The Mass of AtomsOxford AQA IGCSE Chemistry: Revision notes
Section 1
What are the relative masses of subatomic particles?
Atoms are made of three types of subatomic particle, and each has its own relative mass:
| Particle | Relative mass |
|---|---|
| Proton | 1 |
| Neutron | 1 |
| Electron | very small (often treated as 0) |
Because protons and neutrons have virtually all the mass of an atom, and electrons contribute almost nothing, the mass of an atom is concentrated in its nucleus.
If asked to explain why the nucleus contains almost all the mass of an atom, state clearly that electrons have negligible mass compared with protons and neutrons.
Section 2
What is relative atomic mass (Ar)?
The relative atomic mass (Ar) of an element compares the mass of its atoms with the mass of atoms of the carbon-12 (12C) isotope, which is used as the international standard.
Relative atomic mass has no units, because it is a comparison (a ratio), not an absolute mass.
Section 3
Why is relative atomic mass an average value?
Most elements exist as a mixture of isotopes — atoms with the same number of protons but different numbers of neutrons, so with different masses. Relative atomic mass is therefore an average value for all the isotopes of that element, taking into account how much of each isotope is naturally present.
This is why many Ar values are not whole numbers — for example chlorine has an Ar of 35.5, because it is a mixture of chlorine-35 and chlorine-37 atoms.
Do not confuse relative atomic mass with mass number. Mass number is a whole number for one specific atom; Ar is an average across all isotopes of the element and can be a decimal.
Must Know
- Relative mass of a proton = 1, neutron = 1, electron = very small
- Almost all the mass of an atom is in the nucleus (protons + neutrons)
- Relative atomic mass (Ar) compares an element's atoms with the 12C isotope
- Ar is an average value across all the isotopes of an element
- Ar has no units, because it is a ratio/comparison
That's the notes covered.
Carry on to the next subtopic.