Part 1: Particles, ions and isotopes

Use proton number, mass number and charge to count particles, then explain what changes between isotopes and ions.

The structure of an atom

The tiny nucleus contains protons and neutrons; electrons occupy orbitals around it. Nearly all the mass is in the nucleus, but most of the atom’s volume is the region occupied by electrons. Atomic number Z counts protons and identifies the element. Mass number A counts protons plus neutrons.

Read the three numbers in an atom symbol

In a full nuclide symbol the mass number belongs at the upper left and atomic number at the lower left; the ionic charge goes at the upper right. These numbers answer different questions. The periodic table supplies atomic number even if it is omitted from the symbol. Its decimal relative atomic mass must not be rounded and used as the mass number of an unspecified isotope.

For an ion, calculate protons first, neutrons second and electrons last. Protons identify the element, A − Z gives neutrons, and the charge tells you how electrons differ from protons. A 3+ charge means three fewer electrons, not three extra protons.

Charge changes electrons, not the nucleus

A neutral atom has Z electrons. A positive ion has lost electrons; a negative ion has gained them. For an ion of charge +q, electrons = Z − q. Forming an ordinary ion does not change the element or its mass number.

Worked example: ²⁷Al³⁺ has Z = 13, so it contains 13 protons, 27 − 13 = 14 neutrons and 13 − 3 = 10 electrons. For ³⁴S²⁻, Z = 16 gives 16 protons, 18 neutrons and 18 electrons.

Work backwards from a particle inventory

An unknown ion contains 18 electrons, 20 neutrons and has charge 2+. Because p − e = +2, p = 20. Element 20 is calcium. Its mass number is 20 + 20 = 40, giving ⁴⁰Ca²⁺. The 18 electrons alone would also fit neutral Ar or K⁺; the charge resolves the identity.

Compare ³⁶Ar, ³⁹K⁺ and ⁴⁰Ca²⁺: all contain 18 electrons, but have different proton numbers. They are isoelectronic, not isotopes. Compare ³⁵Cl⁻ and ³⁷Cl⁻ instead: same element and charge, same electron count, different neutron counts. This distinction prevents the common mistake of defining an element by its electrons.

Same element, different neutron count

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Their mass numbers differ. Neutral isotopes have the same electron configuration and broadly the same chemistry, but their masses and some physical properties differ.

³⁵Cl and ³⁷Cl each have 17 protons. Their neutron counts are 18 and 20. Chloride ions of those isotopes each have 18 electrons: isotope and ionic charge describe different features.

Turn a fact into a connected explanation

If asked why neutral isotopes have similar chemical properties, connect proton number to the electron arrangement: equal proton numbers mean equal electron numbers in neutral atoms; the same arrangement, particularly the outer electrons, produces similar bonding and reactions. Merely saying “they are the same element” restates the question.

If the question asks for the difference between isotopes, answer with neutron number or mass number as appropriate. Do not introduce electron loss: that describes ion formation. A nuclear change can alter the element or isotope; ordinary chemical reactions rearrange electrons and atoms without changing nuclei.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources