Covalent Bonding (HSC SSCE Chemistry): Revision Notes

Covalent Bonding

What is covalent bonding?

Covalent bonding is a type of chemical bonding where atoms share pairs of electrons to form stable compounds. This sharing of electrons allows atoms to achieve a stable electron configuration, similar to that of noble gases.

When atoms form covalent bonds, each shared pair of electrons is considered to belong to both atoms involved in the bond. This means both atoms can "count" the shared electrons as part of their own electron configuration, helping them achieve stability.

Examples of covalent bonding

Chlorine molecule ($\text{Cl}_2$)

A chlorine atom has the electron configuration $2,8,7$, meaning it needs one more electron to achieve the stable configuration of argon ($2,8,8$). When two chlorine atoms come together, they can both achieve this stable configuration by sharing a pair of electrons.

Hydrogen chloride ($\text{HCl}$)

Hydrogen has only one electron and needs one more to become like helium (a stable configuration). Chlorine, with configuration $2,8,7$, also needs one electron to become like argon ($2,8,8$).

When these atoms combine:

• Each atom contributes one electron to form a shared pair
• The hydrogen atom achieves a helium-like configuration
• The chlorine atom achieves an argon-like configuration
• The covalent molecule $\text{HCl}$ is formed

Structural formula: $\text{H—Cl}$

Water ($\text{H}_2\text{O}$)

Oxygen has the electron configuration $2,6$ and needs two more electrons to become like neon ($2,8$). Hydrogen needs one electron to become like helium.

To satisfy both requirements:

• One oxygen atom bonds with two hydrogen atoms
• Each hydrogen shares its electron with the oxygen
• The oxygen shares one electron with each hydrogen
• All three atoms achieve stable noble gas configurations

Structural formula: $\text{H—O—H}$

The molecular formula is $\text{H}_2\text{O}$, showing that each water molecule contains two hydrogen atoms and one oxygen atom.

Representing covalent bonds

Electron-dot structures (Lewis structures)

Electron-dot structures are diagrams that show the valence electrons (outer shell electrons) as dots around element symbols. These structures help us visualise how electrons are shared in covalent bonds.

Structural formulae

Structural formulae use dashes to represent covalent bonds between atoms. Each dash represents one shared pair of electrons. For example:

• $\text{Cl—Cl}$ for chlorine
• $\text{H—Cl}$ for hydrogen chloride
• $\text{H—O—H}$ for water

Shapes of simple covalent molecules

Different covalent molecules have different three-dimensional shapes:

• Chlorine ($\text{Cl}_2$): linear molecule
• Hydrogen chloride ($\text{HCl}$): linear molecule
• Water ($\text{H}_2\text{O}$): bent or V-shaped molecule

Covalent molecular substances and molecular formulae

Covalent molecular substances (or simply molecular substances) are materials made up of discrete covalent molecules. Examples include $\text{Cl}_2$, $\text{HCl}$, and $\text{H}_2\text{O}$.

A molecular formula tells us the exact number of each type of atom present in one molecule of the substance. This differs from an empirical formula, which only shows the simplest ratio of atoms.

Covalency and the periodic table

Covalent bonding occurs when both elements involved need to gain electrons to achieve noble gas configurations. Elements in the centre and right side of the periodic table tend to form covalent compounds, including:

• Carbon ($\text{C}$)
• Silicon ($\text{Si}$)
• Nitrogen ($\text{N}$)
• Phosphorus ($\text{P}$)
• Oxygen ($\text{O}$)
• Sulfur ($\text{S}$)
• Fluorine ($\text{F}$)
• Chlorine ($\text{Cl}$)

Valency in covalent compounds

Valency (or valence) in covalent compounds refers to the number of covalent bonds an element forms. This is equivalent to the number of electrons the atom needs to gain to achieve a noble gas configuration.

Examples:

• Chlorine in $\text{HCl}$: valency = 1
• Oxygen in $\text{H}_2\text{O}$: valency = 2
• Nitrogen in $\text{NH}_3$: valency = 3
• Carbon in $\text{CH}_4$: valency = 4

Some elements always display the same valency (fixed valency), while others can have different valencies in different compounds (variable valency).

Common valencies:

Fixed valencies:

• Valency 1: $\text{H}$, $\text{F}$, $\text{Cl}$, $\text{Br}$, $\text{I}$
• Valency 2: $\text{O}$
• Valency 4: $\text{C}$, $\text{Si}$

Variable valencies:

• Sulfur ($\text{S}$): valency 2, 4, or 6
• Nitrogen ($\text{N}$): valency 3 or 5 (can also be 1, 2, or 4 in oxygen compounds)
• Phosphorus ($\text{P}$): valency 3 or 5

Determining formulae for covalent binary compounds

If we know the valencies of two elements, we can determine the formula for the covalent compound they form. For a compound with formula $\text{A}_x\text{B}_y$, we choose the smallest whole number values of $x$ and $y$ that satisfy:

$x \times \text{valency of A} = y \times \text{valency of B}$

Remember!