How to write the electron configuration of an atom

Writing the electron arrangement of an atom is one of the first real skills you learn in matter and the atomic structure, and it quietly powers everything that comes after it, bonding, the groups and periods of the periodic table, and why some elements are reactive while others are inert. In SPM Chemistry the skill is completely mechanical once you know the rules, so this guide walks through the method step by step, then shows you how to handle ions and how to read the group and period straight off the arrangement.

What “electron configuration” means at SPM

In the KSSM syllabus we usually call this the electron arrangement (susunan elektron). It describes how the electrons of an atom are spread across shells, the energy levels around the nucleus. We write it as numbers separated by dots, from the innermost shell outwards. Sodium, for example, is written 2.8.1: two electrons in the first shell, eight in the second, one in the third.

Two facts make the whole thing work. First, a neutral atom has the same number of electrons as protons, so the proton number tells you how many electrons to place. Second, each shell can only hold so many electrons before it fills.

The shell rules you must know

For the first 20 elements, hydrogen to calcium, which is all SPM asks you to arrange, the filling limits are simple:

• First shell: maximum 2 electrons
• Second shell: maximum 8 electrons
• Third shell: maximum 8 electrons (for the first 20 elements)
• Fourth shell: begins filling once the third has 8

You fill the shells from the inside out, completing each one before moving to the next. Note the special point about the third shell: potassium (proton number 19) is 2.8.8.1 and calcium (20) is 2.8.8.2, the fourth shell starts even though the third could in theory hold more. SPM only requires the first 20 elements, so this simple pattern is all you need.

The step-by-step method

1. Find the proton number of the element from the periodic table. This equals the number of electrons in the neutral atom.
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, but if you reach 18 total electrons and still have more, start the fourth shell.
5. Write it out with dots between the shells, innermost first.

Worked examples

Oxygen, proton number 8. Place 2 in the first shell, leaving 6. The second shell takes all 6. Arrangement: 2.6.

Aluminium, proton number 13. First shell 2, second shell 8, that uses 10, leaving 3 for the third shell. Arrangement: 2.8.3.

Chlorine, proton number 17. First shell 2, second shell 8 (total 10), third shell takes the remaining 7. Arrangement: 2.8.7.

Argon, proton number 18. 2, then 8, then 8. Arrangement: 2.8.8, a full outer shell, which is why argon is unreactive.

Calcium, proton number 20. 2, 8, 8 uses 18, and the last 2 go into the fourth shell. Arrangement: 2.8.8.2.

Electron arrangements of ions

Ions are just atoms that have lost or gained electrons, so the method barely changes, you only adjust the electron count.

• A positive ion (cation) has lost electrons. A sodium atom is 2.8.1; a sodium ion Na⁺ has lost one electron and becomes 2.8.
• A negative ion (anion) has gained electrons. A chlorine atom is 2.8.7; a chloride ion Cl⁻ has gained one and becomes 2.8.8.

Notice that both Na⁺ and Cl⁻ end up with a stable, full outer shell, the same arrangement as a noble gas. That is the whole reason atoms form ions, and it is the bridge from this topic into chemical bonding.

Reading the group and period from the arrangement

This is where the skill pays off, because the arrangement tells you exactly where the element sits on the periodic table:

• The number of the outermost electrons gives the group (for Groups 1, 2 and 13–18). Chlorine, 2.8.7, has 7 valence electrons, so it is in Group 17.
• The number of occupied shells gives the period. Chlorine has three shells, so it is in Period 3.

So from one arrangement, 2.8.7, you can say: this is chlorine, Group 17, Period 3, with 7 valence electrons, a very common Paper 2 answer.

Common mistakes to avoid

The most frequent error is overfilling a shell, writing something like 2.9 or 2.8.9, so always respect the 2, 8, 8 limits. The second is confusing the number of shells with the number of valence electrons: the period comes from the shell count, the group from the outer electrons. The third is forgetting to change the electron count for ions. If you keep these straight, you will rarely lose a mark here.

Practise with the real elements

The fastest way to make this automatic is to arrange every element from hydrogen to calcium until you no longer need to think about the shell limits. Our interactive periodic table lets you check each arrangement as you go, and you will see the patterns down the groups and across the periods emerge on their own. If the topic still feels shaky, often because the link between arrangement, group and period has not clicked, that is a quick thing to 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 it taught against your own weak spots.