The electronic configuration of noble gases is shown below;

Helium (He): Atomic number=2, Electronic configuration=2

Neon (Ne): Atomic number=10, Electronic configuration=2.8

Argon (Ar): Atomic number=18, Electronic configuration=2.8.8

  The noble gases are very un-reactive because each has a completely filled outermost shell of 8 electrons except Helium which has 2.

  Other elements undergo chemical bonding because they don’t have completely filled outermost shell and are therefore very unstable. They combine with other elements to have a completely filled outermost shell.

  This is done by loss, gain or sharing of electrons. Metals usually give out electrons to non-metals. Non-metals can receive or share electrons. The force of attraction between the elements is called a chemical bond. There are three types of bonds:

• Ionic or electrovalent bond
• Covalent bond
• Metallic bond

Recall:

An ion is a charged particle. There are two types of ions.

• Cations: They are positively charged and are formed when metals lose one or more electrons from their outermost shell.
• Anions: They are negative and are formed when atoms of non-metals gain one or more electrons in their valence shell.

TYPES OF BONDS

1. Ionic or Electrovalent Bond

It occurs between a metal and a non-metal. The metal completely transfers one or more electrons to the non-metal. The metal becomes very charged. The positive and negative charges attract each other. This force of attraction between the two charges forms the ionic bond. Eg: NaCl, MgCl₂.

Properties of Ionic Compounds

• Crystalline: They are crystalline solids with high melting and boiling point because of strong electrostatic force existing between them.
• They do not conduct electricity in the solid state because the ions are held together by strong electrostatic force.
• When in molten state or in solution, they conduct electricity because the ions are free to move.
• They are usually very soluble in polar solvents. Eg: Benzene

2. Covalent Bonding

It occurs between atoms of non-metals. It involves the sharing of electrons, each atom will donate an electron and the electron pair is shared between the two atoms.

Dative or Coordinate Covalent Bonding

This is a special type of covalent bonding. It involves the sharing of electrons, but the electron pair shared is donated by only one of the atoms. The atom that donates the electron pair is called the donor atom while the atom that accepts the electron pair is called the acceptor atom. Eg: Ammonium ion (NH₄)

Properties of Covalent Compounds

• They usually have low melting and boiling points. This is because of weak Vander Waals forces between them.
• They are made up of molecules.
• They do not conduct electricity both in molten or solid state because they are made up of molecules.
•  They are insoluble in polar solvents but soluble in non-polar solvents because they are non-polar (made up of molecules). Eg: Benzene, Cl₂.
• The compounds formed are volatile liquids or gases.

Differences between Ionic Bonding and Covalent Bonding

Shapes of Some Simple Molecules

Some covalent compounds have shapes because the covalent bond is always between particular atoms and electrons pairs. This causes molecules to be far apart at possible. Examples of shapes are:

6. Tetrahedral
6. Pyramidal
6. V-shape
6. Linear

Some molecules and their shapes are:

• Methane (CH₄)

Methane has a tetrahedral shape with a bond angle of 109.5°. This is due to repulsion of equal strength. Other examples with the same shape are Carbon tetrachloride (CCl₄), Ammonium ion (NH4+).

• Carbon dioxide

It has a linear shape with a bond angle of 180°.

Other examples are: H-H, H-Cl, and Nitrogen.

• Ammonia (NH₃)

It is having a pyramidal shape with a bond angle of 107° or 107.5°.

• Water (H₂O)

It has a V-shape with a bond angle of 104.5. This reduced angle is due o repulsion of the lone pair of electrons on the oxygen atom.