Periodic Table Explained: Groups, Periods & How to Read It
By the Periodixy Editorial Team · Last reviewed July 11, 2026
The periodic table is a chart of all 118 known chemical elements, arranged by atomic number into 18 vertical groups and 7 horizontal periods so that elements with similar properties line up in the same column. It looks intimidating — 118 boxes packed with numbers and letters — but it is really a cheat sheet: once you know how to read it, it hands you an element's structure, behaviour and even its likely charge without memorising anything.
This guide walks through what each part of an element tile means, how the rows and columns are organised, and how to pull useful predictions straight from the layout.
What each element tile tells you
Every tile on the interactive periodic table carries at least four pieces of information:
- Atomic number — the number of protons in the nucleus. This defines the element. Carbon is carbon because it has 6 protons.
- Element symbol — one or two letters, like O for oxygen or Na for sodium. The first letter is always a capital; the second is always lowercase.
- Element name — sometimes the symbol comes from a Latin name (Fe for ferrum, iron), so the name is worth reading.
- Atomic mass — the weighted average mass of the element's natural isotopes, in atomic mass units (amu).
Rows are periods, columns are groups
The horizontal rows are called periods, numbered 1 to 7. The period number tells you how many electron shells an element's atoms use. Sodium in period 3 has three shells: 2, 8, 1.
The vertical columns are called groups, numbered 1 to 18. A group of elements is a single column whose members share the same number of outer (valence) electrons, so they behave alike in reactions. That is the single most useful fact about the table: lithium, sodium and potassium all react like family members because each has one valence electron.
The big regions: metals, nonmetals, metalloids
Roughly three quarters of the elements are metals, filling the left and centre of the table. Nonmetals cluster in the upper right. Between them runs a staircase of metalloids — elements like silicon that share properties of both. The categories are colour-coded on most tables, including ours.
For details and examples, see Metals, Nonmetals, and Metalloids.
Predicting behaviour from position
Position is prediction. Here are three shortcuts you can start using immediately:
- Group 1 and 2 metals form +1 and +2 ions respectively; group 17 elements form −1 ions; group 18 barely reacts at all.
- Reactivity of metals increases down a group (potassium is more violent with water than sodium); reactivity of nonmetals increases up a group (fluorine beats chlorine).
- Elements diagonally close on the table often have similar properties — this is why lithium behaves a little like magnesium.
Worked example: predicting a formula
What formula would you predict for the compound of calcium and chlorine?
- Calcium is in group 2, so it forms Ca²⁺ ions.
- Chlorine is in group 17, so it forms Cl⁻ ions.
- To make the charges cancel, you need two Cl⁻ for every Ca²⁺.
Answer: CaCl₂ — calcium chloride.
The two detached rows
The two rows floating below the main table — the lanthanides (57–71) and actinides (89–103) — actually belong inside periods 6 and 7. They are printed separately only so the table fits on a page. All the actinides are radioactive; the lanthanides are the “rare earth” metals in magnets and screens.
Summary
- Atomic number = protons = electrons in a neutral atom.
- Periods (rows) tell you the number of electron shells.
- Groups (columns) tell you the number of valence electrons, so same group = similar chemistry.
- Metals sit left/centre, nonmetals upper right, metalloids along the staircase between them.
- Position on the table lets you predict ion charges, formulas and reactivity trends.