COVALENT COMPOUNDS

Noadswood Science, 2012

Wednesday, April 19, 2023

To understand the properties of covalent compounds

Organic

Many covalent substances come from living materials – organic in this sense means anything containing carbon (C)

Organic compounds tend to have covalent bonding…

Covalent Bonding

A covalent bond forms when two non-metal atoms share a pair of electrons – the electrons involved are in the highest occupied energy levels (outer shells) of the atoms

An atom that shares one or more of its electrons will complete its highest occupied energy level

Covalent bonds are strong – a lot of energy is needed to break them

Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water (as they have weak forces of attraction between the molecules)

Hydrogen Chloride

After bonding, the chlorine atom is now in contact with eight electrons in its highest energy level causing it to become stable

The hydrogen atom is now in contact with two electrons in its highest energy level resulting in the hydrogen also being stable

Bonds

Atoms may form multiple covalent bonds – that is, share not just one pair of electrons but two or more pairs

Atoms of different elements will form either one, two, three or four covalent bonds with other atoms – there is a quick way to work out how many covalent bonds an element will form…

The number of covalent bonds is equal to eight minus the group number

Group 4 Group 5 Group 6 Group 7

Example Carbon Nitrogen Oxygen Chlorine

Number of bonds

8 – 4 = 4 8 – 5 = 3 8 – 6 = 2 8 – 7 = 1

Models

Element FormulaChemical Structure

Ball & Stick Model

Hydrogen H2

Water H2O

Ammonia NH3

Methane CH4

Covalent Compounds

Covalent bonds form between non-metal atoms

Each bond consists of a shared pair of electrons and is very strong – covalently bonded substances fall into two main types: -

Simple molecules

Giant covalent structures

Simple Molecules

These contain only a few atoms held together by strong covalent bonds

An example is carbon dioxide (CO2), the molecules of which contain one atom of carbon bonded with two atoms of oxygen

OO COOH

O

H

Oxygen Water Carbon dioxide

Simple Properties

Simple molecular substances have the following properties: -

Low melting and boiling points – this is because the weak intermolecular forces break down easily

Non-conductive – substances with a simple molecular structure do not conduct electricity because they do not have any free electrons or an overall electric charge

Solids are usually soft and brittle, shattering when hit, and insoluble in water (but soluble in other solvents, e.g. petrol)

Simple molecular substances are gases, liquids or solids with low melting and boiling points

Intermolecular Forces

Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds

All have very strong bonds between the atoms, but much weaker forces holding the molecules together – when one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds

Atomic bonding

Giant Covalent

In some substances, millions of atoms join together by covalent bonding – this produces giant covalent structures, not molecules

All the bonds are covalent, which means that giant covalent structures have a very high melting and boiling point

This also means that almost all giant covalent structure are hard but brittle

Giant Covalent

Giant covalent structures contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds

The atoms are usually arranged into giant regular lattices – extremely strong structures because of the many bonds involved

Giant covalent structures can vary, for example the molecular structure of carbon can have two allotropes – diamond and graphite (allotropes are structurally different forms of an element)…

Graphite (left) and diamond (right) – allotropes of carbon

Giant Covalent Properties

Giant covalent structures have the following properties: -

Very high melting points – substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken (graphite, for example, has a melting point of more than 3,600ºC)

Variable conductivity – diamond does no conduct electricity whilst graphite contains free electrons so it does conduct electricity and silicon is semi-conductive (midway between non-conductive and conductive

Many are very hard

They are chemically un-reactive

Covalent Differences

Covalent Compounds Summary Covalent compounds usually exist as small molecules Many organic compounds have covalent bonds Covalent compounds have low melting points and boiling points

(very little energy is needed to break the forces of attraction between the molecules (even though the covalent bonds are extremely strong))

Covalent compounds are usually gases and liquids at room temperature

Weak forces of attraction

Weak forces of attraction

Weak forces of attraction

CC

C

CO

O

O

O

O

O

O

O