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Atomic Structure: an interactive chemistry course

Published 5 min read

Atomic Structure is an interactive atomic structure course about what atoms are made of and why that decides how elements behave. Its 23 short sections cover how the model of the atom changed, the gold foil experiment, protons, neutrons and electrons, atomic and mass numbers, isotopes, relative atomic mass, electron shells and subshells, the periodic table and ions.

It opens with an atom builder: add protons, neutrons and electrons and watch the element, mass number, charge and shells update as you go.

What the course covers

SectionsTopicWhat you do
1 to 4What an atom is, models of the atom and the gold foil experimentMatch each gold foil result to what it showed
5 to 7Protons, neutrons and electronsSort facts about mass, charge and place
8 to 9Atomic number and mass numberRead nuclear notation and count the particles
10 to 13Isotopes and relative atomic massSpot an isotope, then calculate a relative atomic mass
14 to 17Electron shells, filling them, and s, p and d subshellsFill the shells for an element
18 to 19Shells and the periodic tableFind the element from its electron configuration
20 to 23Ions, why the outer shell matters, and a reviewMake an ion with a full outer shell and read its charge

Inside the atom

The model of the atom changed as experiments disagreed with it. In 1909 Geiger and Marsden fired alpha particles at thin gold foil; a few bounced back, which showed that the atom has a tiny, dense, positive nucleus. Protons and neutrons, each with a relative mass of 1, sit in the nucleus, while electrons, with almost no mass and a charge of −1, are arranged around it.

Numbers that describe an atom

The proton count of an atom, its atomic number, fixes which element it is; add the neutrons and you get the mass number. Isotopes share the proton count of their element but carry different numbers of neutrons, and the relative atomic mass is the average mass of an element's atoms, weighted by how common each isotope is.

Electron shells

Electrons fill the lowest shell with room first: up to 2 in the first shell and 8 in the second. The number of outer electrons decides an element's group in the periodic table and how it reacts, and ions form when atoms lose or gain electrons to reach a full outer shell.

24
Carbon has 6 protons and 6 neutrons in its nucleus, and 6 electrons arranged 2, 4

Try a question from the course

Isotopes match in protons and differ in neutrons. Carbon-12 and carbon-14 are isotopes: both have 6 protons, but carbon-12 has 6 neutrons and carbon-14 has 8. Nitrogen-14 has the same mass number as carbon-14 but 7 protons, so it is a different element, not an isotope of carbon.

Now pick the isotope of a new atom, then press Mark to check your answer in the course.

Pick the atom that is an isotope of the one shown.

Press Mark to have your answer marked in the Atomic Structure course.

Who this course is for

  • GCSE chemistry and combined science students revising atomic structure, isotopes and electron configurations.
  • Students starting A-level chemistry who want subshells and ions on a firm footing.
  • Anyone who wants to understand what the numbers on the periodic table mean.

What you need to know first

Nothing beyond basic science. The relative atomic mass practice uses percentages and a weighted average, which the course works through. Our guides to the periodic table and ions go deeper on two topics.

How long it takes

There are 23 sections and most take a few minutes. Learners without a plan can open two sections of a course a day, in order, so the whole course takes 12 days. A plan removes the daily limit and lets you go straight to any topic, such as relative atomic mass.

Guided courseChemical BondingThe next step: how outer electrons make ionic, covalent and metallic bonds.

Common questions

Is this atomic structure course free?
You can take every section without paying, two sections a day, in order. A plan removes the daily limit and opens any section straight away.
What are the three particles in an atom?
Protons, which are positive and in the nucleus; neutrons, which have no charge and are also in the nucleus; and electrons, which are negative, have almost no mass and are arranged in shells around the nucleus.
What is the difference between atomic number and mass number?
The atomic number counts protons only, while the mass number adds the neutrons on top. Taking the first from the second leaves the neutron count.
What is an isotope?
A version of an element whose atoms carry a different number of neutrons: same atomic number, different mass number. Isotopes of an element react the same way chemically.
What did the gold foil experiment show?
Most alpha particles passed straight through gold foil, so the atom is mostly empty space. A few bounced back, so its positive charge and most of its mass are in a tiny nucleus.
How many electrons go in each shell?
In the simple model used at GCSE, you can fit 2 electrons in the innermost shell, then 8 in each of the next two. The course also explains s, p and d subshells for those who want more.

Revision cards

Say the answer first, then flip the card to check it.

Where to go next

  • Solve it

    Electron Configuration Tool

    Fill the shells and orbitals of any element

    Open the tool
  • Learn it

    Atomic Structure

    Electron shells, orbitals and the periodic table: the foundation of all chemistry

    Start the course
  • Test it

    GCSE Chemistry Higher

    A new exam-style paper every day, with instant marking and a mark scheme

    Start today's paper
Read next
Chemistry · Atoms and the periodic tableWhat is relative atomic mass? Definition and calculationRelative atomic mass is the weighted average mass of an element's atoms compared with carbon-12. The definition and how to calculate it from isotopes.