How to read the periodic table

A 10-minute orientation for students seeing the table for the first time — or the hundredth.

The periodic table looks intimidating: 118 boxes, cryptic symbols, a weird castle-with-towers shape. But it is built from just a few simple rules, and once you know them, the table stops being a chart you memorize and becomes a tool you read. Everything below can be tried live on the interactive periodic table.

The one rule that orders everything

Elements are arranged by atomic number — the count of protons in the nucleus. Hydrogen has 1 proton, so it comes first. Helium has 2, lithium has 3, and so on to oganesson at 118. The atomic number is not just a ranking: it determines how many electrons a neutral atom has, and the electrons determine essentially all of an element's chemistry.

What each tile shows

On this site — and most tables — every tile carries three or four things:

• Atomic number (top left): the proton count, e.g. 26.
• Symbol: one or two letters, e.g. Fe for iron. Some symbols come from Latin names — Fe from ferrum, Au from aurum for gold, Na from natrium for sodium.
• Name: the everyday name of the element.
• Atomic mass: the weighted average mass of the element's naturally occurring isotopes. When you see a value in parentheses, like (98) for technetium, the element has no stable form and the number is the mass number of its most stable isotope.

Rows are periods

Each horizontal row is a period. Moving left to right across a period, every element has one more proton — and one more electron — than the last. All the elements in a period are filling the same outermost electron shell. Period 1 has just two elements (hydrogen and helium) because the first shell only holds two electrons; later periods are longer because their shells hold more.

Crucially, properties change gradually across a period: period 3 starts with the soft, violently reactive metal sodium, passes through sturdy aluminium and the semiconductor silicon, and ends with the reactive gas chlorine and the inert gas argon. That repeat-every-row pattern is the "periodicity" the table is named for.

Columns are groups — the table's superpower

Each vertical column is a group (numbered 1–18). Elements in a group have the same number of outer-shell electrons, and outer electrons are what do chemistry — so group-mates behave alike. This is the single most useful fact about the table:

• Group 1, the alkali metals (Li, Na, K…), all have one outer electron they give up eagerly — all react with water, all form +1 ions.
• Group 17, the halogens (F, Cl, Br…), are one electron short of a full shell — all are reactive, all form −1 ions and salts.
• Group 18, the noble gases (He, Ne, Ar…), have full shells — and barely react at all.

If you know how one member of a group behaves, you can predict a lot about the others. That predictive power is why Mendeleev could leave gaps in his 1869 table and correctly forecast the properties of elements nobody had discovered yet.

Why the shape? Blocks.

The table's odd silhouette traces which type of orbital is being filled:

• The tall towers on each end are the s-block (groups 1–2) and p-block (groups 13–18).
• The long middle section is the d-block — the transition metals like iron, copper, and gold.
• The two detached rows at the bottom are the f-block: the lanthanides and actinides. They actually belong inside periods 6 and 7 (that's what the 57–71 and 89–103 markers in the main table mean) but are pulled out so the table fits on a page.

Metals left, nonmetals right

Roughly three-quarters of the elements are metals, and they fill the left and center of the table. Nonmetals cluster in the upper right. Between them runs a diagonal "staircase" of metalloids — boron, silicon, germanium, arsenic, antimony, tellurium — with in-between properties. (More on this in metals, nonmetals & metalloids.)

Put it together

Reading the table is answering three questions about any element: Which period? (how many shells), which group? (how many outer electrons, therefore how it reacts), and which region? (metal, nonmetal, or metalloid). With those three reads, an unfamiliar tile like caesium — period 6, group 1, deep metal territory — is instantly legible: a large, soft, extremely reactive metal that forms +1 ions. Which is exactly what it is.

Next steps: see the patterns quantified in periodic trends, or take the tour of the element families. Better yet, open the interactive table and click around — the filter buttons highlight every group discussed above.