ELECTROVALENT BONDING

Atoms chemically bond with other atoms in order to achieve a stable configuration of their electrons. The group of elements called the noble gases were once referred to as the inert gases because none of them are readily chemically reactive. The electronic structure of each member of the group is remarkably stable. During chemical bonding, atoms acquire the electronic structure of one of the noble gases. There are two main ways in which atoms bond with each other

• Electrovalent bonding and

• Covalent bonding

Electrovalent bonding involves the transfer of electrons. The example shown below illustrates how a sodium atom and a chloride atom form an electrovalent bond between them, producing the salt, sodium chloride.

Sodium, a metal, loses an electron. You should note that the nucleus (the number of protons and neutrons) remains unchanged. The new structure still represents sodium but it is a sodium ion and carries one positive charge.

It is also important to note that the sodium ion has the electronic structure of the noble gas, neon.

Chlorine, a non-metal, gains an electron. Again, the nucleus remains unchanged; a chloride ion has been formed that carries one negative charge. The chloride ion has the electronic structure of the noble gas, argon.

Sodium belongs to vertical group 1 of the periodic table, a group known as the alkali metals. Other members of this group include lithium and potassium

Lithium atom	Potassium atom

When these atoms are involved in an electrovalent bond they also lose one electron. It is easy to see why these metals are grouped together. All of the members of group 1 need only lose one electron to gain a stable electronic structure. This also means that these metals tend to react with non-metals in the same way.

The more complex the atom, the more reactive it is. The larger atoms, such as potassium and rubidium, tend to lose the single, outer electron more easily since it is further away from the influence of the positively charged nucleus. The easier it is to lose the outer electron then the faster the reaction.

As you might expect, the vertical group 2 of the periodic table contains atoms with two electrons in their outer shell. Each of these atoms needs to lose 2 electrons to gain the electronic structure of a noble gas. The metals in this group are called alkali earth metals. Calcium and magnesium are members of the alkali earth group. The diagram below shows the compound formed when calcium electrovalently bonds with oxygen.

Calcium oxide

When calcium bonds with chlorine, two chlorine atoms are needed to take up the two electrons from the calcium atom’s outer shell.

Calcium chloride

This is the reason that calcium chloride has the chemical formula CaCl₂

During electrovalent bonding, non-metals, such as chlorine (vertical group VII) and oxygen (vertical group VI), add to their outer shell the number of electrons needed to gain a stable electronic structure.
 

Chlorine atom	Chloride ion
Oxygen atom	oxide ion

It is rare for group 4 atoms, such as carbon and silicon, to form electrovalent bonds. These atoms are usually involved in covalent bonding that does not involve the formation of ions.

Electrovalent bonding forms compounds. The description “molecule” is never used with electrovalent bonding.

Valency or valency electrons

In electrovalent compounds the valency is (a) the number of electrons lost to form the ion (to give positive ions or cations) or (b) the number of electrons gained to form the ion (to give negative ions or anions).

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