Mendeleev
ScienceElements
EnglishEN
Languages
EnglishENБългарскиBGCatalàCAČeštinaCSDanskDADeutschDEΕλληνικάELEspañolESEestiETSuomiFIFrançaisFRHrvatskiHRMagyarHUItalianoIT日本語JALëtzebuergeschLBLietuviųLTLatviešuLVNederlandsNLNorskNOPolskiPLPortuguêsPTRomânăROРусскийRUSlovenčinaSKSlovenščinaSLСрпскиSRSvenskaSVTürkçeTRУкраїнськаUK中文ZH
Periodic table›Science›Periods in the periodic table
Mendeleev · Science

Periods in the periodic table

What the horizontal rows reveal about atomic structure and changing properties.

← Science topicsElementsLearn

Overview

Moving across a period increases atomic number one step at a time. Electron structure changes systematically, which is why several properties show recognizable trends across a row.

What the horizontal rows reveal about atomic structure and changing properties.

How to use this idea

What the horizontal rows reveal about atomic structure and changing properties.

Explore further

Science topicsElementsLearnReference

Why periods matter

The horizontal rows are periods. Moving from left to right increases atomic number one proton at a time and progressively fills available electron states. Period 1 contains only hydrogen and helium; periods 2 and 3 contain eight elements each, while later periods are longer because d and f subshells become involved.

A period is therefore more than a row number: it indicates the highest principal electron shell occupied in the ground-state atom for the main-group elements and helps explain why chemical behaviour changes systematically across the table.

Understand the idea

Periods in the periodic table

1 · Understand the idea2 · How to use this concept3 · Try it yourself

Start by identifying what the page is describing, which quantities or structures are involved, and what changes when one parameter changes. Separate the definition from the consequence: a scientific term is useful only when you can connect it to an observation, a calculation or a position in the periodic table.

Worked reading

How to use this concept

Read the statement once without calculating. Then list the known information, the unknown, the units and the relation that connects them. If a diagram is present, name every symbol before using it. Finally, check whether the result is physically and chemically plausible rather than accepting a number simply because the arithmetic worked.

Key point

The horizontal rows are periods. Moving from left to right increases atomic number one proton at a time and progressively fills available electron states. Period 1 contains only hydrogen and helium; periods 2 and 3 contain eight elements each, while later periods are longer because d and f subshells become involved.

Try it yourself

Periods in the periodic table

Use the explanation above to restate the concept in one sentence, then identify one concrete example from the page. If a formula is involved, explain what each symbol means before substituting values.

Show the answer

A period is therefore more than a row number: it indicates the highest principal electron shell occupied in the ground-state atom for the main-group elements and helps explain why chemical behaviour changes systematically across the table.