Periods: The Horizontal Rows

Introduction

Read a line of text from left to right and the story moves forward one word at a time. The periodic table's horizontal rows, called periods , work in a similar way. Moving one box to the right adds one proton to the nucleus and one electron to the atom. Across each row, the elements change gradually from reactive metals to non-metals and finally to an unreactive noble gas, before a new row begins and the pattern starts again.

Core explanation

Seven periods. The modern table has seven periods, numbered 1 to 7 from top to bottom. They are not all the same length:

Period Number of elements First and last element --- --- --- 1 2 hydrogen to helium 2 8 lithium to neon 3 8 sodium to argon 4 18 potassium to krypton 5 18 rubidium to xenon 6 32 caesium to radon 7 32 francium to oganesson

Periods 6 and 7 include the lanthanides and actinides, which are usually printed in a separate block below the main table so the chart is not too wide.

Period number and shells. The period number tells you how many electron shells are occupied in an atom of that element. Every element in period 2 has electrons in two shells; every element in period 3 has electrons in three shells. Sodium (2,8,1) and chlorine (2,8,7) both have three occupied shells, so both are in period 3.

Moving along a period. Each step to the right increases the atomic number by one. In the main groups, this adds one electron to the outer shell. At the end of the row the outer shell is full, giving a noble gas. The next element must begin a new shell, so it starts the next period.

Properties change across a period. Period 3 shows the typical pattern clearly:

- Sodium, magnesium and aluminium are metals: shiny, good conductors. - Silicon is a metalloid with properties in between. - Phosphorus, sulfur and chlorine are non-metals. - Argon is an unreactive noble gas.

So moving from left to right, elements change from metals to non-metals. The number of outer electrons rises from 1 to 8, and the atoms actually become slightly smaller, because the increasing positive charge of the nucleus pulls the electrons in the same shell closer.

Similar in shells, different in behaviour. Elements in the same period are not a family. They share the number of shells, but because their outer electrons differ, their chemistry differs widely. Families of similar elements run vertically, not horizontally.

Step-by-step reasoning

To find the period of an element from its electron arrangement:

1. Write the electron arrangement, for example 2,8,3 for aluminium. 2. Count the number of numbers (shells) in the arrangement: three. 3. The period number equals the number of shells: aluminium is in period 3. 4. Check on the table: aluminium is in the third row.

Visual explanation

Picture each period as a row of houses on a street, numbered from left to right. Each house has one more resident (electron) than the house before. The last house on each street has a full outer floor, and the next street begins with a brand-new floor being built.

Real-world analogy

A period is like a school year group. Everyone in Year 9 has spent the same number of years at school (same number of shells), but their personalities vary enormously. Being in the same year does not make students alike, just as being in the same period does not make elements alike.

Real-world example

Period 3 contains both the lightweight metal aluminium, used in drinks cans and aircraft, and chlorine, a toxic gas used in controlled amounts to disinfect drinking water. Two elements from the same row can have completely different uses.

Why?

Why does a new period start after each noble gas? A noble gas has a full outer shell. The next electron cannot fit into that shell, so it must go into a new shell further from the nucleus. That new shell marks the beginning of the next row.

Common misconception

"Elements in the same period have similar chemical properties." They do not; elements in the same group are the similar ones. A period runs from a reactive metal to a noble gas, covering a wide range of behaviour.

Worked example

Question: An element has the electron arrangement 2,8,8,2. Which period is it in, and is it more likely to be a metal or a non-metal?

Reasoning: There are four numbers, so four occupied shells, placing it in period 4. It has 2 outer electrons, so it is near the left of the row, where metals are found. The total, 20 electrons, identifies calcium.

Answer: Period 4; it is a metal (calcium).

Quick check

1. How many elements are in period 2, and what are the first and last? Answer: Eight, from lithium to neon.

Exam focus

Learn that the period number equals the number of occupied electron shells. Be ready to describe the change from metals on the left to non-metals on the right of a period, using period 3 as an example.

Advanced insight

The period lengths 2, 8, 8, 18, 18, 32, 32 reflect how many electrons can be added before a noble-gas arrangement is reached. From period 4 onwards, inner d sub-shells fill within the row (adding 10 elements), and from period 6 inner f sub-shells also fill (adding a further 14).

Summary

Periods are the seven horizontal rows of the periodic table, containing 2, 8, 8, 18, 18, 32 and 32 elements. The period number equals the number of occupied electron shells. Moving across a period, atomic number rises by one each step, the outer shell fills, and elements change from metals to non-metals, ending with a noble gas.

Practice questions

1. State what the period number tells you about an atom. Answer: The number of occupied electron shells in the atom. 2. Chlorine has the arrangement 2,8,7. Which period is it in? Explain. Answer: Period 3, because its electrons occupy three shells. 3. Describe how the type of element changes across period 3 from sodium to argon. Answer: It changes from metals (sodium, magnesium, aluminium) through a metalloid (silicon) to non-metals (phosphorus, sulfur, chlorine) and ends with a noble gas (argon). 4. Why does potassium start period 4 rather than finishing period 3? Answer: Argon's outer shell is full at the end of period 3, so potassium's extra electron must go into a new, fourth shell, beginning period 4.