Elements, Compounds and MixturesCambridge IGCSE Chemistry: Revision notes
Section 1
What Distinguishes Elements, Compounds and Mixtures?
All substances can be classified into three categories based on what they are made of and how their particles are held together.
| Term | Definition | Can it be separated by chemical means? |
|---|---|---|
| Element | A substance made of only one type of atom | No — it is already a single substance |
| Compound | Two or more elements chemically combined together, in fixed proportions, forming a new substance | Only by a chemical reaction, breaking the bonds |
| Mixture | Two or more substances (elements and/or compounds) that are not chemically combined | Yes — by a physical method (e.g. filtration, distillation) |
- A compound has different properties from the elements it is made from
- A mixture keeps the properties of its individual parts, and its composition can vary
Iron and sulfur mixed together is a mixture (still shows properties of both, can be separated with a magnet). Heated together, they form iron sulfide, a compound with completely different properties.
Section 2
What Is the Structure of an Atom?
All elements are made of atoms. Every atom has the same basic structure:
- A tiny, dense nucleus at the centre, containing protons and neutrons
- Electrons moving around the nucleus, arranged in shells (energy levels)
- Almost all of the atom's mass is concentrated in the nucleus, but almost all of its volume is empty space occupied by the electrons
The three subatomic particles have different relative charges and relative masses:
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | -1 | very small (often taken as 0) |
An atom overall has no charge, because the number of protons (positive) always equals the number of electrons (negative).
Section 3
What Do Proton Number and Mass Number Tell Us?
Two key numbers describe every atom:
- Proton (atomic) number — the number of protons in the nucleus of an atom; this defines which element it is
- Mass (nucleon) number — the total number of protons and neutrons in the nucleus
To find the number of neutrons in an atom: neutrons = mass number − proton number
Every atom of a given element has the same proton number, which is what makes it that element and no other.
An atom of sodium, Na, has proton number 11 and mass number 23. It therefore has 11 protons, 11 electrons, and 23 − 11 = 12 neutrons.
Section 4
How Are Electrons Arranged in Shells?
Electrons occupy shells around the nucleus, filling from the shell closest to the nucleus outwards.
- The maximum number of electrons in the first three shells is 2, 8, 8
- Electronic configurations are written listing the number of electrons in each shell, e.g. 2,8,3 for an atom with 13 electrons
- The electronic configuration of an atom can be worked out directly from its proton number (which equals the number of electrons in a neutral atom)
- This configuration also applies to ions — an ion has gained or lost electrons compared with the neutral atom
Don't fill shells unevenly — always fill the innermost shell first (up to 2), then the next (up to 8), before starting the next shell out.
Must Know
- An element contains only one type of atom; a compound is two or more elements chemically combined; a mixture is not chemically combined and can be separated physically
- Atoms have a central nucleus (protons + neutrons) surrounded by electrons in shells
- Proton: charge +1, mass 1. Neutron: charge 0, mass 1. Electron: charge -1, mass ~0
- Proton number = number of protons; mass number = protons + neutrons; neutrons = mass number − proton number
- Electron shells fill from the nucleus outwards, maximum 2, then 8, then 8
- An atom is neutral overall because it has equal numbers of protons and electrons
That's the notes covered.
Carry on to the next subtopic.