How Is the Periodic Table Organized (Periods; Groups; Blocks)?

The periodic table is organized into horizontal periods, vertical groups, and electron-configuration blocks. For both SL and HL IB Chemistry, an element’s position connects its atomic structure to its chemical and physical properties.

Periods, Groups, and Blocks

The period number identifies the highest principal energy level occupied by electrons in the ground-state atom. For example, sodium has the configuration $1s^2 2s^2 2p^6 3s^1$, so its highest occupied level is $n$; therefore, sodium is in period 3.

Elements within a group often react similarly because they have related valence-electron configurations. Group 1 elements have one valence electron and commonly form $+1$ ions, while group 17 elements have seven valence electrons and commonly form $-1$ ions. Group 18 elements have filled outer shells and are generally unreactive.

The blocks reflect electron configuration:

A common misconception is that helium belongs to the $p$-block because it is in group 18. Its configuration is $1s^2$, so it belongs to the $s$-block despite its group position.

Exam Technique

For an IB explain question, connect position to electron configuration rather than merely naming the period, group, or block. State the relevant occupied energy level or sublevel and then link the valence electrons to the element’s expected properties or ion charge.