Subatomic particles and atomic notation
An atom has three particles. Relative mass and charge: proton mass 1, charge +1; neutron mass 1, charge 0; electron mass negligible (~1/2000), charge −1. Protons and neutrons sit in the nucleus; electrons occupy energy levels (shells) around it. In a neutral atom protons = electrons. Proton number (Z) = number of protons, which identifies the element. Nucleon number (A) = protons + neutrons, so neutrons = . Notation: .
Electronic configuration, group and period
Shells fill lowest-energy first: shell 1 holds up to 2, shell 2 up to 8, shell 3 up to 8 (for the first 20 elements), then shell 4. The total electrons (= proton number for a neutral atom) split across the shells: e.g. Na (11) is 2.8.1, P (15) is 2.8.5, Ca (20) is 2.8.8.2. Read position from the configuration: period = number of occupied shells; group = electrons in the outer shell. Noble gases have a full outer shell but sit in Group 0.
(Extended) Isotopes — same element, different neutrons
Isotopes are atoms of the same element with different numbers of neutrons; both parts are needed for credit. They react identically because chemistry depends on electrons, and isotopes share the same electron configuration; they differ only physically (mass, density). Chlorine's and are both 2.8.7. Because is the weighted average over isotope abundances, chlorine's , not a whole number.
Drawn from real examiner reports.
Electrons = proton number, not nucleon
The number of electrons equals the proton number, not the nucleon number. Carbon (proton number 6) is 2.4, not 2.8.2 — writing 2.8.2 wrongly uses the nucleon number 12 as the electron count. Always place a number of electrons equal to the proton number, then fill shells in order 2, 8, 8.
W22 P31 Q2b; S22 P31 Q5e(iv)
Shell 3 fills to 8 before shell 4
For the first 20 elements shell 3 holds a maximum of 8 before shell 4 begins. Calcium (proton number 20) is 2.8.8.2, not 2.8.10.2 (overfilling shell 3) and not 2.8.8.4 (confusing the group number with the outer count). Check: the shell electrons must total the proton number.
W22 P31 Q2b; S22 P31 Q5e(iv)
(Extended) Isotopes: "different neutrons", not masses
The definition must say atoms of the same element with different numbers of neutrons. "Same element but different masses" scores zero — it omits neutrons, the atomic-level distinction. "Different mass numbers" is weaker than "different neutrons". Dropping "atom" (writing "particles" or "species") also loses credit.
S23 P41 Q2b
Period is shells; group is outer electrons
The period is the number of occupied electron shells; the group is the number of electrons in the outer shell. Asked which element has the most occupied shells, candidates wrongly pick the highest group number — e.g. choosing fluorine (2 shells) over potassium (4 shells). The number of shells gives the period, not the group.
W22 P13 Q21; S23 P31 Q1c
Ions: adjust electrons, not protons
When an ion forms the proton number never changes — only electrons are adjusted. A positive ion has fewer electrons (subtract the charge): has 10 electrons, not 11. A negative ion has more (add the charge). Also, do not confuse the charge with the nucleon (mass) number — they are unrelated.
W22 P13 Q4; W22 P23 Q4
Four-step atomic-structure scaffold
For any : (1) use to name the element and give a neutral atom's electron count; (2) neutrons ; (3) write the configuration by filling 2, 8, 8; (4) shells give the period, outer electrons the group. Check the shells total .
Ions: adjust electron count only
For an ion, keep the proton number fixed and change only electrons: a positive ion loses electrons (subtract the charge), a negative ion gains them (add the charge). E.g. has 18 electrons (2.8.8), not 19.
Shells give period, outer gives group
Get the order right: count the occupied shells first to give the period, then read the outer-shell electrons to give the group (Group 0 if the outer shell is full). Reversing these is one of the most penalised 0620 errors.
Atom: The smallest particle of an element that can exist. An atom consists of a central nucleus containing protons and neutrons, surrounded by electrons arranged in energy levels (shells).
Proton number (atomic number): The number of protons in the nucleus. Every atom of the same element has the same proton number. For a neutral atom, proton number = number of electrons.
Nucleon number (mass number): The total number of protons and neutrons in the nucleus.
Standard notation: An atom of element X is written , where = nucleon number and = proton number.
Subatomic particles — relative mass and charge:
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | negligible () | −1 | Shells around nucleus |
In a neutral atom: number of protons = number of electrons.
(Extended) Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons. Both elements of this definition are required for full marks. Isotopes have identical chemical properties (same electron configuration) but different physical properties (different mass).
(Extended) Relative atomic mass (): The weighted average mass of an atom of an element relative to 1/12 the mass of a carbon-12 atom, taking into account the abundance of each isotope. Because chlorine has two isotopes ( and ), its .
Define proton number (atomic number).
An atom of sodium is represented as .
(a) State the number of protons, neutrons and electrons in this atom.
(b) State the location of the protons and neutrons in the atom.
(c) State the relative charge of an electron.
