# Covalent Bonding

> Non-metals share electrons. Single, double and triple bonds, electron dot diagrams, and naming molecules.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/covalent-bonding/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: Two friends and one umbrella
It starts raining. Ali and Sam each have half an umbrella, which is useless on its own. So they stand close together and hold both halves over their heads. Now they are both dry, but they cannot walk apart without one of them getting wet.

Non-metal atoms do the same with electrons. Two chlorine atoms each have 7 outer electrons and each need 8. Neither can steal from the other, because they pull equally hard. So each puts one electron into a shared pair. Both now count 8, and the shared pair holds them together. That is a **covalent bond**.

**In short:** A covalent bond is a pair of electrons shared between two non-metal atoms, so each atom fills its outer shell. Atoms can share one, two or three pairs (single, double and triple bonds), and covalent molecular substances have low melting points and do not conduct.

## What is a covalent bond?

A **covalent bond** is a **shared pair of electrons** between two **non-metal** atoms. Both nuclei are attracted to the shared pair, and that holds the atoms together.

A group of atoms held together by covalent bonds is a **molecule**. Examples: $\mathrm{H_2}$, $\mathrm{Cl_2}$, $\mathrm{H_2O}$, $\mathrm{CO_2}$, $\mathrm{CH_4}$.

## How many bonds can an atom make?

An atom shares enough electrons to fill its outer shell. The number of bonds it usually makes is its **valency**.

| Atom | Valence electrons | Needs | Bonds (valency) |
|---|---|---|---|
| H | 1 | 1 more (to reach 2) | 1 |
| Cl, F (group 17) | 7 | 1 | 1 |
| O, S (group 16) | 6 | 2 | 2 |
| N, P (group 15) | 5 | 3 | 3 |
| C, Si (group 14) | 4 | 4 | 4 |

## Electron dot diagrams

An **electron dot diagram** (Lewis diagram) shows only the **outer** electrons. Use dots for one atom and crosses for the other so you can see where each shared electron came from.

*Diagram: Electron dot diagram of water: oxygen in the middle sharing one pair with each of two hydrogen atoms, with two lone pairs on oxygen*
*Water, H₂O. Dots are oxygen's electrons, crosses are hydrogen's. Oxygen ends up with 8, each hydrogen with 2.*

Pairs of outer electrons that are **not** shared are called **lone pairs**. Oxygen in water has two lone pairs.

## Structural formulas

A **structural formula** shows each shared pair as a line.

| Molecule | Structural formula | Bond type |
|---|---|---|
| Hydrogen, $\mathrm{H_2}$ | H−H | Single |
| Water, $\mathrm{H_2O}$ | H−O−H | Two single |
| Oxygen, $\mathrm{O_2}$ | O=O | **Double** (2 shared pairs) |
| Carbon dioxide, $\mathrm{CO_2}$ | O=C=O | Two double |
| Nitrogen, $\mathrm{N_2}$ | N≡N | **Triple** (3 shared pairs) |
| Methane, $\mathrm{CH_4}$ | C in the middle with four H, each by a single bond | Four single |

Triple bonds are stronger and shorter than double bonds, which are stronger than single bonds. The triple bond in $\mathrm{N_2}$ is so strong that nitrogen gas, 78% of our air, hardly reacts.

## Diatomic elements

Seven elements exist as pairs of atoms ($\mathrm{X_2}$) joined by covalent bonds: **H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂**.

### Exam tip: Remember the seven
"**H**ave **N**o **F**ear **O**f **I**ce **C**old **Br**occoli" gives H, N, F, O, I, Cl, Br. Or picture a "7" on the periodic table: starting at nitrogen (element 7), go across to fluorine and down the halogens, plus hydrogen.

## Naming covalent compounds

For two non-metals, use **prefixes** to show how many of each atom.

| Number | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| Prefix | mono | di | tri | tetra | penta | hexa |

- The second element ends in **-ide**.
- Leave out **mono** on the first element.

$\mathrm{CO}$ is carbon **mon**oxide. $\mathrm{CO_2}$ is carbon **di**oxide. $\mathrm{N_2O_4}$ is **di**nitrogen **tetr**oxide. $\mathrm{SF_6}$ is sulfur **hexa**fluoride.

Some molecules keep everyday names: water ($\mathrm{H_2O}$), ammonia ($\mathrm{NH_3}$), methane ($\mathrm{CH_4}$).

### Watch out: Prefixes are for covalent only
Never use prefixes for ionic compounds. $\mathrm{MgCl_2}$ is magnesium chloride, **not** magnesium dichloride.

## Naming acids

An **acid** is a compound that releases hydrogen ions, $\mathrm{H^+}$, when dissolved in water. Its formula usually starts with H. Acids have their own naming rules.

**Binary acids** contain hydrogen and **one** other element. Use **hydro-**, the root of the element, **-ic**, then "acid".

| Formula | Name |
|---|---|
| HCl | **hydro**chlor**ic** acid |
| HF | **hydro**fluor**ic** acid |
| HBr | **hydro**brom**ic** acid |

**Oxyacids** contain hydrogen, oxygen and another element (a polyatomic ion). Name them from the ion, with **no** "hydro":

- Ion ending in **-ate** becomes **-ic** acid. "I **ate** something **ic**ky."
- Ion ending in **-ite** becomes **-ous** acid.

| Ion | Acid | Name |
|---|---|---|
| sulf**ate**, $\mathrm{SO_4^{2-}}$ | $\mathrm{H_2SO_4}$ | sulfur**ic** acid |
| sulf**ite**, $\mathrm{SO_3^{2-}}$ | $\mathrm{H_2SO_3}$ | sulfur**ous** acid |
| nitr**ate**, $\mathrm{NO_3^-}$ | $\mathrm{HNO_3}$ | nitr**ic** acid |
| carbon**ate**, $\mathrm{CO_3^{2-}}$ | $\mathrm{H_2CO_3}$ | carbon**ic** acid |
| phosph**ate**, $\mathrm{PO_4^{3-}}$ | $\mathrm{H_3PO_4}$ | phosphor**ic** acid |

## Properties of covalent molecular substances

The covalent bonds **inside** each molecule are strong. But the forces **between** molecules (intermolecular forces) are weak.

- **Low melting and boiling points**: many are gases or liquids at room temperature. Only the weak forces between molecules need to break.
- **Do not conduct electricity**: there are no ions or free electrons.
- Often soft as solids, like wax or sugar.

**Mind map**

```text
Covalent bonding
  What
    Shared pair of electrons
    Between non-metals
  Bonds
    Single: 1 pair
    Double: 2 pairs
    Triple: 3 pairs
  Diagrams
    Dot and cross
    Lone pairs
    Structural: lines
  Naming
    mono, di, tri, tetra
    Second element -ide
    Acids: hydro...ic, -ate to -ic
  Properties
    Low melting point
    Do not conduct
```

### Year 11: Bond length and bond energy
In a covalent bond, the two nuclei are pulled towards the shared electrons but repel each other. The bond settles at the distance where these forces balance: the **bond length**. The atoms vibrate back and forth around it, as if joined by a spring.

The **bond energy** is the energy needed to break one mole of that bond.

| Bond | Bond length (pm) | Bond energy (kJ/mol) |
|---|---|---|
| C−C (single) | 154 | 346 |
| C=C (double) | 134 | 614 |
| C≡C (triple) | 120 | 839 |

More shared pairs give a **shorter and stronger** bond. Down group 17 the atoms get bigger, so the bonds get **longer and weaker**: H−F is much stronger than H−I.

Molecule shapes and polarity are covered in the next chapter, Molecular Shapes and Polarity.

### Year 12: Intermolecular forces
Three types of force act between molecules, from weakest to strongest:

1. **Dispersion forces**: in every molecule, from brief uneven spreads of electrons. They grow with molecule size, which is why iodine is a solid but fluorine is a gas.
2. **Dipole-dipole forces**: between polar molecules, $\delta+$ end attracted to $\delta-$ end.
3. **Hydrogen bonds**: a strong dipole attraction when H is bonded to **N, O or F**. Hydrogen bonding gives water its unusually high boiling point (100 °C, compared with $-60\,^\circ\text{C}$ for $\mathrm{H_2S}$), and makes ice float.

Boiling a covalent molecular substance breaks the intermolecular forces, **not** the covalent bonds.

## Quick summary

- A **covalent bond** is a shared pair of electrons between two non-metal atoms.
- Atoms share enough electrons to fill their outer shell. H makes 1 bond, O makes 2, N makes 3, C makes 4.
- Single, double and triple bonds share 1, 2 and 3 pairs.
- Seven diatomic elements: $\mathrm{H_2, N_2, O_2, F_2, Cl_2, Br_2, I_2}$.
- Name covalent compounds with prefixes: carbon dioxide, dinitrogen tetroxide.
- Covalent molecular substances have low melting points and do not conduct.

## Quiz

Try these questions from the chapter quiz.

1. A covalent bond is:
   - A. A transfer of electrons from a metal to a non-metal
   - B. A shared pair of electrons between two non-metal atoms
   - C. The attraction between positive and negative ions
   - D. A sea of electrons
   - Answer: B. A shared pair of electrons between two non-metal atoms. Covalent means sharing.

2. Which compound is held together by covalent bonds?
   - A. NaCl
   - B. MgO
   - C. $\mathrm{CH_4}$
   - D. KBr
   - Answer: C. $\mathrm{CH_4}$. Carbon and hydrogen are both non-metals.

3. How many covalent bonds does a carbon atom usually form?
   - A. 1
   - B. 2
   - C. 3
   - D. 4
   - Answer: D. 4. Carbon has 4 valence electrons and needs 4 more.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/covalent-bonding/#quiz