Edexcel International A Level Chemistry

Topic 2: Atomic Structure and the Periodic Table

What is inside an atom, how we know, and how the answer explains the shape of the Periodic Table and the pattern of properties across a period.

The one idea

You cannot see inside an atom, so chemists read the evidence. Two experiments carry this topic.

The mass spectrometer weighs atoms and molecules. It shows that most elements are mixtures of isotopes, and it explains why the relative atomic mass of chlorine is 35.5 when no chlorine atom has a mass of 35.5.

Ionisation energies measure how tightly electrons are held. They show that electrons sit in shells, that the shells are divided into sub-shells, and that the sub-shells are made of orbitals holding at most two electrons each.

That model predicts the electronic configuration of every element from hydrogen to krypton, the shape of the Periodic Table, and the repeating pattern of properties across a period.

How to use these notes

• Read the pages in order the first time. Later pages build on the earlier ones.
• Each page opens with its one idea in a sentence. If you can explain that sentence to someone else, you know the page.
• Coloured boxes mean the same thing on every page: definitions you may be asked to quote, key ideas, common mistakes, exam technique, and beyond the spec (interesting, but you will not be examined on it).
• Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
• Use the topic on a page the night before an exam.

The most heavily marked skill in this topic is explanation. Every ionisation energy trend, and every melting or boiling temperature trend, has a pattern of reasons that earns the marks. Learn the patterns, then apply them to new data.

Relative atomic masses used in these notes are the values printed on the Edexcel IAL periodic table, to one decimal place. Isotope masses in the mass spectra calculations are the mass numbers, unless a question gives exact masses.

Pages

1. Inside the atom

   An atom is a tiny nucleus of protons and neutrons with electrons around it. Two whole numbers, the atomic number and the mass number, tell you how many of each particle an atom or ion contains.

   12 min 2.12.22.32.4

2. Isotopes

   Atoms of one element can have different numbers of neutrons. They behave the same way chemically, but they have different masses.

   8 min 2.5

3. The mass spectrometer

   A mass spectrometer sorts ions by their mass-to-charge ratio and counts them. The result is a spectrum with one peak for each isotope, and its height shows how much of that isotope there is.

   13 min 2.62.6(i)

4. Relative atomic mass from mass spectra

   The relative atomic mass of an element is the weighted mean of its isotope masses. You can work it out from a spectrum, and work backwards from it to the abundances of two isotopes.

   14 min 2.6(ii)

5. Molecules and 2+ ions

   For a molecule, the peak with the largest m/z is the molecular ion, and its m/z is the relative molecular mass. Ions with a 2+ charge give extra peaks at half the mass.

   13 min 2.6(iii)2.6(iv)

6. Predicting spectra of diatomic molecules

   A molecule made of two atoms of an element with two isotopes gives three molecular ion peaks. Their heights come from the chance of each pairing, which for chlorine gives the ratio 9 to 6 to 1.

   12 min 2.7

7. Ionisation energy

   Ionisation energy measures how tightly an atom holds an electron. It is always endothermic, and its size depends on the nuclear charge, the shielding, and the sub-shell the electron comes from.

   12 min 2.82.10

8. Successive ionisation energies

   Take the electrons off an atom one at a time and the energies jump each time you start on a new shell. The jumps are evidence for shells, and the position of the first jump gives the group.

   12 min 2.11(i)

9. First ionisation energy of elements 1 to 36

   Plot the first ionisation energy of the first 36 elements and the same pattern repeats after each noble gas. That repeat defines a periodic property, and the small dips inside each period are evidence for sub-shells.

   14 min 2.172.11(ii)

10. Orbitals, sub-shells and shells

    Electrons live in orbitals, each holding at most two. Orbitals are grouped into sub-shells (s, p, d) and sub-shells into shells. This page gives the shapes, the capacities and the energy order you need to fill them.

    13 min 2.92.122.16

11. Filling the orbitals

    Electrons fill the lowest-energy sub-shell first, spread out singly across the orbitals of a sub-shell, and only then pair up with opposite spins. Use these rules to write configurations from hydrogen to argon.

    14 min 2.13(i)2.13(ii)2.14

12. Potassium to krypton, and ions

    After argon the 4s sub-shell fills, then 3d, then 4p. Chromium and copper break the pattern. Positive ions lose their outer electrons first, which means 4s electrons before 3d in the d block.

    14 min 2.14

13. Blocks of the Periodic Table, and why configuration matters

    The Periodic Table is organised by electronic configuration. Elements fall into s, p and d blocks by the sub-shell being filled, and the outer electrons decide how an element reacts.

    11 min 2.162.15

14. Explaining ionisation energy trends

    Across a period the first ionisation energy rises overall, with two dips at the start of each p sub-shell and at the first pair. Down a group it falls. Each trend is explained by nuclear charge, shielding and distance, and sub-shell.

    15 min 2.18(ii)2.18(iii)2.10

15. Melting and boiling temperatures of Periods 2 and 3

    Melting temperature rises, peaks at the giant covalent elements, then falls sharply at the simple molecules. The pattern is explained by the structure of each element and the forces that must be overcome.

    15 min 2.18(i)

Also in this topic: Topic 2 on a page

Spec checklist

Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.