Inside out

The idea

Electrons fill shells stepwise in the order K, L, M, N…, starting from the shell closest to the nucleus; a shell is filled only after the one inside it is complete.

Which shell first?

Electrons are added one by one to an atom. The K shell is closest to the nucleus and the L shell comes next.

The K shell is full with 2 electrons. Where does the next electron go?

• The L shell
• The M shell
• Back into the K shell

Add electrons one by one

Press Add an electron and watch which shell it joins.

Stepwise filling

Electrons enter the shell closest to the nucleus first. A shell fills only after the one inside it is complete.

Electrons fill shells stepwise, in the order K, L, M, N…, starting from the shell closest to the nucleus. The L shell is filled only after the K shell is complete, and so on.

Notes

Electrons fill shells stepwise, K, L, M, N…, starting closest to the nucleus. A shell fills only after the one inside it is complete.

Check yourself

An atom has 3 electrons. Where is the third electron?

In which order are the shells filled?

When does the L shell start to fill?

An atom has 10 electrons. The K shell takes 2. How many go into the L shell?

Answer: 8

The K shell is full with 2, and the L shell takes the next 8: 10 − 2 = 8.

• In the K shell. The K shell is complete with 2 electrons, so the third electron cannot go there.
• In the L shell — correct. Yes!
• In the M shell. The M shell is filled only after the L shell is complete.
• N, M, L, K. Filling starts from the shell closest to the nucleus, the K shell.
• Any order. Shells are filled stepwise, starting from the one closest to the nucleus.
• K, L, M, N — correct. Yes!
• After the K shell is complete — correct. Yes!
• Before the K shell is complete. The L shell is filled only after the K shell is complete.
• Only after the M shell is complete. The M shell lies outside the L shell and is filled later.