How to draw electron arrangement diagrams

An electron arrangement diagram shows how the electrons of an atom are spread across its shells. It is a small skill with a big payoff, because it underpins matter and the atomic structure, explains an element’s place in the periodic table of elements, and is the starting point for chemical bonding. Once you know the rules, you can draw the diagram for any of the first twenty elements in under a minute.

What the diagram represents

An atom has a tiny central nucleus containing protons and neutrons, surrounded by electrons that occupy shells (energy levels) at increasing distances. In a diagram we draw the nucleus as a small circle in the centre, often labelled with the proton number and nucleon number, and the shells as concentric circles around it. Electrons sit on the shells as dots or crosses.

The key facts you must fix in memory:

• In a neutral atom, number of electrons = number of protons = proton number.
• Electrons fill the innermost shell first, then the next, and so on.
• The maximum each shell holds (for the first 20 elements) is 2 in the first shell, 8 in the second, and 8 in the third.

The step-by-step method

1. Find the proton number from the periodic table. That equals the number of electrons to place.
2. Fill the first shell with up to 2 electrons.
3. Fill the second shell with up to 8.
4. Fill the third shell with up to 8, and put any remaining electrons in the fourth shell.
5. Write the arrangement as numbers separated by dots, for example 2.8.1.
6. Draw it: nucleus in the centre, one circle per occupied shell, and the correct number of electrons on each, spacing them out neatly.

Worked examples

Sodium (proton number 11). Place 11 electrons: 2 in the first shell, 8 in the second, 1 left over in the third. The arrangement is 2.8.1. The diagram is three circles, with 2, 8 and 1 electrons.

Chlorine (proton number 17). Place 17 electrons: 2, then 8, then 7. The arrangement is 2.8.7.

Calcium (proton number 20). Place 20 electrons: 2, 8, 8, then 2 in the fourth shell. The arrangement is 2.8.8.2.

Oxygen (proton number 8). Place 8 electrons: 2, then 6. The arrangement is 2.6.

Notice the pattern: you never overfill an inner shell before moving outward. If you find yourself writing 2.10 or 9 in the first shell, you have broken a rule.

Reading the periodic table from your diagram

The diagram tells you two things at a glance:

• The number of occupied shells = the period the element is in. Sodium (2.8.1) has three shells, so it is in Period 3.
• The number of electrons in the outermost shell = the valence electrons, which fixes the group (for Groups 1, 2 and 13–18). Sodium has 1 valence electron, so it is in Group 1; chlorine has 7, so it is in Group 17.

This is why electron arrangement and the periodic table are really one topic. You can check any element’s arrangement against the SPM periodic table as you practise.

Drawing ions, not just atoms

Ions are where students lose easy marks, so slow down here. When an atom forms an ion it gains or loses electrons, and you must redraw the shells for the new number.

• A metal loses electrons. Sodium, 2.8.1, loses its single outer electron to become Na⁺, which is 2.8. The outer shell disappears from the diagram. Magnesium, 2.8.2, loses two to become Mg²⁺, also 2.8.
• A non-metal gains electrons. Chlorine, 2.8.7, gains one to become Cl⁻, which is 2.8.8. Oxygen, 2.6, gains two to become O²⁻, which is 2.8.

Two habits earn the marks: write the charge clearly (Na⁺, O²⁻), and for a bonding diagram, show which electrons came from which atom using dots for one element and crosses for the other. When you draw an ion, the nucleus and its proton number do not change, only the electrons do. That is the whole idea of an ion.

Common mistakes to avoid

• Overfilling the first shell (it holds only 2) or the second shell (only 8).
• Forgetting that the proton number sets the electron count, then miscounting.
• Drawing an ion with the same number of electrons as the atom, the point of an ion is that the number has changed.
• Leaving off the charge on an ion, or mixing up gain and loss (metals lose, non-metals gain).

Practise until it is automatic

Electron arrangement is one of the fastest topics to master because the rules never change and there are only twenty atoms to practise. Draw a few each day, say the arrangement aloud, then predict the period and group before checking. Once atoms feel easy, move on to ions and simple bonding diagrams for chemical bonding. If shell-filling or ion charges keep tripping you up, that is a quick fix with a teacher, our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial; see how it works if you would like the diagrams checked against exam wording.