Tenth Grade · Science · week 8

Electron configuration: the Aufbau principle

Write the electron configuration of an atom using the Aufbau principle, and state the maximum number of electrons each sublevel can hold.

Electrons fill the space around the nucleus in a predictable order, and electron configuration is the notation chemists use to record exactly where every electron in an atom sits. Electrons occupy sublevels labeled s, p, d, and f, and each sublevel holds a maximum number of electrons: s holds 2, p holds 6, d holds 10, and f holds 14.

The Aufbau principle (German for "building up") says electrons fill the lowest-energy sublevels first, in this order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, and so on. Notice that 4s fills before 3d -- energy order does not always match the number order you would expect, which is a common surprise the first time through.

A configuration is written as a string of sublevels with the electron count as a superscript, like 1s^2 2s^2 2p^4 for oxygen -- and every superscript in the whole configuration must add up to that element's atomic number, since that is simply the total electron count in a neutral atom.

📋 For the teacher
Teach it (10–15 min):
  • Draw the Aufbau filling order as a diagonal arrow chart together (a quick web search for "Aufbau diagram" shows the classic diagonal-line version), since seeing the zigzag path explains why 4s fills before 3d far better than memorizing the order as a flat list.
  • Keep a small reference card with the max capacity of each sublevel (s=2, p=6, d=10, f=14) next to them while they practice -- fluency comes after several repetitions, not before.
  • Make "do the superscripts add up to the atomic number" a mandatory last step on every problem, every time -- it catches almost every mistake before it becomes a habit.
  • Connect back to the periodic table: point out that the table's row (period) number matches the highest principal energy level (the number before each letter) an atom is filling, which previews next week's pattern.
Talk about it:
  • What does the Aufbau principle tell you about the order electrons fill?
  • Why does 4s fill before 3d even though 3 is a smaller number than 4?
  • How do you check that a completed electron configuration is correct?

Mastery looks like: They write correct electron configurations for elements up through at least atomic number 36, following Aufbau order and correct sublevel capacities.

Common stumbles: Filling sublevels out of Aufbau order (for example, filling all of 3d before starting 4s); exceeding a sublevel's maximum capacity.

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