Home › Learning Hub › IGCSE Chemistry › 2a Atoms, elements and isotopes
Topic 2 · 2.1–2.3

Atoms, elements and isotopes

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe the differences between elements, compounds and mixtures.
  • Describe the atom (nucleus of protons and neutrons, electrons in shells) and state the relative charges and masses of the particles.
  • Use proton number (atomic number) and mass number (nucleon number), and symbols such as 126C and 3517Cl−.
  • Write the electronic configuration of elements and ions with proton numbers 1 to 20, and link it to group and period.
  • Define isotopes; explain why isotopes have the same chemical properties EXTENDED.
  • Calculate relative atomic mass from isotope masses and abundances EXTENDED.

📚The chemistry

Elements, compounds and mixtures

  • An element contains only one type of atom.
  • A compound contains two or more elements chemically combined in a fixed ratio; it has different properties from its elements and can only be separated by a chemical reaction.
  • A mixture contains two or more substances not chemically combined; the proportions can vary and it can be separated by physical methods such as filtration or distillation (topic 12a).
Four boxes of particles. An element: single blue atoms and blue two-atom molecules of the same kind. A compound: identical molecules each made of one blue and two orange atoms joined together. A mixture of elements: separate blue molecules and orange atoms. A mixture of an element and a compound: blue molecules mixed with blue-orange compound molecules.
Look for (1) how many kinds of atom, and (2) whether different kinds are joined together.

Inside the atom

proton: relative charge \(+1\), relative mass 1
neutron: relative charge 0, relative mass 1
electron: relative charge \(-1\), relative mass \( \tfrac{1}{1836} \) (almost zero)

The proton number (atomic number, \(Z\)) is the number of protons in the nucleus; it identifies the element. The mass number (nucleon number, \(A\)) is the total number of protons and neutrons. So neutrons \( = A - Z \). An atom is neutral, so it has as many electrons as protons; an ion has gained or lost electrons, never protons.

Electrons fill shells in order: 2 in the first, 8 in the second, 8 in the third, then the fourth begins (for proton numbers up to 20). The configuration of sodium is 2,8,1 and of calcium 2,8,8,2. For ions, add or remove electrons: Na+ is 2,8 and Cl− is 2,8,8.

  • number of outer-shell electrons \( = \) group number (Groups I to VII);
  • number of occupied shells \( = \) period number;
  • Group VIII noble gases have a full outer shell, which is why they are unreactive.
Two shell diagrams. Left: an oxygen atom with a nucleus of 8 protons and 8 neutrons, 2 electrons in the first shell and 6 in the second, configuration 2,6. Right: an oxide ion with the same nucleus and 2 and 8 electrons, configuration 2,8, drawn in square brackets with a 2 minus charge; the two gained electrons are shown as crosses.
The oxide ion has the same nucleus as the atom; it has gained 2 electrons (crosses) to fill its outer shell.

Isotopes

Isotopes are different atoms of the same element with the same number of protons but different numbers of neutrons: 35Cl has 18 neutrons and 37Cl has 20. EXTENDED Isotopes have the same chemical properties, because they have the same number of electrons and so the same electronic configuration — and chemistry is about electrons.

EXTENDED The relative atomic mass, Ar, is the average mass of an element’s atoms, weighted by the abundance of each isotope:

\[ A_\mathrm{r} = \frac{\sum (\text{isotope mass} \times \text{abundance})}{\sum \text{abundances}} \]
A bar chart of relative abundance against mass for chlorine: a bar of 75 percent at mass 35 and a bar of 25 percent at mass 37. The weighted mean, 35.5, is marked with a dashed line between them, nearer 35.
Chlorine: 75% 35Cl and 25% 37Cl. The average, 35.5, sits nearer the more abundant isotope.

✏️Worked example

(a) State the numbers of protons, neutrons and electrons in 2713Al3+, and give the electronic configuration of the ion. [3] (b) An element has electronic configuration 2,8,6. State its group and period and identify it. [2] (c) EXTENDED Chlorine is 75% 35Cl and 25% 37Cl. Calculate its relative atomic mass. [2]

(a) 13 protons; \( 27 - 13 = 14 \) neutrons; \( 13 - 3 = 10 \) electrons. The atom is 2,8,3; the ion has lost its 3 outer electrons: 2,8.

(b) 6 outer electrons: Group VI. 3 shells: Period 3. Proton number \( 2 + 8 + 6 = 16 \): sulfur.

(c) \( A_\mathrm{r} = \dfrac{35 \times 75 + 37 \times 25}{100} = \dfrac{2625 + 925}{100} = 35.5 \).

Check it. Ar must lie between the isotope masses and nearer the more common one: 35.5 is between 35 and 37, nearer 35 ✓.
Changing the protons. A 3+ ion has lost electrons. Answering “10 protons” would make it a different element (neon).

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) How many protons, neutrons and electrons are in 199F−? A: 9, 10, 9. B: 9, 10, 10. C: 10, 9, 10. D: 9, 19, 10.
B. 9 protons, \( 19 - 9 = 10 \) neutrons, and one extra electron: 10.
2. (Multiple choice.) Which is a mixture? A: air. B: water. C: sodium chloride. D: carbon dioxide.
A. Air is several gases not chemically combined; the others are compounds.
3. (Theory.) Give the electronic configurations of a calcium atom and a calcium ion, Ca2+. [2]
Ca: 2,8,8,2. Ca2+: 2,8,8.
4. (Theory.) Element X has three occupied shells and five electrons in its outer shell. State its period and group, and name it. [3]
Period 3, Group V; configuration 2,8,5 means proton number 15: phosphorus.
5. (Theory.) Define the term isotopes. [2]
Different atoms of the same element that have the same number of protons but different numbers of neutrons.
6. (Theory.) EXTENDED Explain why 35Cl and 37Cl have the same chemical properties. [2]
They have the same number of electrons, so the same electronic configuration; chemical reactions depend on the electrons.
7. (Theory.) EXTENDED Boron is 20% 10B and 80% 11B. Calculate the relative atomic mass of boron. [2]
\( \dfrac{10 \times 20 + 11 \times 80}{100} = \dfrac{1080}{100} = 10.8 \).
8. (Theory.) EXTENDED Copper has Ar 63.5 and two isotopes, 63Cu and 65Cu. Calculate the percentage abundance of 63Cu. [3]
Let \(x\)% be 63Cu: \( 63x + 65(100 - x) = 6350 \Rightarrow 6500 - 2x = 6350 \Rightarrow x = 75 \). So 75% 63Cu.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • PhET “Build an Atom” — add protons, neutrons and electrons and watch the symbol and charge change
  • Royal Society of Chemistry — the interactive Periodic Table, with isotope data for every element