Electronegativity and Polarity (OCR A-Level Chemistry A): Revision Notes

Electronegativity and Polarity

What is electronegativity?

When atoms form covalent bonds, they share electrons between them. In the simplest molecules made from just one element (like $\text{H}_2$, $\text{O}_2$, $\text{N}_2$, or $\text{Cl}_2$), both atoms are identical, so the bonding electrons are shared equally between them. Both atomic nuclei attract the electron pair with the same strength.

However, when the bonded atoms are different elements, the situation changes. The atoms now have different properties:

• Different nuclear charges (different number of protons)
• Different atomic sizes
• Different abilities to attract electrons

This means the shared electron pair may be closer to one nucleus than the other. One atom attracts the bonding electrons more strongly than its partner.

Measuring electronegativity: the Pauling scale

Electronegativity is measured using the Pauling scale, which assigns numerical values to different elements. These values allow us to compare how strongly different atoms attract bonding electrons.

Pauling electronegativity values

Here are some important Pauling electronegativity values you should know:

Element	Pauling Value
Hydrogen (H)	2.1
Lithium (Li)	1.0
Beryllium (Be)	1.5
Boron (B)	2.0
Carbon (C)	2.5
Nitrogen (N)	3.0
Oxygen (O)	3.5
Fluorine (F)	4.0
Sodium (Na)	0.9
Magnesium (Mg)	1.2
Aluminium (Al)	1.5
Silicon (Si)	1.8
Phosphorus (P)	2.1
Sulfur (S)	2.5
Chlorine (Cl)	3.0
Potassium (K)	0.8
Bromine (Br)	2.8

Trends in electronegativity

Most and least electronegative elements

Classifying bonds by electronegativity difference

The type of chemical bond formed between two atoms depends on the difference in their electronegativity values. When the electronegativity difference is large, one atom has a much greater pull on the electrons than the other, and the more electronegative atom gains control of the electrons.

Bond polarity

Non-polar bonds

A non-polar bond is one where the bonding electron pair is shared equally between the atoms. This happens when:

• The bonded atoms are the same element, OR
• The bonded atoms have the same or very similar electronegativity values

For example, in molecules like hydrogen ($\text{H}_2$) and chlorine ($\text{Cl}_2$), both atoms are identical, so the electron pair is attracted equally to each nucleus. The bond is perfectly non-polar.

Polar bonds

A polar bond is one where the bonding electron pair is shared unequally between the atoms. This happens when the bonded atoms are different elements with different electronegativity values, creating a polar covalent bond.

Partial charges

When a bond is polar, the unequal sharing of electrons creates small electrical charges on the atoms:

• The more electronegative atom gains a greater share of the electrons and develops a partial negative charge, written as $\delta^-$
• The less electronegative atom has a reduced share of the electrons and develops a partial positive charge, written as $\delta^+$

Example: Hydrogen chloride

Hydrogen chloride (HCl) is a classic example of a polar molecule:

• From the Pauling scale: hydrogen has electronegativity 2.1, chlorine has electronegativity 3.0
• Chlorine is more electronegative than hydrogen
• The chlorine atom attracts the bonding electrons more strongly
• This creates a polar H—Cl bond

The H—Cl bond becomes polarised with:

• A partial positive charge ($\delta^+$) on the hydrogen atom
• A partial negative charge ($\delta^-$) on the chlorine atom

Permanent dipoles

This separation of opposite charges along a bond is called a dipole. In polar covalent bonds, this dipole doesn't change - it's always there as long as the molecule exists. We call this a permanent dipole to distinguish it from induced dipoles (which you'll meet in the topic on intermolecular forces).

The hydrogen chloride molecule is polar, with $\delta^+$ and $\delta^-$ charges at different ends of the H—Cl bond.

Molecular polarity

For molecules with just two atoms (like HCl), a polar bond automatically means a polar molecule. But what about molecules with more than two atoms?

When a molecule contains multiple bonds, it may have two or more polar bonds creating multiple dipoles. Whether the overall molecule is polar or non-polar depends on the shape of the molecule and whether these dipoles cancel out or reinforce each other.

Polar molecules: Water

A water molecule ($\text{H}_2\text{O}$) is polar.

Water has:

• Two O—H bonds, each with its own permanent dipole
• The two dipoles point in different directions but don't exactly oppose each other
• The bent shape of the water molecule means the dipoles don't cancel

Non-polar molecules: Carbon dioxide

A carbon dioxide molecule ($\text{CO}_2$) is non-polar, even though it contains polar bonds.

Carbon dioxide has:

• Two C=O bonds, each with its own permanent dipole
• The two dipoles point in exactly opposite directions
• The linear shape of the molecule means the dipoles exactly cancel each other

Polar solvents and ionic solubility

Water is a polar solvent, which makes it excellent at dissolving ionic compounds. When an ionic compound like sodium chloride dissolves in water, the ionic lattice breaks down and the ions become surrounded by water molecules.

The dissolution of sodium chloride can be represented by the equation:

$\text{NaCl(s)} + \text{aq} \rightarrow \text{Na}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$

How water dissolves ionic compounds

The process works through ion-dipole interactions:

1. Water molecules are attracted to the ions at the surface of the ionic lattice
2. The ionic lattice structure breaks down as ions are pulled away
3. In the resulting solution, water molecules surround each ion

The ability of polar solvents to dissolve ionic compounds is one reason why water is such an important solvent in chemistry and in biological systems.

Remember!