Atomic Structure
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge IGCSE Chemistry (0620)
The Structure of the Atom
An atom has a small, dense nucleus containing protons and neutrons, surrounded by electrons arranged in shells.
| Particle | Location | Relative mass | Relative charge |
|---|---|---|---|
| Proton | Nucleus | 1 | +1 |
| Neutron | Nucleus | 1 | 0 |
| Electron | Shells around the nucleus | Very small (negligible) | -1 |
Almost all of an atom's mass is packed into a nucleus that occupies only a tiny fraction of its total volume
- The nucleus contains almost all of the atom's mass, since protons and neutrons are about 1840 times heavier than electrons
- Electrons occupy shells (energy levels) around the nucleus; the space they move through is mostly empty
- Atoms are electrically neutral because the number of protons (positive) equals the number of electrons (negative)
Atomic Number and Mass Number
- Atomic (proton) number, Z: the number of protons in the nucleus
- Defines which element the atom is - no two elements share an atomic number
- In a neutral atom, atomic number = number of electrons
- Mass (nucleon) number, A: the total number of protons and neutrons in the nucleus
- Number of neutrons = mass number - atomic number
Atoms are represented using the notation AZX, with the mass number as a superscript and the atomic number as a subscript, e.g. 23₁₁Na or 35₁₇Cl⁻.
Worked Example: Finding Subatomic Particles
- Question: an atom of chlorine is represented as chlorine-35 (atomic number 17). State the number of protons, neutrons, and electrons it contains. [3]
- Answer: protons = 17 (the atomic number) [1]; neutrons = mass number - atomic number = 35 - 17 = 18 [1]; electrons = 17, the same as the number of protons, since the atom is neutral [1]
Common Mistakes
- ❌ Subtracting the wrong way round when finding the number of neutrons
- ✓ neutrons = mass number − atomic number, always the larger number (mass number) minus the smaller (atomic number)
- ❌ Thinking an ion has a different atomic number from its parent atom
- ✓ forming an ion only changes the number of electrons; the number of protons (and so the atomic number/element) never changes
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