Intermolecular forces (4) Solid structures of simple molecular lattices: Iodine and Ice

OCR A level H432 Chemistry A (from 2017)

Learning objectives

2.2.2 Intermolecular forces

(n) explanation of the solid structures of simple molecular lattices, as covalently bonded molecules attracted by intermolecular forces, e.g. I2, ice 


(o) explanation of the effect of structure and bonding on the physical properties of covalent compounds with simple molecular lattice structures including melting and boiling points, solubility and electrical conductivity. 


Solid structures of simple molecular lattices

Simple molecular lattices are very common especially in organic chemistry.

Think of sugar molecules, alkane molecules or alcohol molecules: when they freeze they exist in simple molecular lattices.

Two examples that come from inorganic chemistry are Iodine and ice.

Iodine I₂(s)

Iodine is a grey, shiny solid at room temperature and pressure.

With seven electrons in its outer electron energy level, an atom of iodine is covalently bonded to another to form a diatomic molecule I_2.

Such an arrangement means that each iodine atom possesses a full outer energy level of eight electrons.

The bond between these two atoms is a σ bond where the electron density sits along the axis joining the two iodine atoms.

The bond is formed by the end on overlap of two atomic orbitals gving a molecular orbital that is complete with two electrons.

The picture to the left  is an attempt at a representation of this covalent bond.

A weak induced dipole—induced dipole force holds the iodine molecules in the molecular lattice since both atoms have the same electronegativity and so do not possess permanent bond polarity.

The weak intermolecular force is called by its generic name in the diagram above as a van der Waals force.

The final picture attempts to show the arrangement of iodine molecules in the crystal lattice as they are close packed together.

Ice H₂O(s)

Ice is a hard, colourless solid at temperatures below 0^oC or +273K.

It is quite brittle and breaks easily to produce sharp shards that can cut through skin.

Ice is composed of two elements of significantly different electronegativity:  oxygen (3.44) and hydrogen (2.20. 

So ice molecules are polar covalent molecules.

The covalent σ bonds that exist between oxygen and hydrogen are polar with the oxygen atom pulling the bonding electron pairs towards itself. 

Because of the covalent bond polarity molecules hydrogen bonds are found between molecules of water in ice.

There are two hydrogen bonds per water molecules that mean the structure of ice is very similar to that of diamond.

Compare the two structures here:

                                                         Diamond   

                                                                Ice

Two hydrogen bonds per molecule are possible because the molecule of water has two lone pairs on the oxygen atom.

The intermolecular forces in ice are permanent dipole forces and the much weaker induced dipole forces that we find exist between all molecules at some level. 

Substance	Structure	Bonding	Solubility in water	Mp and bp /oC	Electrical conductivity
Iodine	Diatomic molecular Lattice	Covalent	Partially soluble	387K 457K	no
Ice	Simple molecular lattice	Polar covalent	Fully miscible	273K 373K	yes