I/GCSE Chemistry: Electronegativity and Polarity

Measured in “Pauling Units” “δ”- means PARTIAL or incomplete charge

Permanent dipole: A small charge difference across a bond that results from a difference in the electronegativities of bonded atoms

Polar covalent bond: Has a permanent dipole

Electronegativity depends on 2 things:

• Proportional to proton number of an atom= the larger the atomic number, the more electronegative the atom is → ELECTRONEGATIVITY INCREASES ACROSS A PERIOD
• Proportional to 1÷(atomic radius)²= the more shells of electron an atom has, the less electronegative the atom → ELECTRONEGATIVITY DECREASES DOWN A GROUP

1. PERFECTLY COVALENT BONDING (NON-POLAR):

• Exist usually in molecules of elements e.g. H₂, O₂, Cl₂, S₈, P₄
• E.g. H₂- Both hydrogen nuclei has got “FAIR SHARE” of two electrons- which gives rise to a “UNIFORM (EVEN) CHARGE DENSITY” over bond
• There is equal and opposite forces of attraction on the shared pair
• Each nucleus is fighting for control of shared pair
• So there’s zero electronegativity difference

2. POLAR COVALENT BONDING:

• Occurs when there’s a small difference in electronegativity between two atoms in bond
• Hydrogen= 2.1 Pauling units; Chlorine= 3.0 Pauling units
• Chlorine has more electronegative than hydrogen

• Chlorine has greater attraction of bonding pair than hydrogen
• Bonding pair is closer to chlorine atom than hydrogen atom
• There’s a small charge difference across H – Cl bond
• This charge is called a PERMANENT DIPOLE or POLAR bond
• It’s shown by: a small positive charge on H atom (δ+ ), a small negative charge on Cl atom (δ-)

BOND BETWEEN ATOMS OF 2 DIFFERENT ELEMENTS WITH SMALL DIFFERENCE IN ELECTRONEGATIVITY

• The more electronegative atom will have slightly more than its fair share of bonded electrons
• The result is a POLAR COVALENT BOND

BOND BETWEEN ATOMS OF 2 DIFFERENT ELEMENTS WITH LARGE DIFFERENCE IN ELECTRONEGATIVITY

• The more electronegative atom will effectively have captured both bonding electrons
• The result is an IONIC BOND

The GREATER the difference in electronegativity between bonded atoms, the GREATER the permanent dipole. The more electronegative atom will take the δ- charge.

Therefore,

• The GREATER the DIFFERENCE in between the electronegativities of the  bonding atoms, the GREATER the ionic character of the bond
• The GREATER the SIMILARITY in the electronegativities of the bonding atoms, the GREATER the covalent character of the bond.

That's all!!

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