Chemical Combination

The force which holds together the ions, atoms and molecules in a chemical substance is called as chemical bond. It is the same force which makes substances combine with each other in many different ways to give newer products.

The combining tendency of atoms to form chemical bonds can be explained in term of the electronic theory of valency, which states that:

• The tendency of an atom to take part in chemical combination is determined by the number of valence electrons (electrons in the outermost shell of an atom).

• The atoms acquire the stable noble gas configuration of having eight electrons in the outermost shell (called octect rule) by mutual sharing or by transfer of one or more electrons.

• The valency (number of electrons an atom loses, gains or mutually shares to attain noble gas configuration) of an element is either equal to the number of valence electrons or equal to 8 minus the number of valence electrons.

Lewis electron dot symbols

In order to represent the structures of the combining elements in the most simplest way in terms of the valence electrons on paper, Lewis proposed the following notation:

• The symbols of the element is written first. This represents the nucleus of the element with all the inner electrons that do not take part in the bond formation.

• The valence electrons are then written as dots or (small cross marks) around the symbol. They are spread in a pair on four sides of the symbol.

• In case of ions the charge is shown with the symbol.

This type of representation is called as Lewis symbols or Lewis structures or electron dot symbol.

For example, the Lewis structure for hydrogen is:

H represents the nucleus of hydrogen atom and the dot () represents the one valence electrons present in the atom of hydrogen.

Similarly the Lewis dot structure of Chlorine can be written as: