Particles
Constituents of the Atom
AQA A-level Physics
Protons, neutrons and electrons
| Particle | Charge / C | Relative charge |
|---|---|---|
| Proton | +1.60 × 10−19 | +1 |
| Neutron | 0 | 0 |
| Electron | −1.60 × 10−19 | −1 |
| Particle | Mass / kg | Relative mass | Specific charge / C kg−1 |
|---|---|---|---|
| Proton | 1.673 × 10−27 | 1 | 9.58 × 107 |
| Neutron | 1.675 × 10−27 | 1 | 0 |
| Electron | 9.11 × 10−31 | 1/1836 | 1.76 × 1011 |
- The charge on a proton is e, 1.6 × 10−19 C.
- Mass of a nucleon, a proton or a neutron: 1.67 × 10−27 kg.
Proton number, nucleon number and nuclide notation
- Proton number Z: the number of protons in the nucleus.
- Nucleon number A: the number of nucleons, protons and neutrons together.
- Nuclide notation: AZX, with the nucleon number A on top, the proton number Z below and X the symbol of the element.
number of neutrons = nucleon number − proton numberN = A − Z
Isotopes
- Isotopes: nuclei with same number of protons but different numbers of neutrons.
- Comparing two isotopes of one element: the heavier has more neutrons; the lighter has less mass and fewer neutrons.
- Nuclei of isotopes have the same charge and different masses, so different specific charges.
Isotopes differ only in neutrons
Two isotopes of one element have the same number of protons, so the whole difference between them is in neutrons. Write that the heavier isotope has more neutrons. Never write isotopes have different numbers of protons, and never write isotopes have the same atomic number: name the protons themselves.
Specific charge
specific charge = charge ÷ massspecific charge = Q ÷ m
- Unit: C kg−1.
- Specific charge is inversely proportional to mass for the same charge: same charge and less mass gives a greater specific charge.
- Ions of two isotopes with equal charge: the specific charge on the heavier ion is less than the specific charge on the lighter ion.
- Charge of nucleus = proton number × e.
- Charge of an ion = number of electrons removed or added × e.
- Mass of nucleus = nucleon number × 1.67 × 10−27 kg.
- A nucleus of n nucleons with one proton: specific charge = proton specific charge ÷ n.
Method: number of neutrons from specific charge
- Charge on the nucleus = specific charge × mass.
- Number of protons = charge ÷ e.
- Number of nucleons = mass ÷ 1.67 × 10−27 kg, rounded to a whole number.
- Number of neutrons = nucleon number − proton number.
Quantities and units
| Quantity | Symbol | Unit |
|---|---|---|
| Proton number | Z | no unit |
| Nucleon number | A | no unit |
| Number of neutrons | N | no unit |
| Charge | Q | C |
| Elementary charge | e | C |
| Mass | m | kg |
| Specific charge | none | C kg−1 |
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