Electron arrangement

Electron arrangement is the way the electrons of an atom are distributed among the shells, written as numbers separated by dots, such as 2.8.1. Malay: susunan elektron · Chinese: 电子排布.

Electrons fill the shells starting from the one nearest the nucleus. For the first twenty elements, the first shell holds a maximum of 2 electrons and the second and third shells hold up to 8 each. You only start filling the next shell once the one before it is full, so the arrangement is built up shell by shell from the proton number.

Example. Sodium has proton number 11, so it has 11 electrons arranged 2.8.1: two in the first shell, eight in the second, one in the third. Magnesium (12) is 2.8.2, and argon (18) is 2.8.8. The dots separate the shells, and the numbers must add up to the total number of electrons.

Confusion to avoid. Do not confuse the whole electron arrangement with the valence electrons. The electron arrangement lists every shell, while the valence electrons are only the electrons in the outermost occupied shell, the last number in the arrangement. A common slip is to overfill a shell; keep to the 2, 8, 8 limits for the first twenty elements.

You can check an arrangement quickly by adding the numbers: they must equal the proton number of a neutral atom. If they do not, either a shell has been overfilled or an electron has been missed. Writing the arrangement neatly, with clear dots between the shells, also makes the group and period easy to read off afterwards, and helps when you draw the electron-shell diagram.

In the exam you use the electron arrangement to draw atomic-structure diagrams and to read off an element’s group (from the valence electrons) and period (from the number of occupied shells). It is one of the most practical skills in the Matter and the Atomic Structure chapter, and getting it fluent is something our teachers drill early because the Periodic Table chapter depends on it entirely.