AQA GCSE Chemistry 8462: A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes – Revision Notes

These notes condense section 4.1.1 A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 onwards and exams in 2018 onwards (version 1.1). They cover spec points 4.1.1.1 to 4.1.1.7, examined on Paper 1 at Foundation and Higher Tier (Paper 2 can draw on them too). The one (HT only) statement is marked Higher tier only.

For full explanations and worked examples, use the study guide for this section. Then test yourself with the practice questions. The topic 4.1 revision notes cover the periodic table and group trends, which this page does not. The course hub is AQA GCSE Chemistry and the printable checklist lists every specification point.

4.1.1.1 Atoms, elements and compounds

Key definitions

• Atom: the smallest part of an element that can exist.
• Element: made of only one type of atom; about 100 exist, shown in the periodic table, each with a symbol.
• Compound: two or more elements chemically combined in fixed proportions. Separated only by chemical reactions.
• Chemical reaction: always makes one or more new substances; often has a detectable energy change.

Symbols to know: the first 20 elements, Groups 1 and 7, and other elements named in the specification.

Naming rules

Formula part	Name
Cl, O, S, Br, I on their own	chloride, oxide, sulfide, bromide, iodide
SO₄	sulfate
CO₃	carbonate
NO₃	nitrate
OH	hydroxide

Method: balancing an equation

1. Word equation first.
2. Correct formulae (never change them).
3. Big numbers in front only.
4. Count every element on both sides.

Worked reminder. Magnesium + hydrochloric acid → magnesium chloride + hydrogen: Mg + 2HCl → MgCl₂ + H₂. Check: Mg 1 = 1, H 2 = 2, Cl 2 = 2.

Higher tier only: ionic and half equations

• Ionic equation: leave out spectator ions (unchanged on both sides).
• Half equation: one species with electrons, e⁻.
• Both must balance in atoms and charge.

Worked reminder: Na → Na⁺ + e⁻ (electron lost, written on the right). Br₂ + 2e⁻ → 2Br⁻ (electrons gained, written on the left).

4.1.1.2 Mixtures

A mixture is two or more elements or compounds not chemically combined. Each keeps its own chemical properties. Separation is physical: no reaction, no new substance.

Mixture	Technique	Why it works
Insoluble solid + liquid	Filtration	Solid particles are trapped by filter paper
Solution, want the solid	Crystallisation	Solvent evaporates, solute crystallises
Solution, want the solvent	Simple distillation	Solvent boils off and is condensed
Liquids that mix, different boiling points	Fractional distillation	Each liquid condenses at its own temperature
Several dissolved dyes	Chromatography	Substances move different distances

Method: choosing a technique

1. Is anything insoluble?            -> filter it out first
2. Want a dissolved solid?           -> crystallisation
3. Want the liquid from a solution?  -> simple distillation
4. Two or more liquids that mix?     -> fractional distillation
5. Several coloured substances?      -> chromatography

Must-know distinction: mixture vs compound

Mixture	Compound
Not chemically combined	Chemically combined
Any proportions	Fixed proportions
Properties of each part kept	New properties
Separated by physical processes	Separated only by chemical reactions

4.1.1.3 The model of the atom

indivisible spheres
  -> electron discovered -> plum pudding (positive ball, electrons embedded)
  -> alpha scattering    -> nuclear model (mass and charge concentrated in a nucleus)
  -> Bohr                -> electrons orbit at specific distances
  -> later experiments   -> positive charge = whole number of protons
  -> Chadwick            -> neutrons in the nucleus (about 20 years after the nucleus was accepted)

• Why the model changed: the plum pudding model could not explain the scattering results, so new evidence forced a new model.
• Plum pudding vs nuclear: spread-out positive charge and mass vs a tiny, dense, charged nucleus with electrons outside it and mostly empty space.
• Bohr’s calculations agreed with experimental observations. You do not need the details of Bohr’s or Chadwick’s experiments.

4.1.1.4 and 4.1.1.5 Particles, size and mass

Particle	Relative charge	Relative mass	Where
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	-1	Very small	Shells

• Atoms: electrons = protons, so no overall charge.
• Atomic number = protons (defines the element). Mass number = protons + neutrons.
• Isotopes: same element, same protons, different neutrons.
• Atomic radius about 0.1 nm = 1 × 10⁻¹⁰ m. Nuclear radius less than 1/10 000 of that, about 1 × 10⁻¹⁴ m.
• Almost all the mass is in the nucleus.

Method: protons, neutrons, electrons

p = atomic number (bottom number)
n = mass number - atomic number
e = p - ion charge      (2+ ion: p - 2; 1- ion: p + 1)

Worked reminder: ²⁴₁₂Mg²⁺ has 12 protons, 24 - 12 = 12 neutrons and 12 - 2 = 10 electrons.

Unit reminder: 1 nm = 1 × 10⁻⁹ m, so 0.1 nm = 0.1 × 10⁻⁹ m = 1 × 10⁻¹⁰ m.

Worked scale reminder. Scale an atom up to a radius of 50 m. The nucleus radius is 50 ÷ 10 000 = 0.005 m = 5 mm. Almost all the mass sits in that tiny centre.

Must-know distinctions

Pair	Difference
Atomic number vs mass number	Protons only vs protons + neutrons
Isotope vs ion	Different neutrons (still neutral) vs different electrons (charged)
Nucleus vs shells	Protons and neutrons, almost all the mass vs electrons, very little mass
Simple vs fractional distillation	Solvent from a solution vs liquids that mix with different boiling points

4.1.1.6 Relative atomic mass

Quantity	Formula
Relative atomic mass, Ar	(Σ isotope mass × percentage abundance) ÷ 100
Neutrons in an isotope	mass number - atomic number

Ar is an average that takes account of the abundance of each isotope, so it is usually not a whole number and always lies between the lightest and heaviest isotope masses; with two isotopes it is nearer the more abundant one.

Worked reminder with three isotopes. Silicon is 92% silicon-28, 5% silicon-29 and 3% silicon-30.

Ar = (28 x 92 + 29 x 5 + 30 x 3) / 100
   = (2576 + 145 + 90) / 100
   = 28.11  ->  28.1 (1 d.p.)

Check the percentages add up to 100 before you start; if one is missing, it is 100 minus the others.

4.1.1.7 Electronic structure

• Electrons fill the lowest available energy levels (innermost shells) first.
• Up to element 20: 2 in the first shell, 8 in the second, 8 in the third, then the fourth.
• Give it as numbers (sodium 2,8,1) or as a diagram (circles round a nucleus with crosses for electrons).
• “Shells” and “energy levels” are both accepted.

Element	Atomic number	Structure
Lithium	3	2,1
Oxygen	8	2,6
Sodium	11	2,8,1
Sulfur	16	2,8,6
Potassium	19	2,8,8,1

Quick self-test

1. What is the smallest part of an element that can exist?
2. Name Na₂SO₄.
3. How many atoms are shown in the formula Al₂(SO₄)₃?
4. Balance: __Na + __Cl₂ → __NaCl
5. Two liquids that mix boil at 80 °C and 110 °C. Which technique separates them?
6. Why can a mixture be separated by physical processes but a compound cannot?
7. What did the plum pudding model say an atom was like?
8. Whose experimental work gave evidence for the neutron?
9. Give the numbers of protons, neutrons and electrons in ¹⁹₉F⁻.
10. Boron is 20% boron-10 and 80% boron-11. Calculate its relative atomic mass.
11. Write the electronic structures of beryllium (4) and phosphorus (15).
12. Higher tier only: write the half equation for a potassium atom forming a potassium ion.

Answers

1. An atom.
2. Sodium sulfate.
3. 2 + 3 + (3 × 4) = 17 atoms.
4. 2Na + Cl₂ → 2NaCl.
5. Fractional distillation.
6. In a mixture the substances are not chemically combined; in a compound they are, so only a chemical reaction can separate them.
7. A ball of positive charge with negative electrons embedded in it.
8. James Chadwick.
9. 9 protons, 10 neutrons, 10 electrons.
10. (10 × 20 + 11 × 80) ÷ 100 = (200 + 880) ÷ 100 = 10.8.
11. Beryllium 2,2; phosphorus 2,8,5.
12. K → K⁺ + e⁻

Where marks are usually lost

• Writing “Mg(NO₃)₂ has 6 atoms”: the 2 multiplies everything inside the bracket.
• Balancing by changing subscripts, for example writing NaCl₂.
• Naming CaCO₃ “calcium carbide” or “calcium oxide”: CO₃ is carbonate.
• Higher tier: putting electrons on the wrong side of a half equation, or leaving the charges unbalanced.
• Choosing evaporation when the question asks for the pure liquid from a solution. That needs distillation.
• Calling a separation process a chemical reaction. It is physical; no new substance forms.
• Describing the plum pudding model as “electrons orbiting a positive centre”. It had no nucleus.
• Saying Bohr discovered the nucleus. The nucleus came from the scattering experiment; Bohr placed electrons at specific distances.
• Subtracting the charge the wrong way for negative ions: a 2- ion has two more electrons than protons.
• Giving Ar as a whole number when the data give a decimal, or forgetting to divide by 100.

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016, exams from June 2018 (version 1.1), published by AQA: section 4.1.1, A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes.