Proton, neutron, electron: mass & charge
An atom has a central nucleus of protons and neutrons, with electrons in shells around it. Recall exactly: a proton has relative mass , charge ; a neutron has relative mass , charge ; an electron has relative mass (negligible), charge . Almost all the mass is in the nucleus, so the mass number counts only protons and neutrons. A neutral atom has equal protons and electrons, so the charges cancel.
Atomic number, mass number, neutrons
The atomic number () is the number of protons — it defines which element the atom is. The mass number () is the total protons + neutrons. So the number of neutrons . In a neutral atom, electrons protons . The notation puts the mass number on top and the atomic number below: has 11 protons, 11 electrons and neutrons.
Isotopes and relative atomic mass
Isotopes are atoms of the same element with the same protons but different numbers of neutrons (same atomic number, different mass numbers). They react identically because they share the same electrons; they differ only physically. The relative atomic mass () is the average mass relative to of a carbon-12 atom, weighted by abundance: .
Drawn from real examiner reports.
Isotopes: state both halves
A definition of isotopes needs TWO parts: (1) atoms of the same element / same number of protons, and (2) different numbers of neutrons (different mass numbers). Vague answers — "atoms that are different", "different versions", "a different mass" — score zero. Writing only "different neutrons" without "same protons" also loses a mark. State both halves.
Recurring across the 4CH1 Paper 1CR examiner reports for June 2024 and June 2023: candidates lose marks on the definition of isotopes by giving an imprecise "different forms of an atom" answer rather than the two-part same-protons / different-neutrons statement.
Mass number ≠ relative atomic mass
Mass number () is protons + neutrons in one atom — always whole. Relative atomic mass () is the weighted average over all isotopes, so usually NOT whole (chlorine from its and ). "The mass number of chlorine is 35.5" is wrong — no single atom has that mass number. Candidates also forget to divide by 100.
Recurring in 4CH1 Paper 1CR examiner reports (June 2024, June 2023): in relative-atomic-mass calculations, weaker candidates multiply mass by abundance but omit the final division by 100, and conflate mass number with relative atomic mass.
Neutrons = mass − atomic number
For , neutrons , NOT and NOT . A common slip is "neutrons = mass number", forgetting to subtract the protons: for neutrons , not 40. Another takes electrons as the mass number; in a neutral atom electrons atomic number. For ions: a positive ion has FEWER electrons than protons, a negative ion MORE.
Recurring in 4CH1 Paper 1CR examiner reports (June 2024, June 2023): candidates state the number of neutrons as the mass number rather than mass number minus atomic number, and confuse electron count with mass number.
Atom vs molecule vs element
Keep these terms separate. An atom is the smallest particle of an element. A molecule is two or more atoms bonded together — it may be an element () or a compound (), so "molecule" does not mean "compound". An element is made of one type of atom; a compound is two or more elements chemically combined.
(common Edexcel chemistry error)
Electrons do not count in mass number
The mass number counts only protons and neutrons. An electron has relative mass about — negligible — so electrons add nothing to it and are never included. A related error is thinking that gaining or losing electrons (forming an ion) changes the mass number; it does not, because only the electron count changes, not the protons or neutrons.
(common Edexcel chemistry error)
Divide by 100, not the isotope count
In a relative atomic mass calculation, divide the sum of (isotope mass percentage abundance) by 100, not by the number of isotopes. Magnesium has three isotopes but you still divide by 100, never by 3 — the abundances are percentages totalling 100. Dividing by the isotope count gives a value outside the isotope-mass range, which cannot be right.
(common Edexcel chemistry error)
Ar: multiply, add, divide by 100
Fixed routine for : (1) multiply each isotope mass by its percentage abundance; (2) add the products; (3) divide by 100. Sanity check — the answer lies between the smallest and largest isotope mass, closer to the more abundant one. Outside that range means a slip.
Read Z and A, then subtract for neutrons
For any , read directly (protons, and electrons if neutral), then subtract for neutrons: . For an ion, change only the electrons — add for negative, subtract for positive — never the protons. Setting it out in three lines avoids slips.
Explain isotope chemistry via electrons
Asked why isotopes react identically, link to electrons, not "same element" alone. Same protons means the same electrons; reactions depend on the outer electrons, so all isotopes react alike. They differ only in neutrons — physical properties, not chemistry.
An atom is the smallest particle of an element that can exist. A molecule is two or more atoms chemically bonded (joined by covalent bonds), e.g. or .
An atom has a tiny, dense central nucleus (containing protons and neutrons) surrounded by electrons in shells (energy levels). Almost all the atom's mass is in the nucleus.
Relative mass and relative charge of the sub-atomic particles (learn this table exactly):
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | In the nucleus | ||
| Neutron | In the nucleus | ||
| Electron | (≈ negligible) | In shells around the nucleus |
Two-element mark-scheme definitions:
Counting particles from the symbol :
Relative atomic mass from isotope abundances:
Define isotopes.
An atom of sodium is represented as .
Calculate the number of neutrons in this atom.
