Structure and Bonding - Covalent Bonding

Introduction to Covalent Bonding

Sharing electrons to hold atoms together

Understanding how atoms bond together helps explain why different substances have such different properties. While metals lose electrons to form ionic bonds, non-metal atoms prefer to share electrons instead.

Covalent Bonds

Atoms of non-metals can achieve stable electronic structures by sharing pairs of electrons rather than losing or gaining them. Each shared pair of electrons forms a covalent bond.

Think of it like two people sharing a blanket - both benefit from having the shared electrons between them. The positive nuclei of both atoms are attracted to the same pair of negative electrons, creating a strong electrostatic attraction that holds the atoms together.

Within a molecule, covalent bonds are very strong and difficult to break. However, the forces between separate molecules (called intermolecular forces) are much weaker - like the difference between breaking a steel chain link versus separating two magnets.

Representing Covalent Bonds

We can show covalent bonds in several ways:

• Dot-and-cross diagrams - show individual electrons as dots and crosses

• Displayed formulas - show bonds as lines, for example H–ClH–Cl

• Structural formulas - simplified version: HClHCl

All these methods highlight the key point: each covalent bond contains exactly two electrons.

Small Molecular Substances

Many covalently bonded substances exist as small, separate molecules containing just a few atoms. Common examples include:

Comparison table

Because these molecules are electrically neutral and only weakly attracted to each other, they typically:

• Have low melting and boiling points

• Are gases or liquids at room temperature

• Do not conduct electricity (no free ions or mobile electrons)

How to Recognise Small Molecules from Their Formula

1. Count the atoms - usually fewer than 10-20 total

2. Check that all elements are non-metals

3. Look for the absence of brackets with subscript nn (which indicates a polymer)

For example, C2H6OC2​H6​O (ethanol) has 9 atoms total, contains only non-metals (C, H, O), and has no repeating units - so it's a small molecular substance.

Worked example

Question: Classify the substance with formula N2H4N2​H4​.

Solution:

1. Count atoms: 2+4=62+4=6 atoms total - this is a small number

2. Check elements: N and H are both non-metals ✓

3. Look for polymer signs: No brackets or subscript nn ✓

Conclusion: N2H4N2​H4​ (hydrazine) is a small molecular substance.

Very Large Molecules - Polymers

Sometimes covalent bonding continues in long chains, joining thousands of small units together. These polymers have very high relative molecular masses. We represent them using shorthand notation:

–CH2–CH2–CH2––CH2​–CH2​–CH2​–

or more simply: (CH2)n(CH2​)n​

where nn represents a very large number (often thousands).

Although the bonds within each polymer chain remain strong covalent bonds, the forces between different polymer chains are stronger than those between small molecules. This gives polymers higher melting points and makes them tougher materials.

Common Polymer Examples:

• Poly(ethene) - used in shopping bags and plastic bottles because it's flexible and waterproof

• Poly(chloroethene) (PVC) - used in window frames and water pipes because it's rigid and durable

• Proteins and DNA - biological polymers that make up living organisms

Giant Covalent Structures

When covalent bonding extends in all three dimensions, it creates a giant covalent lattice. Imagine a 3D network where millions of atoms are all connected by strong covalent bonds - breaking this structure requires enormous amounts of energy.

Diamond

• Each carbon atom forms four single covalent bonds in a tetrahedral arrangement

• Creates an extremely hard material with a very high melting point

• Does not conduct electricity because all electrons are locked in bonds

Silicon Dioxide (Silica)

• Each silicon bonds to four oxygen atoms; each oxygen bonds to two silicon atoms

• Forms the main component of sand and quartz

• Has a rigid lattice structure with a high melting point

• Acts as an electrical insulator

Comparison table

Case Study: Why does dry ice sublimate?

Dry ice is solid carbon dioxide (CO2CO2​). When you see it "smoking" at parties or in science demonstrations, it's actually changing directly from solid to gas without melting.

• CO2CO2​ molecules are small and non-polar

• The intermolecular forces between molecules are very weak

• At normal atmospheric pressure, the solid converts straight to gas at −78.5°C−78.5°C

• The energy needed to separate the molecules is much less than for substances like water ice

This is why dry ice is useful for keeping things cold - it doesn't leave any liquid mess behind!

Key terms

Covalent bond - A strong bond formed by the electrostatic attraction between nuclei and a shared pair of electrons.

Intermolecular forces - Weak attractions between separate molecules.

Polymer - A very large molecule made by joining many small repeating units (monomers) together with covalent bonds.

Giant covalent structure - A 3D network containing millions of atoms, each bonded covalently to its neighbours.

Displayed formula - A diagram showing every bond in a molecule as a line.