There are three main types of chemical bonds – covalent, ionic and metallic bonds. Three types of bonding can result from the manner in which the atoms can combine:

• Ionic bond is formed by electron transfer from a metal to a non-metal with different electronegativity values.
• Covalent bond is formed as a result of electron sharing between two non-metals. If the electronegativity values are very similar then it is non-polar covalent bonding but if the electronegativity values are much different, then it is a polar covalent bonding.
• Metallic bond refers to the interaction between the delocalised electrons and the metal nuclei.

Factors affecting formation of ionic bond

There three main factors that affect the formation of affinity and Lattice energy.

1. Ionization energy: As we have seen in chapter 4, ionization energy is the amount of energy required to remove the outermost electron in the atom when the gas atom is isolated in free space. For the formation of ionic bond there must exist one atom which forms a cation by losing one or more electrons. Alkali and alkaline earth metals have greater tendencey to form ionic bond because of ionization energy associated with the higher electron affinity.
2. Electron affinity: In the formation of an ionic compound the atom participating form an anion by gaining an electron(s). Therefore, formation of an ionic bond will be greater in which the elements have higher electron affinity. Halogens have greater tendency to form ionic compounds because of having high electron affinities.
3. Lattic energy: Lattice energy is the amount of energy released when cations and anions are brought together in the crystal lattice to form one made of the ionic compound. The higher the lattic energy, the greater is the tendency of the formation of an ionic bond.

General properties of ionic compounds

Summary of the general properties of ionic compounds:

• Ionic compounds do not contain molecules. They are aggregates of positive ions and negative ions. In the solid state, each ion is surrounded by ions of the opposite charge, producing an orderly array of ions called crystal.
• At room temperature ionic compounds are hard and rigid crystalline solids. This is due to the existence of strong electrostatic forces of attraction between the ions.

• Ionic compounds have relatively high melting and boiling points. This is due to the presence of strong electrostatic forces between the ions. These forces can be overcome only by applying very large amounts of energy.
• Ionic compounds are soluble in polar solvents such as water. They are insoluble in non-polar solvents such as benzene.

Factors affecting formation of covalent bond

Electron Affinity - A covalent bond is generally favoured between the two atoms having high electronic affinity.
Ionization energy - Ionization energy of both the atoms participating in covalent bond formation should be high.
Atomic size - Atomic size of atoms forming covalent bond should be smaller. Smaller the atomic radii, more stronger the covalent bond will be.
Electronegativity - The electronegativities of both the atoms should be high. The difference of electronegativity between the two atoms should be minimum.

General properties of covalent compounds

1. Covalent compounds are generally liquids or gases at ordinary temperature. For example : water and ethyl alcohol are liquids. Hydrogen chloride, methane and cabon dioxide are gases. Some covalent compounds are solids (e.g. sugar)
2. Compared to ionic compounds, covalent compounds have relatively lower melting points and boiling points.
3. They do not conduct electric current when molten or in aqueous solution, because they consist of molecules rather than of ions.

Observations and analysis

Draw general conclusions on the (a) melting points, (b) solubility in polar and non-polar solvents of the covalent compounds given.