A-level Chemistry/AQA/Module 1/Bonding

Intermolecular Forces

The bonds which act between a molecules are called intermolecular forces. The word intermolecular means, "between molecules". There are three kinds of intermolecular forces.

Van Der Waals

Temporary Induced Dipole-Dipole Attractions Also known as London forces. These are the weakest of the intermolecular forces.

Dipole-Dipole

Also known as Dispersion forces.

Hydrogen Bonding

This is the strongest type of intermolecular force. This only occurs between a lone pair of electrons on a Nitrogen (N), Oxygen (O) or Fluorine (F)atom and a hydrogen atom that has a strong partial charge(δ+). The electronegative atom pulls electrons away from the hydrogen so that, on the opposite side to the bond, the hydrogen appears almost like an unshielded proton.

Bonds

Ionic

This occurs between oppositely charged ions (atoms or molecules which have gained or lost electrons). An example of an ionically bonded compound is NaCl.

The sodium exists as a positive ion, $Na^{+}$ (positively charged). It loses the electron in its $3s^{1}$ sub-energy level, giving it a stable electron configuration of $1s^{2}2s^{2}2p^{6}$, or $[Ne]$. This electron is donated into the 2p sub-energy level of the chlorine atom, which then forms a chloride ion, $Cl^{-}$. This also now has a stable electron configuration of $1s^{2}2s^{2}2p^{6}$.

The force of attraction (called the "electrostatic force of attraction") between these two ions, caused by their opposite charges, is the ionic bond.

The ions arrange themselves into "giant ionic lattices", which are regular, repeating structures. Ionic compounds are usually white and crystalline in appearance; they have high melting and boiling points, as a lot of energy is required to overcome the electrostatic forces of attraction. They can, however, be disrupted by polar solvents, such as water; they are, therefore, water soluble.

Covalent

The term covalent bond is used to describe the bonds in compounds that result from the sharing of one or more pairs of electrons. This is usually indicated as H-F. The electron pair creates a 'bond' between the two atoms because it attracts the nucleus of each atom and therefore resist the separation of the two atoms.

A co-ordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.

Metallic

Positive ions are arranged in a lattice with a sea of delocalised (free) electrons. The force of attraction between the delocalised electrons and positively charged ions holds the structure together.