Atomic Structure
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
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
Filling Electron Shells
Electrons occupy shells (energy levels) around the nucleus, filling the innermost shell first.
- 1st shell: maximum 2 electrons
- 2nd shell: maximum 8 electrons
- 3rd shell: maximum 8 electrons (for elements 1-20)
- Only once a shell is full does the next shell start to fill
You need to know the electronic configuration of elements with proton number 1 to 20, written as a sequence such as 2.8.1 (sodium: 2 electrons in the 1st shell, 8 in the 2nd, 1 in the 3rd).
Shell diagrams show electrons filling the innermost shell first, with only the outermost shell partially filled
Worked Example: Working Out a Configuration
- Question: aluminium has atomic number 13. Work out its electronic configuration. [2]
- Answer: 13 electrons to place: 1st shell takes 2 (11 remaining), 2nd shell takes 8 (3 remaining), 3rd shell takes the remaining 3 [1]; electronic configuration = 2.8.3 [1]
Electronic Configuration and the Periodic Table
- Group VIII (noble gases): have a full outer electron shell (He: 2, Ne: 2.8, Ar: 2.8.8)
- Group number (for Groups I-VII) = number of electrons in the outer shell
- Group I: 1 outer electron (Li: 2.1, Na: 2.8.1, K: 2.8.8.1)
- Group VII: 7 outer electrons (F: 2.7, Cl: 2.8.7)
- Period number = number of occupied electron shells
- Period 2 elements (Li to Ne) all have 2 shells; Period 3 elements (Na to Ar) all have 3 shells
Common Mistakes
- ❌ Starting to fill the 3rd shell before the 2nd shell is full
- ✓ each shell must reach its maximum (2, then 8, then 8) before the next shell receives any electrons - e.g. oxygen is 2.6, not 2.2.4
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