Bonding, Molecular Shape & Structure

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Electronegativity is defined as the ability of an atom in a molecule to attract electrons to itself

Prof. Linus Pauling

Nobel Prize for Chemistry 1954

Nobel Prize for Peace 1962

Electronegativity is a function of two properties of isolated atoms;

The atom’s ionization energy (how strongly an atom holds onto its own electrons)

The atom’s electron affinity (how strongly the atom attracts other electrons)

For example, an element which has:

A large (negative) electron affinity

A high ionization (always endothermic, or positive for neutral atoms)

Will: Attract electrons from other atoms and Resist having electrons attracted away

Such atoms will be highly electronegative

Slide 18

Pauling scale of electronegativity;

Fluorine is the most electronegative element followed by O and N, Cl are equal third. Cs is least.

Electronegativity increases from left to right along the Periodic Table.

For the representative elements (s & p block), the electronegativity decreases as you go down a group.

No trend in the transition metals.

Slide 19

Slide 20

Slide 21

Electronegativity is dictated by

The number of protons in the nucleus

across a period you are increasing the number of protons, but filling electrons in the same Bohr quantized energy level. You are only filling sub-shells, so electronegativity increases from left to right

The distance from the nucleus

down groups, you are placing electrons into new quantized energy levels, so moving further away from the attractive power of the nucleus. Outer shell becomes further away from the nucleus.

The amount of screening by the inner electrons

level of screening upon bonding electrons increases down groups, and adds to the reduction in electronegativity. Screening is caused by repulsion of electrons for each other.

In hydrogen atom, energy of orbital depends on the principle quantum number, n.

But in many electron atoms, electron-repulsions cause different sub-shells to have different energies,

Sub-shell energy increases (with increasing l)

s < p < d

Slide 22

The three major types of intramolecular bond can be described by the electronegativity difference:

Non-Polar Covalent – Bonds which occur between atoms with little or no electronegativity difference (less than 0.5).

Polar Covalent – Bonds which occur between atoms with a definite electronegativity difference (between 0.5 and 2.0).

Ionic – Bonds which occur between atoms with a large electronegativity difference (2.0 or greater), where electron transfer can occur.