ΓΌ  Electronegativity: The tendency of an atom in a molecule to attract the shared pair of electrons towards itself is termed as its electronegativity. It is not a measurable quantity and so it has no unit. There are different scales for measuring the Electronegativity of elements. The most commonly used is the Pauling Electronegativity scale developed by Linus Pauling.
Ø  In period- The electro-negativity increases from left to right in a period.
Ø  In group- The electro-negativity decreases from top to bottom in a group.




                        Difference between Electronegativity and Electron Gain Enthalpy



Diagonal relationship: Some elements of second period show similarities with elements of third period placed diagonally to each other due to same charge / radius ratio.

Anomalous Behaviour of first element of every group is due to
1.  Small size and high electronegativity: N can form p Ο€ – p Ο€ multiple bonds whereas P cannot.
2. High IE:  They form only covalent compounds and not ionic compounds.
3. Absence of vacant d orbitals: N cannot form NCl5 or R3N = O since it cannot expand its covalence beyond 4 whereas P can form PCl5 and R3P = O.

    ΓΌ  Valence Electrons: The electrons present in outermost shell are called as valence electron. Be            cause the electrons in the outermost shell determine the valency of an element.
Valency of an Element: According to the electronic concept of valency, “the number of electrons which an atom loses or gains or shares with other atom to attain the noble gas configuration is termed as its valency.”
            Periodicity:
(i)       In period- The valency first increases then decreases from left to right in a period.
(ii)     In group- The valency remains constant from top to bottom in a group.