Covalent compounds

A covalent compound is a substance whose atoms are joined by covalent bonds — shared pairs of electrons. The atoms are almost always non-metals (e.g. carbon, hydrogen, oxygen, nitrogen, chlorine).

Each covalent bond is one shared pair of electrons, with one electron donated by each atom. The shared pair is attracted to both nuclei, and this electrostatic attraction is what holds the atoms together — a covalent bond is a strong bond.

The two-question test for any compound:

1. What elements are present? Two (or more) non-metals → covalent. A metal + a non-metal → ionic.
2. What structure does it have? Covalent compounds are either simple molecular or giant covalent — and these behave very differently (next sections).

Examples of covalent compounds: water (H₂O), carbon dioxide (CO₂), methane (CH₄), ammonia (NH₃), hydrogen chloride (HCl), and giant structures such as silicon dioxide (SiO₂). Note that diamond and graphite are giant covalent elements (pure carbon), not compounds, but they show the same bonding.

Key distinction examiners test: covalent = electrons SHARED; ionic = electrons TRANSFERRED (one atom loses, the other gains). Using the wrong word loses the mark.

Both structure types use strong covalent bonds, but they are arranged very differently — and that decides their properties.

Simple molecular substances are made of small, separate molecules (e.g. H₂O, CO₂, CH₄, Cl₂). Inside each molecule the atoms are held by strong covalent bonds, but the molecules are held to each other only by weak intermolecular forces.

Giant covalent substances are one continuous lattice of atoms, all joined by strong covalent bonds throughout — there are no separate molecules (e.g. diamond, graphite, silicon dioxide).

	Simple molecular	Giant covalent
Structure	small separate molecules	one continuous lattice
What's broken on melting	weak intermolecular forces	strong covalent bonds
Melting / boiling point	low (often gas or liquid at rt)	very high (always solid at rt)
Hardness	soft / no fixed hardness	hard (graphite is soft — layers)
Conducts?	no	no (graphite conducts)
Examples	H₂O, CO₂, CH₄, Cl₂	diamond, graphite, SiO₂

The crucial idea for the marks: when you melt a simple molecular substance you do NOT break the covalent bonds inside the molecules — you only overcome the weak forces between the molecules. That is why so little energy is needed and m.p./b.p. are low.

This is the single most-tested comparison on the paper. Learn the contrasts and you can answer almost any "compare" or "classify" question.

Property	Ionic	Covalent (simple molecular)	Covalent (giant)
Bonding	transfer of electrons; strong attraction between oppositely charged ions	shared pairs of electrons	shared pairs of electrons
Particles	positive + negative ions	neutral molecules	neutral atoms
Structure	giant lattice	small separate molecules	giant lattice
Elements	metal + non-metal	non-metal + non-metal	non-metals
m.p. / b.p.	high	low	very high
Conducts when solid?	no (ions fixed)	no	no (graphite yes)
Conducts when molten/aqueous?	yes (ions free to move)	no	no (graphite yes)
Solubility in water	often soluble	varies (polar yes, non-polar no)	insoluble
Examples	NaCl, MgO	H₂O, CO₂, CH₄	diamond, SiO₂

Why ionic conducts but simple molecular covalent doesn't: ionic compounds contain charged ions. When molten or dissolved, the ions are free to move and carry charge. Simple molecular covalent substances have no ions and no free electrons, so they cannot conduct in any state.

In the exam you are often given a data table (melting point, conductivity, solubility) and asked to decide the bonding. Use this decision routine:

Step 1 — Look at the elements (if given).

• Metal + non-metal → ionic.
• Non-metal + non-metal → covalent.

Step 2 — Look at the melting/boiling point.

• Very low m.p. (gas or liquid at room temperature) → simple molecular covalent.
• Very high m.p. → ionic OR giant covalent (use conductivity to separate them).

Step 3 — Look at conductivity (this separates ionic from giant covalent).

• High m.p. + conducts when molten or in solution (but not solid) → ionic (free ions when molten/aqueous).
• High m.p. + does NOT conduct (in any state) → giant covalent (no free ions or electrons — except graphite).
• Low m.p. + does not conduct → simple molecular covalent.

Step 4 — Solubility (a useful extra clue).

• Many ionic compounds dissolve in water; giant covalent substances are insoluble; simple molecular ones vary.

Quick reference:

Clue	Ionic	Simple molecular	Giant covalent
m.p.	high	low	very high
conducts when solid	no	no	no
conducts when molten/aqueous	yes	no	no

A high melting point alone is not enough to prove ionic — you must check conductivity to rule out giant covalent. That single point is where most marks are won.