Elements, Compounds, Mixtures and Atomic Structure

Q: What is the main difference between a compound and a mixture?

A compound is formed when two or more elements chemically combine in fixed proportions, has a unique chemical formula, and can only be separated by chemical reactions. A mixture has variable proportions of unbonded components that retain their individual properties, and can be separated by physical methods like filtration or distillation.

Chemistry· 2.1, 2.2 (2026-2028 syllabus)· 25 min read

1. Classifying Elements, Compounds and Mixtures (Core)★★☆☆☆⏱ 8 min

📘 Definition

Element

A pure substance made of only one type of atom, that cannot be broken down into simpler substances by chemical means. All elements are listed on the periodic table.

Example:

Oxygen ( $O_{2}$ ), iron (Fe)

📘 Definition

Compound

A pure substance formed when two or more elements are chemically bonded together in fixed mass proportions, with properties different from its constituent elements.

Example:

Water ( $H_{2} O$ ), sodium chloride (NaCl)

📘 Definition

Mixture

An impure combination of two or more substances (elements or compounds) that are not chemically bonded, present in variable proportions, retaining their individual properties.

Example:

Air, salt water

📐 Worked Example

Classify each of the following as element, compound or mixture, with justification: a) Magnesium ribbon, b) Carbon dioxide, c) Orange juice

1
1. Magnesium ribbon is made of only magnesium atoms, cannot be broken down chemically: it is an element.
2
1. Carbon dioxide is made of carbon and oxygen chemically bonded in a fixed 1:2 ratio: it is a compound.
3
1. Orange juice contains water, sugar, citric acid and other components in variable proportions, with no chemical bonds between them: it is a mixture.

Exam tip:

Always give justification for classification questions: markers award marks for stating the key property (fixed ratio, one atom type, no chemical bonds) not just the label.

2. Basic Atomic Structure (Core)★★☆☆☆⏱ 7 min

All matter is made of tiny particles called atoms. Atoms have a central positively charged nucleus containing protons and neutrons, surrounded by negatively charged electrons arranged in shells (energy levels).

Subatomic particle	Relative charge	Relative mass
Proton	+1	1
Neutron	0	1
Electron	-1	1/1840 (negligible)

📘 Definition

Atomic number (proton number, Z)

The number of protons in the nucleus of an atom of an element. All atoms of the same element have the same atomic number.

📘 Definition

Mass number (nucleon number, A)

The total number of protons and neutrons in the nucleus of an atom.

📐 Worked Example

An atom of carbon has atomic number 6 and mass number 12. Calculate the number of protons, neutrons and electrons in this neutral atom.

1
1. Number of protons = atomic number = 6
2
1. In a neutral atom, number of electrons = number of protons = 6 (charges cancel out overall)
3
1. Number of neutrons = mass number - atomic number = 12 - 6 = 6

✓ Quick check

An atom of aluminium has atomic number 13 and mass number 27. How many neutrons does it contain?
Reveal answer
14 —
27 - 13 = 14 neutrons. Remember neutrons = mass number minus atomic number.

3. Electron Configuration / Arrangement (Core)★★☆☆☆⏱ 6 min

Electrons occupy shells (energy levels) around the nucleus. Shells are filled from the innermost outwards: the first shell holds up to 2 electrons, the second up to 8, and the third up to 8 for the first 20 elements. The electron configuration is written by listing the number of electrons in each shell, separated by commas (for example 2,8,3 for aluminium). You must be able to work out the electron configuration of any element from proton number 1 to 20.

📐 Worked Example

Write the electron configuration of an atom of aluminium (proton number 13).

1
1. A neutral aluminium atom has 13 electrons, equal to its 13 protons.
2
1. Fill the shells from the inside out: 2 in the first shell, 8 in the second shell, leaving 13 - 2 - 8 = 3 electrons.
3
1. Place the remaining 3 electrons in the third shell. Electron configuration = 2,8,3.

📐 Worked Example

An element has the electron configuration 2,8,7. State its group and period in the Periodic Table.

1
1. The outer shell contains 7 electrons, so the element is in Group VII.
2
1. There are 3 occupied shells, so the element is in Period 3.
3
1. The element is chlorine (proton number 17, electron configuration 2,8,7).

Exam tip:

For Groups I to VII, the number of outer shell electrons equals the group number, and the number of occupied shells equals the period number — use these to place an element from its configuration.

4. Extended: Particle Level Substance Classification★★★☆☆Extended only⏱ 5 min

For Extended tier questions, you will be asked to explain differences between elements, compounds and mixtures at the microscopic particle level, and interpret particle diagrams for each classification.

Element diagrams show only one type of atom, either as single atoms or molecules made of identical atoms
Compound diagrams show molecules or lattices made of two or more different types of atoms chemically bonded together, with all identical units
Mixture diagrams show a mix of different atoms, molecules or compounds with no fixed arrangement or ratio

📐 Worked Example

A particle diagram shows two distinct types of molecules: one made of two hydrogen atoms, one made of two oxygen atoms. Classify the substance shown, justify your answer.

1
1. Identify the particles present: separate $H_{2}$ and $O_{2}$ molecules, no chemical bonds between them
2
1. There is no fixed ratio of hydrogen to oxygen molecules, and each retains its own properties
3
1. This is a mixture of hydrogen and oxygen elements

Exam tip:

If a diagram shows multiple types of particles with no bonds between them, it is always a mixture, regardless of whether the individual particles are elements or compounds.

5. Exam Application: Classify Substances from Data★★★☆☆⏱ 5 min

Exam questions often give you melting point or boiling point data to classify substances as pure or impure (mixtures).

📐 Worked Example

Substance X melts at 801°C and boils at 1413°C. Substance Y melts between 120°C and 140°C, and boils between 250°C and 280°C. Classify each as pure or impure, justify your answer.

1
1. Substance X has fixed, sharp melting and boiling points: it is a pure substance (either element or compound)
2
1. Substance Y melts and boils over a range of temperatures: it is an impure mixture

6. Common Pitfalls

Wrong move:

Classifying $O_{2}$ as a compound because it has two atoms

Why:

Compounds require two different types of elements. $O_{2}$ only contains oxygen atoms, so it is an element.

Correct move:

Only classify a substance as a compound if it contains two or more different elements chemically bonded.

Wrong move:

Counting electrons when calculating mass number

Why:

Electrons have negligible mass, so they are not included in the mass number calculation.

Correct move:

Calculate mass number as the sum of protons and neutrons only.

Wrong move:

Stating mixtures can only be separated by chemical methods

Why:

Mixture components are not chemically bonded, so they can be separated by physical methods like filtration, distillation or chromatography.

Correct move:

Only compounds require chemical reactions to separate into their constituent elements.

Wrong move:

Assuming all atoms have equal numbers of neutrons and protons

Why:

This is only true for some light elements; many atoms have more neutrons than protons (e.g. carbon-14 has 6 protons, 8 neutrons).

Correct move:

Always calculate neutrons as mass number minus atomic number, do not assume it equals the proton count.

7. Quick Reference Cheatsheet

Term	Key Definition	Key Property
Element	Pure substance, one type of atom	Cannot be split by chemical reactions, fixed melting/boiling point
Compound	Pure substance, 2+ elements chemically bonded in fixed ratio	Properties different from constituents, fixed melting/boiling point
Mixture	2+ unbonded substances, variable ratio	Components retain individual properties, melts/boils over range
Atomic number (Z)	Number of protons in nucleus	Same for all atoms of one element
Mass number (A)	Number of protons + neutrons in nucleus	Neutron count = A - Z

8. Frequently Asked

What is the main difference between a compound and a mixture?

A compound is formed when two or more elements chemically combine in fixed proportions, has a unique chemical formula, and can only be separated by chemical reactions. A mixture has variable proportions of unbonded components that retain their individual properties, and can be separated by physical methods like filtration or distillation.

How do I calculate the number of neutrons in an atom?

Subtract the atomic number (number of protons) from the mass number (sum of protons and neutrons): $Neutron count = A - Z$

Going deeper

syllabusCIE IGCSE Chemistry 0620 2026-2028 SyllabusOfficial syllabus reference for this topic

What's Next

Now that you have mastered the classification of elements, compounds and mixtures, basic atomic structure and the arrangement of electrons in shells, you can move on to applying these ideas to bonding and the Periodic Table. Electron configuration is the foundation for all subsequent topics in CIE IGCSE Chemistry, including ionic and covalent bonding, formula writing and chemical equations, because atoms react to achieve full outer shells. Make sure you practice classifying substances from both descriptions and particle diagrams, calculating subatomic particle counts, and writing electron configurations for elements 1 to 20 to build speed for your exam.