Section 1 of 5Subatomic Particles
Atoms are made of three subatomic particles with very different properties.
Proton
- A particle in the nucleus with relative charge +1 and relative mass 1.
Neutron
- A particle in the nucleus with no charge and relative mass 1, which adds mass without changing the element.
Electron
- A particle with relative charge −1 and a mass about 1 ÷ 1840 of a proton, which orbits the nucleus.
Atom as a whole
- An atom is electrically neutral, with equal numbers of protons and electrons.
| Particle | Relative charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | About 1 ÷ 1840 | Orbits the nucleus |
Section 2 of 5Size and Structure
Size and Structure
Atom size
- The radius of an atom is about 1 × 10⁻¹⁰ m, so a very large number of atoms could fit across a thumbnail.
Nucleus size
- The nucleus has a radius of about 1 × 10⁻¹⁴ m and contains almost all of the mass of the atom.
Scale
- The nucleus is over 10 000 times smaller in radius than the whole atom, which is mostly empty space.
Electron shells
- Electrons occupy shells at set distances from the nucleus, arranged around its outside.
Section 3 of 5Atomic Notation
Atomic Notation
Proton number (Z)
- The number of protons in the nucleus, which identifies the element and is written as the bottom number.
Nucleon number (A)
- The total number of protons and neutrons in the nucleus, written as the top number.
Neutron number
- Found by subtracting the proton number from the nucleon number, so neutrons = A − Z.
Electrons in a neutral atom
- A neutral atom has the same number of electrons as protons, so the electron number equals the proton number.
Section 4 of 5Worked Example
Question
- Uranium-235 is written as ²³⁵₉₂U.
- Calculate the number of protons, neutrons and electrons in one neutral atom.
- Protons = proton number = 92.
- Neutrons = nucleon number − proton number = 235 − 92 = 143.
- Electrons = protons in a neutral atom = 92.
Answer The atom contains 92 protons, 143 neutrons and 92 electrons.
Section 5 of 5Exam Checklist & Tips
7.1.1 – Key Points for the Exam
- Proton A positively charged particle in the nucleus.
- Neutron An uncharged particle in the nucleus.
- Electron A negatively charged particle that orbits the nucleus.
- Nucleus The tiny, dense centre of an atom, containing protons and neutrons.
- Proton number The number of protons in an atom, which identifies the element.
- Nucleon number The total number of protons and neutrons in a nucleus.
Common Mistakes
- Saying electrons are inside the nucleus; they orbit around it.
- Confusing proton number with nucleon number.
- Giving the neutron a positive charge; neutrons are neutral.
- Treating the nucleus as roughly the same size as the whole atom.