Atoms and Compounds (Grade 10 NSC Matric Physical Sciences): Revision Notes

Atoms and Compounds

Atoms as building blocks of matter

Understanding matter starts with recognising that everything around us is made up of tiny particles called atoms. Think of atoms like building blocks - just as different building materials create structures with different properties, the type of atoms and how they are arranged determines the properties of different substances.

Most substances we encounter are not made of individual atoms floating around by themselves. Instead, atoms typically join together through chemical bonds to form larger structures. This is similar to how individual building materials are rarely used alone - they are combined to create more complex and useful structures.

What are compounds?

When atoms of different elements combine, they form compounds. Understanding compounds is essential for chemistry because most of the substances we interact with daily are compounds rather than pure elements.

Classification of compounds

Compounds can be organised into different categories based on the type of bonding that holds their atoms together. This classification system helps us understand and predict the properties of different substances.

The main classification divides compounds into two broad categories:

Covalent molecular structures

These compounds consist of small, discrete molecules where atoms share electrons through covalent bonds. The molecules exist as separate units and interact with each other through weaker forces.

Examples include:

• Water ($\text{H}_2\text{O}$)
• Oxygen gas ($\text{O}_2$)
• Sulphur ($\text{S}_8$)
• Buckyballs ($\text{C}_{60}$)

Network structures

Unlike molecular compounds, network structures form continuous networks of bonded atoms extending in all directions. These can be further subdivided into three types:

Covalent network structures

Atoms are connected throughout the entire structure by covalent bonds, creating very strong, three-dimensional networks.

• Examples: Diamond, graphite, silica ($\text{SiO}_2$)

Ionic network structures

Metal and non-metal atoms form a regular, repeating pattern where electrons are transferred from metals to non-metals, creating charged ions held together by electrostatic forces.

• Examples: Sodium chloride ($\text{NaCl}$), barium sulphate ($\text{BaSO}_4$), silver iodide ($\text{AgI}$)

Metallic network structures

Metal atoms are arranged in regular patterns where electrons are delocalised, forming a "sea" of electrons that can move freely throughout the structure.

• Examples: Copper ($\text{Cu}$), iron ($\text{Fe}$), gold ($\text{Au}$)

Representing molecules and compounds

Scientists use several different methods to represent compounds, each showing different aspects of their structure and composition.

Chemical formulae

Chemical formulae provide a shorthand way to show what elements are present in a compound and in what proportions.

For example, the molecular formula $\text{C}_4\text{H}_{10}$ tells us that one molecule of butane contains exactly 4 carbon atoms and 10 hydrogen atoms.

Using the same example, if a compound has the molecular formula $\text{C}_4\text{H}_{10}$, its empirical formula would be $\text{C}_2\text{H}_5$ because this shows the simplest ratio (2:5) of carbon to hydrogen atoms.

Types of molecular models

Visual models help us understand the three-dimensional structure of compounds. There are three main types:

Wireframe or stick models: Show the bonds between atoms as coloured "sticks" or lines. These clearly display how atoms connect but don't show atomic size.

Ball-and-stick models: Represent atoms as coloured spheres connected by rods representing bonds. This type shows both connectivity and gives some indication of atomic size.

Space-filling models: Show atoms as spheres sized according to their actual relative sizes. These models give the best representation of molecular shape and size but make it harder to see individual bonds.

Practical investigations

Investigating elements and compounds experiment

The electrolysis of water