Part 1: Particles, isotopes and relative masses

Start with the particles in one species, then distinguish a nucleon count from a relative mass and a molecular formula from a giant structure.

The nuclear atom

An atom has a small, dense, positively charged nucleus containing protons and neutrons. Electrons occupy regions around that nucleus. Almost all the mass is in the nucleus, but most of the atom’s volume lies outside it. Electrostatic attraction between the positive nucleus and negative electrons holds the atom together.

A neutral atom contains equal numbers of protons and electrons. An ion has gained or lost electrons; its nucleus is unchanged in an ordinary chemical reaction. Losing electrons gives a positive ion because some positive nuclear charge is no longer balanced.

Read a nuclide symbol before counting

In the notation ²⁷₁₃Al³⁺, the upper left number A = 27 is the mass number: the total number of protons and neutrons. The lower left number Z = 13 is the atomic number: the number of protons. The upper right charge compares protons with electrons; it is not an extra number of protons.

Worked example: aluminium-27 has 13 protons and 27 − 13 = 14 neutrons. Al³⁺ has lost three electrons, so it has 13 − 3 = 10 electrons. For ³⁴₁₆S²⁻, the answers are 16 protons, 18 neutrons and 18 electrons. The negative ion has gained electrons.

For a molecule or polyatomic ion, add the nuclear contributions from every atom, then apply the overall charge once. A ¹⁴N¹H₄⁺ ion contains 7 + 4(1) = 11 protons, 7 + 4(0) = 7 neutrons and 11 − 1 = 10 electrons. Its charge does not identify which individual atom lost an electron.

Isotopes change the nucleus, not the element

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Chlorine-35 and chlorine-37 both have 17 protons; they have 18 and 20 neutrons respectively. A different proton number would define a different element.

Neutral isotopes have the same electron configuration and therefore very similar chemical properties. Their masses differ, so some physical properties and reaction rates can differ slightly. Saying they have identical masses or different electron numbers would miss the defining distinction.

If a question describes a change in a nucleus, recalculate both A and Z. Removing two protons and one neutron from ²⁴₁₂Mg gives A = 21 and Z = 10, hence ²¹₁₀Ne. Removing three nucleons lowers A by three, but only the two protons lower Z.

Relative mass uses a common carbon-12 reference

Relative isotopic mass is the mass of one atom of an isotope divided by one twelfth of the mass of one carbon-12 atom. Relative atomic mass, Aᵣ, is the weighted mean mass of an atom of an element divided by that same reference mass. The weighting reflects the isotope abundances in the sample.

These are ratios of masses, so their units cancel. Aᵣ is not a mass in grams and need not be a whole number. Mass number is a whole-number count of nucleons in one isotope; it is not the weighted mean for an element. Exact isotopic masses are close to, but not necessarily exactly equal to, their mass numbers.

Use relative molecular mass, Mᵣ, for a molecule. Use relative formula mass for a giant ionic or covalent structure, where the formula gives an atom or ion ratio rather than one discrete molecule. Both are found by adding the relevant Aᵣ values. Molar mass has the same numerical value but units of g mol⁻¹; it answers a different question.

Worked example: include every atom in a hydrate

Using Aᵣ(Ca) = 40.1, C = 12.0, O = 16.0 and H = 1.0, the relative formula mass of CaCO₃ is 40.1 + 12.0 + 3(16.0) = 100.1. It has a giant ionic structure, so describing this as the mass of a CaCO₃ molecule would be misleading.

Using Mg = 24.3 and S = 32.1, MgSO₄·7H₂O has relative formula mass 24.3 + 32.1 + 4(16.0) + 7[2(1.0) + 16.0] = 120.4 + 126.0 = 246.4. The dot indicates water of crystallisation; the seven multiplies the entire H₂O formula. There are 11 oxygen atoms in total, not five.

Quick checks

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

Sources