Chapter 4: Chemical Bonding and Molecular Structure

“A chemical bond is an attractive force that bonds the constituents of a chemical species together.”

So many theories are suggested for chemical bond formation, such as valence shell electron pair repulsion theory, electronic theory, molecular orbital theory and valence bond theory.

The formation of a chemical bond is credited to the tendency of a system to achieve stability. It was noticed that the inertness of noble gasses is the direct result of their completely filled outermost orbitals. Consequently, it was proposed that the elements having a deficiency of electrons in outermost shells are unstable. Thus, atoms combine with one another and finish their separate octets or duplets to achieve the stable configuration of the closest inert gasses. So, this combination may occur either by sharing of electrons. The chemical bond formed as a result of sharing of electrons among atoms is known as a covalent bond.  Also, an ionic bond is formed as a result of sharing of electrons among atoms.

a) Mg

The magnesium atom contains only 2 valence electrons. Thus, the Lewis dot symbols for Mg is

b) Na

The sodium atom contains only 1 valence electron. Thus, the Lewis dot symbol for Na is

c) B

The boron atom contains only 3 valence electrons. Thus, the Lewis dot symbols for B is

d) O

The oxygen atom contains only 6 valence electrons. Thus, the Lewis dot symbol for O is

e) N

The nitrogen atom contains only 5 valence electrons. Thus, the Lewis dot symbol for N is

f) Br

The bromine atom contains only 7 valence electrons. Thus, the Lewis dot symbols for Br is

For S and S^2-

The sulphur atom contains only 6 valence electrons. Thus, the Lewis dot symbol for S is

The bi-negative charge on sulphur indicates that it has gained 2 electrons. So, there are six valance electrons plus two gained electrons.

Thus the Lewis dot symbol is

For Al and Al³⁺

An aluminium atom contains only 3 valence electrons. Thus, the Lewis dot symbols for Al is

The tri-positive charge on aluminium indicates that it has donated 3 electrons.

Thus, the Lewis dot symbol is

For H and H^–

The hydrogen atom contains only 1 valence electron. Thus, the Lewis dot symbol for H is

The single negative charge on hydrogen indicates that it has gained 1 electron. So, one valance electron plus one gained electron.

Thus lewis dot symbol is

H₂S

SiCl₄

BeF₂

HCOOH

Octet rule says, “Atoms can combine either by transfer of valence electrons from one atom to another or by sharing their valence electrons in order to achieve the nearest inert gas configuration by having an octet in their valence shell.”

Octet rule explains chemical bond formation depending upon the nature of the element.

Limitations:

(a) Octet rule fails to predict the relative stability and shape of the molecules.

(b) It is based on the inert nature of noble gases. But, some inert gases, say krypton(Kr) and xenon(Xe), form compounds like KrF₂, XeF₂ etc.

(c) For elements beyond the 3^rd period, the octet rule cannot be applied. Elements present beyond the 3^rd period have more than 8 valence electrons surrounding the central atom. E.g. SF₆, PF₆ etc.

(d) For atoms in a molecule having an odd number of electrons, the octet rule is not applied. E.g. For No₂ and NO octet rule is not applicable.

(e) If a compound has less than 8 electrons surrounding the central atom, then the octet rule cannot be applied to that compound. E.g. BeH₂, AlCl₃, LiCl etc., does not obey the octet rule.

The formation of an ionic bond takes place through the transfer of 1 or more electrons from one atom to another. Thus, ionic bond formation depends on the flexibility of neutral atoms to lose or gain electrons. The formation of an ionic bond also depends on the lattice energy of the compound which is formed.

The factors that are favourable for ionic bond formation:

(a) High electron affinity of atoms of non-metals.

(b) The high lattice energy of the compound which is formed.

(c) Low ionization enthalpy of an atom of metal.

BeCl₂

The central atom does not have any lone pair but has 2 bond pairs. Thus, its shape is AB_2, i.e., linear shape.

BCl₃

The central atom does not have any lone pair but has 3 bond pairs. Thus, its shape is AB_3, i.e., trigonal planar.

SiCl₄

The central atom does not have any lone pair but has 4 bond pairs. Thus, its shape is AB_4,  i.e., tetrahedral.

AsF₅

The central atom does not have any lone pair but has 5 bond pairs. Thus, its shape is AB₅, i.e., trigonal bipyramidal.

H₂S

The central atom has 1 lone pair and has 2 bond pairs. Thus, its shape is AB₂E, i.e., bent shape.

PH₃

The central atom has 1 lone pair and has 3 bond pairs. Thus, its shape is AB₃E, i.e.,trigonalbipyramidal.

The geometry of H₂O and NH₃:

The central atom(N) in ammonia has 1 lone pair and has 3 bond pairs.

The central atom(O) in water has 2 lone pairs and has 2 bond pairs.

Thus, these 2 lone pairs on O- atom in the water molecule repels the 2 bond pairs. And this repulsion between the lone pair and the bond pair on O- atom of H₂O is stronger than the repulsion between the lone pair and bond pair on the N-atom of NH₃.

Thus, the bond angle in H₂O is less than NH₃, even though they have a distorted tetrahedral structure.

The extent of bonding which occurs between two atoms while forming a molecule is represented by bond strength. As the bond strength increases, the bond becomes stronger, and the bond order increases.

“Bond length is defined as the equilibrium distance between the nuclei of 2 bonded atoms in a molecule.”