How to Read the Periodic Table: Groups, Periods and Blocks Explained
By Periodic Learn Editorial Team · Published August 12, 2026 · Updated October 15, 2026 · 6 min read
The periodic table is not a random grid. Its shape encodes the electron structure of every atom, so once you can read the layout you can predict an element behaviour without memorising it.
Groups are vertical columns
A group is a vertical column. Elements in the same group have the same number of valence electrons, which is why they behave similarly. Group 1 metals all lose one electron easily, and Group 17 halogens all need one more electron to complete an octet.
The group number for main-group elements (1, 2, 13-18) tells you the valence electron count directly, counting from the right for groups 13-18.
- Group 1: alkali metals, one valence electron.
- Group 2: alkaline earth metals, two valence electrons.
- Group 17: halogens, seven valence electrons.
- Group 18: noble gases, full outer shell.
Periods are horizontal rows
A period is a horizontal row and corresponds to the highest principal quantum number occupied by electrons. Period 2 elements fill the 2s and 2p subshells; Period 4 elements fill 4s before 3d, which is why the d-block appears in the fourth row.
Blocks reveal the filling subshell
The s-, p-, d- and f-blocks tell you which subshell is being filled. This is the single most useful reading skill for writing electron configurations quickly under exam time pressure.
Remember the aufbau order: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p.
Predict electron configuration from position
For a main-group element, count the electrons up to the previous noble gas, then place the remaining electrons into the current period subshells. Sodium (Na, Z=11) is Period 3, Group 1: [Ne] 3s1.
Exam tips
- Learn the noble gas shorthand to save time.
- Check the group number before writing the valence shell.
- Watch out for the 4s-before-3d rule in the first transition series.
- Use the interactive table to verify any configuration instantly.