Electronegativity:

Electronegativity is the measure of tendency of an atom to attract the bonding pair of electrons. The electronegativity of an atom is a measure of the affinity for electrons.  Electronegativity is tendency of an atom to attract the electrons towards itself in a molecule of the compound. The value of electronegativity of an element describes the ability of its atom to compete for the electrons with the other atom to which it is bonded. The atoms of various elements differ in their affinity for the electrons.  The electronegativity of an element can be given by average of the valence-state ionization potential and. The electronegativity is not the property of an isolated atom.

Non-polar covalent bonds occur when there is equal or near-equal sharing of electrons between the 2 bonded atoms. It needs a difference of electronegativity between 0.0 and 0.4 indicates a non-polar covalent bond. Polar covalent bonds are formed when there is unequal sharing of the electrons between the atoms. The bonds with an electronegativity difference in between 0.5 and 1.7 are considered polar.  Ionic bonds occur when there is complete transfer of the electrons in the bond. The electronegativity differences of around 1.8 or greater than that create ionic bonds. The electronegativity difference is so large that one atom can attract the electrons enough to "take" them from other atom.

Each element has a typical electronegativity ranging from 0 to 4 on the Pauling scale. A strongly electronegative element, such as fluorine, has an electronegativity of 4 while weakly electronegative elements, like lithium, have values close to 1. Electronegativity trends are to higher values for the elements in top right of the periodic table. Bonds between atoms with the large electronegativity difference that is, greater than or equal to 2.0 on the Pauling scale, are usually considered to be ionic, while values between 2.0 and 0.4 are considered polar covalent. Values below 0.4 are considered non-polar covalent bonds. Electronegativity increases across a period as the number of charges on the nucleus increases. That attracts bonding pair of the electrons much strongly. The electronegativity decreases as we move down the group.