CHEMICAL BONDING

CHM 102 LECTURE NOTE. CHEMICAL BONDING

What is chemical bonding? Atoms rarely occur in free state at ordinary temperatures, except the inert noble gases. The attraction between atoms, forming a cluster is called a CHEMICAL BOND. Thus, a chemical bond is defined as the attractive force that holds two or more atoms together in a molecule of ion.

Why do atoms combine?

1)Net attractive force: when two atoms come close to one another, the attractive and repulsive forces begin to take effect. Attractive forces are between electrons and nuclei (unlike charges) and repulsive forces are between both sets of electrons or between both sets of nuclei. These forces counteract each other. A chemical bond occurs when THE NET ATTRACTIVE FORCE IS GREATER THAN THE NET REPULSIVE FORCE.

2)OCTET RULE OF VALENCY: Lewis, Kossel, and Longmvir (1916) used the rule of octet valency to explain why chemical bonding occurs. Atoms of noble gases are chemically unreactive, and it is reasonable to assume that the outermost shell configuration of the noble gases is a stable one. This configuration is of eight (8) electrons called VALENCE ELECTRONS.

One can reasonably assume that when atoms of other elements combine, the electrons in their outermost shells are arranged between themselves in such a way that they achieve an octet of electrons, which is stable. This is known as OCTET RULE or RULE OF EIGHT.

Note however, that a Helium atom has 2 valence electrons. This is RULE OF TWO. Here, two atoms provide stability, as the first shell can only contain a maximum of two electrons. The main points of this theory is summarized as follows:

a) Atoms with 8 electrons in their outermost shell are chemically stable.

b) Atoms having less than 8 electrons In their outermost shell are chemically reactive. Atoms with less than 4 have a tendency to loose them to become cations. Atoms with more than 4 have a tendency to gain electrons to become anions.

c) Atoms combine chemically as a result of electron transfer, or by electron sharing. The tendency of an atom to transfer or share electrons is a measure of its CHEMICAL ACTIVITY.

3) LOWER ENERGY: When two atoms combine to form a bond, there is an overall decrease in their potential energy. A system of bonded atoms having lower energy levels is more stable than that of single atoms with high energy levels. This principle of stability by lower energy levels is well illustrated, during Nuclear reactions.

TYPES OF BONDS There are different chemical bonds. They are:

1) IONIC BONDING: Occurs as a result if transfer of electrons. This bond is also called POLAR bond. One of the participating atoms has an excess of electrons, which are transferred in order to achieve the octet structure of the nearest noble gas.

Characteristics that favour Ionic bond formation:

a) Number of valence electrons: The atom transformed into a cation should have 1,2 or 3 valence electrons. (Grp IA IIA IIIA). The atom transformed into an anion should have 5,6 or 7 valence electrons. (Grp VA VIA VIIA).

b) Ionization energy of the metal should be low: Ionisation energy is the minimum energy required to remove a valence electron from the outermost shell of a gaseous atom. Low ionisation energies will favour electron transfer.

c) Electron affinity energy of the nonmetal should be high: Electron affinity is the energy required to add a valence electron to the outermost shell of a gaseous atom. High Electron affinity energies will favour electron transfer.

d) High Lattice energies: Lattice energy is the energy released when one mole of a molecular crystal is formed from its ionic components. It is also the minimum energy to slit one mole crystal into its components. Higher lattice energies, make the ionic compound more stable.

e) Electronegativity difference: A and B will form an ionic bond, only if the electronegativity difference between them is high. (Greater than 2)

2) COVALENT BONDING: Occurs as a result of sharing electrons. The shared pair of electrons, are donated by each atom participating. When only one atom, donates the shared pair of electrons, then the bond is called COORDINATE COVALENT BONDING.

....lecture note is being updated.