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.