The electron configurations of potassium and chlorine atoms are;

Note: It is common for chemist to use the noble gas symbol to represent the electron configuration of the inner energy shell of an atom.

To react, the potassium and chlorine atoms must collide with each other, and the impact must have the required energy for the atoms to interact.

When this happen the electron in the potassium atom would move into the vacancy in the orbital of the chlorine atom so that both can have completely electron filled outer shell. That is:

Chemist represent this by   :    K + Cl ® K⁺ + Cl ‾

This is a make believe reaction to clarify the principles for reaction. This is never a way to prepare potassium chloride.

Potassium chloride is a salt or an ionic molecule. The atoms are "bonded" to each other by electrostatic attraction known as ionic bond. (The only salt familiar to non-chemist is sodium chloride, use for cooking.)

COVALENT BONDING

When two atoms of fluorine collide it form fluorine gas. The electron configuration of the fluorine atom is

In this case there is no possibility of giving or taking of electrons. The only solution is for both to share the covalent electrons so that they can both achieve a complete outer shell of electron. The bond formed is known as a covalent bond. The classical way of representing this concept graphically (not chemically) is the Lewis picture.

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Lewis representation just show two features in covalent bonding.

• The two atoms share the electrons drawn between them; and
• Each of the atom has a complete outer shell of electrons.

In the true meaning of the word, only atoms joined by covalent bonds should be called molecules. Because it is only in such molecules are the atoms permanently attached to each other and be separated only by chemical reactions or bombarding with radiation.

The ionic bonding is not permanent in this sense. In the solid form (it normally exist as crystals with definite shape) the positively charged atom (cation) is surrounded on all sides by the negatively charged atoms (anions), which are in turn is surrounded by positively charged atoms. They are arranged in such a way that the charges are balanced at any one location. But we can never say which cation is paired to which specific anion, even though the proportion of the two ions is in a definite ratio. In sodium chloride it will be 1:1, while in beryllium chloride it will be 1:2.

Nevertheless, chemists continued to use the term molecule to mean the smallest unit of any matter - no matter how the atoms are bonded - since they represented a unique entity and have definite properties.

By now you might have noticed that most of the complete outer shells so far has eight electrons. So we sometimes refer to the tendency to form a complete outer shell of eight electrons as the octet rule.

Tutorial 3

Can you explain the following terms: Ionic bond, Salt, Valence electron, and Covalent bond.