Chemical Bonding 1

How are compounds formed?

formation of compounds

Most elements form compounds.

For example: A reaction between sodium and chlorine gives the compound sodium chloride (salt) quite readily.

The noble gases do not usually form compounds. They are different from other elements, since their atoms are described as stable or unreactive. They are stable because their outer electron shell is full. A full outer shell makes an atom more stable.

Only the noble gases have full outer shells. This is why they are stable.

Other elements react with each other in order to obtain full outer shells, this makes them more stable.

How atoms lose and gain electrons

Depending on their electronic configurations, atoms lose or gain electrons in order to achieve a full outer shell.

Losing electrons

The sodium atom has one electron in its outer shell. If it loses this one electron it will achieve a full outer shell. By losing the one electron to another atom, it becomes a sodium ion.

The sodium ion still has 11 protons but by losing one electron it has only 10 electrons compared to the atom. Hence, its overall charge is +1.

This +1 charge is due to the ion having one more proton than electron.

In naming ions, you take the symbol Na and assign a positive charge. This gives us the sodium ion Na⁺.

Gaining electrons

A chlorine atom has seven electrons in its outer shell. It can reach a full outer shell by gaining one electron. It will then become the chloride ion, Cl-.

A negative charge is assigned to the ion to signify that the ion contains one more electron than proton.

Ions

Any atom can become an ion if it gains or loses electrons.

An ion is a charged particle. It is charged due to an unequal number of electrons and protons.

2. IONIC BONDING

Remembering that elements gain or lose electrons, when forming compounds,so that they achieve a full outer shell - let's now look at the reaction between sodium and chlorine.

Example 1: Reaction between sodium and chlorine

A sodium atom loses one electron to achieve a full outer shell and chlorine gains one electron to complete a full outer shell. So when a sodium atom reacts with a chlorine atom, the sodium atom loses its one electron to chlorine. The two ions formed are a sodium ion, Na⁺ and a chlorine ion Cl^-.

The two ions have opposite charges, they attract one another.

The force of attraction between them is an electrostatic one. This type of attraction is strong. It is called an ionic or electrovalent bond.

Example 2: Reaction between magnesium and oxygen

Other metals and non-metals react together to form ionic compounds This is because metals tend to lose electrons, whereas non-metals tend to gain electrons.

A magnesium atom has two electrons in its outer shell, whereas oxygen has six electrons. This means that magnesium wants to lose two (to oxygen) and oxygen wants to gain two (from magnesium) so that they can have full outer shells.

The ions attract each other due to their opposite charges. Magnesium ions and oxide ions are formed. The product is magnesium oxide, MgO.

Example 3: Reaction between magnesium and chlorine

To obtain full outer shells magnesium must lose two electrons and chlorine must gain one electron. So when we react magnesium in chlorine, one magnesium atom reacts with two chlorine atoms to form magnesium chloride, MgCl₂.