How to Read the Periodic Table: Groups, Periods and Blocks Explained

The periodic table looks intimidating the first time you see it — 118 little boxes packed with numbers and letters. But almost all of that information comes down to answering one question: how are this element’s electrons arranged, and what does that mean for how it reacts? Once that clicks, the whole table starts to make sense.

Rows are periods

Each horizontal row is called a period, and it tells you how many electron shells an atom of that element has. Elements in period 1 (hydrogen and helium) have electrons in just one shell. Elements in period 3 (sodium through argon) have electrons spread across three shells. Moving left to right across a period, you’re adding one proton and one electron at a time to the same outermost shell — which is why properties tend to shift gradually across a row, from reactive metal on the left to reactive nonmetal on the right, ending in an unreactive noble gas.

Columns are groups

Each vertical column is a group, numbered 1 to 18. Elements in the same group share the same number of electrons in their outermost shell — and since that outer shell is what does almost all the chemical “talking” when atoms react, elements in a group tend to behave similarly. Group 1 (except hydrogen) is the alkali metals: soft, reactive metals that all form +1 ions. Group 17 is the halogens: reactive nonmetals that all form −1 ions. Group 18 is the noble gases: elements with a full outer shell, which is why they’re famously unreactive.

Try our interactive periodic table — hover any element and you’ll see its group and period called out directly.

Blocks tell you which orbital is filling

You’ll also see the table described in terms of s, p, d and f blocks, based on which type of orbital an atom’s highest-energy electrons occupy:

• s-block (groups 1-2, plus helium): 1 or 2 outer electrons, generally the most reactive metals.
• p-block (groups 13-18): everything from metalloids to nonmetals to the noble gases — the most chemically diverse block.
• d-block (groups 3-12): the transition metals, known for multiple possible charges and often colorful compounds.
• f-block: the lanthanides and actinides, shown as two rows below the main table purely to keep the table a manageable width — chemically they belong between groups 2 and 3 in periods 6 and 7.

Color-coding is category, not group

The background color on most periodic tables (including ours) marks an element’s category — alkali metal, transition metal, halogen, noble gas and so on. Category is closely related to block and group but isn’t identical to either; it’s really a summary of how an element tends to behave chemically, built from real observed properties rather than a strict formula. That’s why, for instance, aluminum and lead are both classified as “post-transition metals” even though they sit in different groups — their chemistry has more in common with each other than with the true transition metals next to them.

Putting it together

Once you can read period, group and block at a glance, a lot becomes predictable without memorizing anything: an element in period 4, group 2 will have two electrons in its fourth shell and behave like a slightly heavier version of magnesium (which sits directly above it in group 2). That’s the real power of the periodic table — it’s not just a list of 118 elements, it’s a map of why they behave the way they do.