Two non-metal atoms share pairs of electrons to reach a stable arrangement, for example H2(g) + Cl2(g) → 2HCl(g); a single, double or triple bond forms depending on the number of shared pairs.
A covalent compound forms when two non-metal atoms share one or more pairs of electrons so that each atom achieves a stable noble-gas electron arrangement. Neither atom gives its electrons away completely, instead the shared pair belongs to both atoms at once. Sharing one pair gives a single bond, two shared pairs give a double bond, and three shared pairs give a triple bond. Covalent bonding is one of the core content standards in Chemical Bond, so it is tested every year in 4541.
The balanced equation
The general pattern is: non-metal + non-metal → covalent compound. With correct formulae and state symbols, the standard examples are:
H2(g) + Cl2(g) → 2HCl(g)
2H2(g) + O2(g) → 2H2O(l)
N2(g) + 3H2(g) → 2NH3(g)
C(s) + O2(g) → CO2(g)
The chemistry behind each is electron sharing rather than transfer. In a hydrogen chloride molecule, hydrogen (1 electron) and chlorine (2.8.7) share one pair, so hydrogen reaches the helium duplet and chlorine reaches an octet. In water, oxygen (2.6) shares one pair with each of two hydrogen atoms. In carbon dioxide, carbon forms two double bonds, one to each oxygen, written O=C=O. In nitrogen, N2, the two atoms share three pairs to give a triple bond, N≡N, which is why nitrogen is so unreactive.
Conditions required
The elements must both be non-metals with high, similar electronegativity so that neither can pull the electrons away completely. Some of these combinations need heat or a spark to start, hydrogen and oxygen combine only when ignited, and nitrogen with hydrogen needs the high temperature, high pressure and catalyst of the Haber process. Others, such as the diatomic molecules of the elements themselves, form as the atoms combine.
What you observe
Because the products are molecular, you often see a gas or a volatile liquid rather than a solid. Hydrogen burns in oxygen with a pale blue flame and a “pop”, forming water that condenses as colourless droplets. Carbon burns in oxygen with a red glow to give the colourless gas carbon dioxide. Covalent compounds generally have low melting and boiling points, do not conduct electricity in any state, and many dissolve better in organic solvents than in water, the opposite of the ionic compounds you meet alongside them.
Where it appears in the SPM exam
In 4541/1 you are asked to identify the number of shared electron pairs or to choose the correct molecular formula. In 4541/2 the standard question asks you to draw the dot-and-cross diagram for a molecule such as H2O, CO2, NH3, CH4, Cl2 or N2, to state whether the bond is single, double or triple, and to write the balanced equation. You must also contrast the physical properties of covalent and ionic compounds, explaining each property in terms of the bonding.
How we teach it
Our teachers make sure you count electrons correctly: hydrogen needs a duplet, most other atoms need an octet, and every bonding pair is shown once, shared between the two atoms. The commonest lost marks come from drawing an ionic-style transfer instead of sharing, forgetting the lone pairs on oxygen or nitrogen, or writing the wrong number of hydrogen atoms. Practising a fixed set of molecules until the diagrams are automatic secures this reliable source of marks.
Quick summary
A covalent compound is held together by shared electron pairs between non-metal atoms. Decide how many pairs each atom needs to share, draw the dot-and-cross diagram with all lone pairs, name the bond as single, double or triple, and link the molecular structure to the low melting point and lack of conductivity. Those linked answers cover almost everything the exam asks on this topic.
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