CONTENTS

(a) Types of chemical bonds

(b) IUPAC system of naming inorganic compounds.

PERIOD 1: PERIODIC TABLE (FIRST 20 ELEMENTS)

Chemical combination: The atoms of noble gases e.g., Helium He, Neon Ne are very stable; because their outermost shells are filled with electrons. The other elements tend to attain this stable structure possessed by the noble gases. This is achieved during chemical combination. Thus, chemical bonding is the coming together of atoms of the same or different elements to form a stable structure.                        

 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

1.  They have high melting and boiling points
2. They are generally soluble in water
3. They are good conductors of electricity when molten or in solution
4. They do not conduct electricity when solid
5. 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 bonde.g. F2, H2, O2, Cl2but 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

1. Low melting and boiling points.
2. Energy required for separation is low

iii. Do not conduct electricity in the solid or molten state, or in solution

1. They generally have a strong, easily noticeable smell
2. 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 aluminum. 

(ii)Weak bonds: These are intermolecular forces of attraction that hold atoms and covalent molecules together in gases, liquids and solids. The most common ones are: (a). Van der Waals forces (b). Hydrogen bond (c) Dipole-Dipole (Dipolar)

(e)Van der Waals forces: They were first described by J.D. van der Waals, and are known as van der Waals forces. They are weak short-ranged attractive forces formed between covalent molecules. They are the only attractive forces between the atoms of the noble gases and non-polar covalent molecules, and are responsible for the low melting and boiling point of covalent compounds. Due to increase in van der Waals forces, there is gradation in the physical properties of the Halogens: Fluorine and chlorine are gases; bromine is a liquid; and iodine is a solid.

Van der Waal force becomes stronger as the relative atomic mass increases among the non-metals. 

(f) Hydrogen bond: This is an intermolecular force which arises when hydrogen is covalently linked to highly electronegative elements like nitrogen, oxygen and fluorine. 

The presence of hydrogen bonds between H2O molecules is responsible for water being a liquid at room temperature and with a high boiling point; if not, it would have been a gas, like hydrogen H2S. HF is a liquid at room temperature, while HCl is a gas.

EVALUATION

What is a chemical bond?

List three (3) general properties of electrovalent compounds.

Define covalent bond and state its characteristics

State two (2) differences between covalent and electrovalent compounds.

Explain the term Metallic bonding