Periodicity :
( i ) In period- The electro-negativity increases from left to right in a period .
( ii ) In group- The electro-negativity decreases from top to bottom in a group .
VALENCE ELECTRONS : The electrons present in outermost shell are called as valence electron . Because the electrons in the outermost shell determine the valency of an element .
VALENCY OF AN ELEMENT : The number of hydrogen or halogen atom or double the number of oxygen atom , which combin with one atom of the element is taken as its valency . 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 .
ELECTROPOSITIVE OR METALLIC CHARACTER : The tendency of an element to lose electrons and forms positive ions ( cations ) is called electropositive or metallic character . The elements having lower ionisation energies have higher tendency to lose electrons , thus they are electropositive or metallic in their behaviour .
Alkali metals are the most highly electropositive elements .
Periodicity : In period- The electropositive or metallic characters decreases from left to right in a period .