# Elements, Compounds, Mixtures and Atomic Structure

> Chemistry · CIE IGCSE 0620
> Source: https://www.owlsprep.com/study/cie-0620-u2-elements-compounds-mixtures-and-atomic/

This guide covers core classification of elements, compounds and mixtures, basic atomic structure, and subatomic particle count calculations, aligned to CIE IGCSE Chemistry 0620 2026-2028 Unit 2 outcomes.

**Prerequisites:** [Basic lab separation techniques](https://www.owlsprep.com/study/cie-0620-u1-separation-techniques/)

## Learning objectives

- Define atoms, elements, compounds and mixtures using official CIE IGCSE terminology
- Distinguish between pure substances and mixtures using melting and boiling point data
- Describe the structure of an atom, including relative charge and mass of protons, neutrons and electrons
- Use atomic number and mass number to calculate subatomic particle counts in neutral atoms
- Determine the electron configuration of elements with proton number 1 to 20, and relate it to group number and period number
- Extended: Interpret particle diagrams to classify elements, compounds and mixtures

## Classifying Elements, Compounds and Mixtures (Core)

**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)

**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 ($H_2O$), sodium chloride (NaCl)

**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

> **info**
>
> Pure substances have fixed, sharp melting and boiling points. Mixtures melt or boil over a range of temperatures, which you can use to distinguish them in exam questions.

**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. 2. Carbon dioxide is made of carbon and oxygen chemically bonded in a fixed 1:2 ratio: it is a compound.
3. 3. 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.

## Basic Atomic Structure (Core)

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) |

**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.

**Mass number (nucleon number, A)** — The total number of protons and neutrons in the nucleus of an atom.

> **mnemonic**
>
> Remember: Atomic number = number of protons, Mass number = mass of the nucleus (protons + neutrons). Electrons have negligible mass so they do not count towards the mass number.

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

**Check your understanding**

1. An atom of aluminium has atomic number 13 and mass number 27. How many neutrons does it contain?

   *Why:* 27 - 13 = 14 neutrons. Remember neutrons = mass number minus atomic number.

## Electron Configuration / Arrangement (Core)

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. 2. Fill the shells from the inside out: 2 in the first shell, 8 in the second shell, leaving 13 - 2 - 8 = 3 electrons.
3. 3. Place the remaining 3 electrons in the third shell. Electron configuration = 2,8,3.

> **info**
>
> Three quick rules for the first 20 elements: Group VIII noble gases have a full outer electron shell (2 for helium, 8 for the others); the number of outer shell electrons equals the group number in Groups I to VII; and the number of occupied electron shells equals the period number.

**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. 2. There are 3 occupied shells, so the element is in Period 3.
3. 3. 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.

## Extended: Particle Level Substance Classification

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. 2. There is no fixed ratio of hydrogen to oxygen molecules, and each retains its own properties
3. 3. 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.

## Exam Application: Classify Substances from Data

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

> **tip**
>
> If a substance melts at exactly 0°C and boils at exactly 100°C, it is pure water. If it melts between -2°C and 1°C, it is a mixture of water and another substance (e.g. salt).

**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. 2. Substance Y melts and boils over a range of temperatures: it is an impure mixture

## Common pitfalls

- **Wrong:** Classifying $O_2$ as a compound because it has two atoms
  - Why it fails: Compounds require two *different* types of elements. $O_2$ only contains oxygen atoms, so it is an element.
  - Correct: Only classify a substance as a compound if it contains two or more different elements chemically bonded.
- **Wrong:** Counting electrons when calculating mass number
  - Why it fails: Electrons have negligible mass, so they are not included in the mass number calculation.
  - Correct: Calculate mass number as the sum of protons and neutrons only.
- **Wrong:** Stating mixtures can only be separated by chemical methods
  - Why it fails: Mixture components are not chemically bonded, so they can be separated by physical methods like filtration, distillation or chromatography.
  - Correct: Only compounds require chemical reactions to separate into their constituent elements.
- **Wrong:** Assuming all atoms have equal numbers of neutrons and protons
  - Why it fails: 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: Always calculate neutrons as mass number minus atomic number, do not assume it equals the proton count.

## 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 |

## 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.

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