What is an element?

An element is a pure substance that cannot be broken down into two or more simpler substances by chemical processes.

Carbon, hydrogen and oxygen are examples of elements. They cannot be broken down further into simpler substances.

Classifying Elements

Elements are often classified based on their
- metallic and non-metallic properties
- physical states

The table below shows the classification of elements by their metallic properties.

Atoms and Molecules

Elements can exist as atoms or molecules.

Atoms are the smallest particles of an element that have the chemical properties of that element. Each element contains only one type of atom. 

Noble gases, such as helium, neon, argon, krypton, xenon and radon, are elements that exist as individual atoms. They are monatomic elements.

A molecule is a group of two or more atoms that are chemically combined.

Diatomic molecules are those that are formed by the combination of two atoms. Polyatomic molecules are those that contain three or more atoms. 

Refer to the video below for a better understanding of atoms and molecules.

The Periodic Table

The periodic table is a list of elements arranged in order of increasing proton (atomic) numbers. 

The Periodic Table divides the elements into periods and groups.

What is a compound?

A compound is a pure substance containing two or more elements that are chemically combined in a fixed ratio. 

The video below explains how compounds are formed from their elements.

Composition of a Compound

A compound may be made up of molecules or ions.

Formation of a Compound

Compounds can be produced from a chemical reaction between elements only, compounds only or between elements and compounds.

Properties of a Compound

A compound has different properties from the elements that form it. 

For example, magnesium burns in oxygen to form the compound magnesium oxide. Magnesium oxide has properties different from magnesium and oxygen.

Chemical Formula of a Compound

The chemical formula of a compound is written by putting together the chemical symbols of the elements that make up the compound. 

The chemical formula shows 
- the types of atoms present
- ratio of different atoms present

The examples of water and lead(II) nitrate are shown below.

Decomposition of Compounds

Chemical processes such as thermal decomposition and electrolysis can be used to decompose compounds. 

Electrolysis is the process of using electricity to break down a compound. 

Thermal decomposition involves heating a compound strongly. For example, when mercury(II) oxide is heated strongly, it decomposes to give the elements mercury and oxygen. 

What is a mixture?

A mixture is made up of two or more substances that are not chemically combined.

- Mixtures can be made up of elements or compounds.
- Components of a mixture are not fixed and can be present in any ratio.

An example of a widely used mixture is alloys, which consist of a mixture of metals with other elements.

Differences Between Compounds and Mixtures

The table below shows the differences between compounds and mixtures.

Differences Between Compounds and Mixtures (Video)

To better understand the differences between compounds and mixtures, watch the video below.