# Atoms, Elements and Compounds

> CIE IGCSE Chemistry · CIE IGCSE Chemistry (0620)
> Source: https://www.owlsprep.com/study/cie-0620-u2-overview/
> Weight: 15-18% of total assessment (MCQ + theory sections)

This unit introduces the fundamental building blocks of all matter, from subatomic particles to core chemical bonding types, explaining how elements combine to form compounds with distinct structures and properties.

**Prerequisites:** [CIE IGCSE Chemistry Unit 1: The Particulate Nature of Matter](https://www.owlsprep.com/study/cie-0620-u1-overview/)

## Learning objectives

- Differentiate between elements, compounds and mixtures, and describe the basic subatomic structure of atoms
- Define isotopes and calculate relative atomic mass from isotopic percentage abundance data
- Compare ionic, covalent and metallic bonding, and predict bond type for common substances
- Relate the structure of simple and giant chemical substances to their observable physical properties

## Unit at a Glance

You will begin by classifying different forms of matter and exploring the structure of atoms, before moving to how isotopes of the same element vary, and how relative atomic mass is calculated from real-world isotopic abundance data. The second half of the unit focuses on the three core types of chemical bonding, and how bond type determines if a substance forms a simple molecular or giant lattice structure.

The content in this unit is foundational for almost all later chemistry topics, including stoichiometry, chemical reactions, and material properties, so a strong grasp of these core concepts will simplify your revision for the rest of the course.

Work through these subtopics in order to build your knowledge sequentially:
- [Elements, Compounds, Mixtures and Atomic Structure](https://www.owlsprep.com/study/cie-0620-u2-elements-compounds-mixtures-and-atomic/) — Classify types of matter, and identify the charge, mass and location of protons, neutrons and electrons in atoms
- [Isotopes and Relative Atomic Mass](https://www.owlsprep.com/study/cie-0620-u2-isotopes-and-relative-atomic-mass/) — Define isotopes, interpret isotopic notation, and calculate relative atomic mass using percentage abundance data
- [Ionic Bonding](https://www.owlsprep.com/study/cie-0620-u2-ionic-bonding/) — Explain ionic bond formation via electron transfer, draw dot-and-cross diagrams, and link ionic structure to properties
- [Covalent Bonding and Simple Molecules](https://www.owlsprep.com/study/cie-0620-u2-covalent-bonding-and-simple-molecules/) — Describe covalent bond formation via electron sharing, draw dot-and-cross diagrams for simple molecules, and link structure to properties
- [Giant Structures: Giant Covalent and Metallic](https://www.owlsprep.com/study/cie-0620-u2-giant-structures-giant-covalent-and/) — Compare giant covalent, ionic and metallic structures, and explain how bonding determines properties like conductivity and melting point

## Common pitfalls

- **Wrong:** Confusing isotopes with different elements because they have different mass numbers
  - Why it fails: Isotopes have the same number of protons, so they are the same element, only differing in neutron count
  - Correct: Use proton (atomic) number to identify an element, regardless of mass number or neutron count
- **Wrong:** Assuming all covalent substances have low melting points
  - Why it fails: Giant covalent structures have strong covalent bonds across their entire lattice, requiring very high energy to break
  - Correct: Distinguish between simple molecular covalent substances (low melting point) and giant covalent substances (very high melting point)
- **Wrong:** Classifying mixtures as compounds because they contain multiple elements
  - Why it fails: Mixtures have non-chemically bonded components that can be separated by physical methods, unlike compounds
  - Correct: Test if components can be separated via physical methods (filtration, distillation) to distinguish mixtures from compounds

## Cheatsheet

| Concept/Formula | Description |
| --- | --- |
| Atomic Structure | Protons (+1 charge, mass 1), neutrons (0 charge, mass 1) are in the nucleus; electrons (-1 charge, mass ~1/2000) occupy outer shells |
| Relative Atomic Mass ($A_r$) | $A_r = \frac{\text{sum (isotopic mass} \times \text{percentage abundance)}}{100}$ |
| Ionic Bond | Electrostatic attraction between oppositely charged ions formed by electron transfer from metal to non-metal |
| Covalent Bond | Electrostatic attraction between a shared pair of electrons and the positive nuclei of two non-metal atoms |
| Metallic Bond | Electrostatic attraction between a lattice of positive metal ions and delocalized free electrons |
| Giant Covalent Structure Properties | Very high melting point, insoluble in all solvents, usually non-conductive (except graphite) |

## What's next

To start this unit, head to the first subtopic on elements, compounds, mixtures and atomic structure, where you will build foundational knowledge of matter classification and subatomic particles that underpins all later content in this unit and the rest of the course. Once you complete all subtopics in this unit, you will move on to Unit 3: Stoichiometry, where you will use your understanding of atoms and bonding to calculate quantities of substances in chemical reactions.

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From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-0620-u2-overview/