How Do You Find an Element's Electron Configuration From Its Position in the Periodic Table?

An element's position identifies the order in which its electrons occupy subshells: the period gives the highest principal energy level, the block gives the subshell receiving the final electron, and the group helps determine the number of outer electrons. This method applies to IB Chemistry at SL and HL.

The Reasoning

The periodic table is divided into s-, p-, d- and f-blocks. These correspond to the subshell containing the differentiating, or final, electron.

Follow the subshell-filling order:

$1s,\ 2s,\ 2p,\ 3s,\ 3p,\ 4s,\ 3d,\ 4p,\ 5s$

Each s subshell holds 2 electrons, each p subshell holds 6, and each d subshell holds 10.

To construct a configuration, first identify the element's atomic number; this is the number of electrons in a neutral atom. Fill subshells in the stated order until that total is reached. The period does not mean that every occupied subshell has the same principal quantum number. For potassium, the first 18 electrons form the argon core, and the nineteenth enters 4s: $[\mathrm{Ar}]4s^1$. For a cation, remove electrons from the highest principal energy level first.

For example, chlorine is in period 3, group 17 and the p-block. Its final subshell is therefore $3p^5$, giving:

$1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^5$

Calcium is in period 4, group 2 and the s-block. Its configuration ends in $4s^2$:

$1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6\ 4s^2$

This can be shortened to $[\mathrm{Ar}]4s^2$ using noble-gas notation.

A common misconception is that subshells fill strictly in numerical shell order. They do not: $4s$ fills before $3d$, although transition-metal ions usually lose their $4s$ electrons first.

Exam Technique

For a deduce question, state the full configuration or noble-gas configuration and check that the superscripts add to the atomic number. Use the element's block and period to verify the final subshell, and remember that some d-block atoms have exceptional ground-state configurations.