine) are found in the upper right corner of the periodic table (ignoring the noble gases.) The least electronegative elements (Caesium) are found in the lower left corner of the table.
When two elements of widely different electronegativities combine, an ionic compound results. The electronegativity values of the non-metals do not differ much. The bonds formed between non-metals are essentially covalent with some polar character. The electronegativity difference gives an indication of the degree of polarity of the covalent bonds. If the difference is zero or very small, an essentially non-polar bond with equal or almost equal sharing of electrons can be assumed. The larger the electronegativity difference, the more polar the covalent bond is. The bond is polarized in the direction of the atom with larger electronegativity.
Atomic radii: The distance between the nucleus and the outermost shell containing the electron cloud is known as atomic radius.
Ionic radii : It is defined as the distance between the nucleus and the point upto which the element has influence over its electron cloud. Generally the cationic radius is smaller than its neutral atom, whereas the anionic radius is larger. An important point to remember here is ions which are having same no. of electrons are called iso electronic species ( eg. S2- K+ Ca2+Cl-).