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TYPES OF CHEMICAL BONDS
The attractive force between atoms when they combine chemically is called a chemical bond. There are two main types of chemical bonds namely (i) Strong bonds (ii) Weak bonds.
(i) Strong bonds are: (a). Electrovalent (or Ionic) (b).Covalent (c). Co-ordinate (or Dative) (d). Metallic
(a) Electrovalent (Ionic) bond is defined as the electrostatic force of attraction between oppositely charged ions. It involves the transfer of electrons from one atom, donor atom, (usually metallic) to another atom, acceptor atom, (usually non-metallic). The electrons involved reside in the outermost shells of the atoms and are called Valence electron(s).
IONIC COMPOUNDS are crystal lattices consisting of aggregates of oppositely charged ions. Examples of such compounds are Sodium chlorides, calcium oxides, ammonium chlorides etc. The formation of sodium chloride is illustrated below:
Na = 2, 8, 1 Cl =2, 8, 7
Na+= 2, 8 Cl– = 2, 8, 8
Na+ + Cl– → NaCl
Characteristics of Electrovalent bonds
- They have high melting and boiling points
- They are generally soluble in water
- They are good conductors of electricity when molten or in solution
- They do not conduct electricity when solid
- The energy needed to separate them is relatively high.
(b) Covalent bonds: This is defined as the bonds formed when two atoms donate equal numbers of electrons and share the donated electrons to attain stable octet structure. In covalent bonding, electrons are shared between atoms of the same or different elements such that each atom contributes the shared electrons so as to attain stable noble gas configuration. During the process, discrete or separate molecules are formed with covalent bonds between the atoms.
If electrons are shared between similar atoms, the donated electrons are equally shared which give rise to a non-polar covalent bond e.g. F2, H2, O2, Cl2 but if the shared pair of electrons involves two different elements with difference in electronegativity, the electrons are then not equally shared, this results in the formation of polar covalent bond, e.g. the formation of HCl, H2O, HF..
Characteristics of Covalent bonds
- Low melting and boiling points.
- Energy required for separation is low
- Do not conduct electricity in the solid or molten state, or in solution
- They generally have a strong, easily noticeable smell
- They are not easily soluble in water, but are usually soluble in organic solvents
(c) Co-ordinate covalent or Dative bond involves sharing of electrons as in the normal covalent bonding, but the shared pair is donated by only one of the participating atoms. For instance, Ammonia and water molecules possess lone pairs and so readily enter into coordinate covalent bonding.
(d) Metallic bond: The electrostatic force of attraction between the positive nuclei and the sea of mobile electrons is called metallic bond. Metallic bonding, therefore, is the process whereby the positively charged nuclei of metal atoms are simultaneously attracted to the sea (or cloud) of mobile electrons. Metallic bond increase with increase in valence electrons of the metal. For example, in period 3, metallic bond increases from sodium to aluminium.
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