Atomic Structure
The atom is mostly empty space — a dense positive nucleus surrounded by orbiting electrons.
Structure of the atom:
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1836 (≈ 0) | −1 | Shells around nucleus |
Key numbers:
- Atomic number (Z) = number of protons in nucleus (defines the element)
- Mass number (A) = number of protons + number of neutrons
- Number of neutrons = A − Z
- In a neutral atom: number of electrons = number of protons (= Z)
Nuclide notation:
ᴬ_Z X
- Top number (A) = mass number
- Bottom number (Z) = atomic number
- Example: ¹⁴_₆C = carbon-14; 6 protons, 8 neutrons, 6 electrons
Isotopes:
- Atoms of the same element with the same number of protons but different numbers of neutrons
- Same atomic number Z; different mass number A
- Same chemical properties (same number of electrons → same electron configuration → same reactions)
- Different physical properties (different mass → different density, melting point, boiling point)
- Example: Carbon-12 (⁶ protons, ⁶ neutrons) and Carbon-14 (⁶ protons, ⁸ neutrons)
- Example: Uranium-235 and Uranium-238 (both 92 protons; 143 or 146 neutrons)
Ions:
- Atom that has gained or lost electrons → charged particle
- If atom loses electrons → positive ion (cation): more protons than electrons
- If atom gains electrons → negative ion (anion): more electrons than protons
- Example: Na⁺ has 11 protons and 10 electrons
- Proton: +1, mass 1. Neutron: 0, mass 1. Electron: −1, mass ≈ 0.
- Z = protons; A = protons + neutrons; neutrons = A − Z.
- Isotopes: same Z, different A, different neutron number. Same chemistry.