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The Periodic Law

Learn how proton number and electron configuration determine Periodic Table position, common ion charges, group similarities and metallic character.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • describe the Periodic Table as an arrangement of the elements in the order of increasing proton (atomic) number
  • describe how the position of an element in the Periodic Table is related to proton number and electronic configuration
  • describe the relationship between number of outer (valence) electrons and the ionic charge of an ion for the first twenty elements
  • explain the similarities between the elements in the same group of the Periodic Table in terms of their electronic configuration
  • describe the change from metallic to non-metallic character from left to right across a period of the Periodic Table

The Periodic Table arranges elements by increasing proton number. For the first 20 elements, electron configuration helps you locate an element, explain group similarities and predict common ion charges.

Reading period, group and ion charge from electron configurationsElectron configuration two eight one has three occupied shells, so it is in Period 3, and one outer electron, so it is in Group 1 and commonly forms a one plus ion. Electron configuration two eight seven is also in Period 3, but has seven outer electrons, so it is in Group 17 and commonly forms a one minus ion.Electron configuration → position → common ionExample A2,8,13 occupied shellsPeriod 31 outer electronGroup 1loses 1 electronM⁺Example: Na → Na⁺ + e⁻Example B2,8,73 occupied shellsPeriod 37 outer electronsGroup 17gains 1 electronX⁻Example: Cl + e⁻ → Cl⁻Modern numbering: Groups 1–2 have 1–2 outer electrons; Groups 13–18 have 3–8 (helium has 2).
Read the period from occupied shells. For the first 20 elements, use the outer-electron pattern to identify the main group and predict common ion charge.

1. Definition

A. Periodic Table arrangement

The Periodic Table arranges elements in order of increasing proton number (atomic number). Elements with similar chemical properties occur in the same vertical group.

2. Key Ideas

  • Atomic number increases by 1 across the table.
  • Group is a vertical column. For the first 20 elements, Groups 1 and 2 have 1 and 2 outer electrons; Groups 13–18 have 3–8, except helium has 2.
  • Period (horizontal row) tells you the number of occupied electron shells.
  • Elements in the same group have similar chemical properties because they have the same number of valence electrons.
  • Across a period, elements change from metallic → non-metallic character as the number of outer electrons increases.

3. Detailed Explanations

Quick Recall (groups, periods, ions)
  • Atomic number = number of protons.
  • Period = occupied shells. Use the modern group pattern for outer electrons.
  • Metals lose electrons → cations; non-metals gain electrons → anions.
  • Typical charges: Group 1 + 1, Group 2 + 2, Group 17 -1, Group 18 no common ions.
  • Across a period: metallic character decreases and non-metallic character increases.

A. Group and period (what they mean)

Periodic Table wordWhat it means (exam-safe)Why it matters
groupvertical column; members have the same number of outer electrons (except helium within Group 18)similar chemical properties
periodhorizontal row; members have the same number of occupied electron shellslocate an element from its electron configuration
You must be able to read electron configuration

Group/period questions are often electron-configuration questions in disguise: Electron Configuration.

B. From group number to ion charge (do not overclaim)

Group 14 trap

Carbon and silicon usually form covalent bonds and do not form simple ions in O-Level questions. Avoid writing “Group 14 → ±4 ions” as a rule.

For many main-group elements, atoms form ions to reach a stable noble gas electron configuration.

GroupValence electronsTypical ion formedExample
11+ 1Na⁺
22+ 2Mg²⁺
133+ 3Al³⁺
155-3N³⁻
166-2O²⁻
177-1Cl⁻
18full outer shell (He has 2)no common ionsAr
Overclaim detector

Do not write “group number = ion charge” as a universal rule.
Use outer-electron count to predict the simplest ion for the first 20 elements. Transition elements can have variable charges, while Group 18 elements do not commonly form ions.

C. Trend: metals to non-metals across a period

Across a period, elements become less metallic and more non-metallic.

The number of outer electrons increases. Elements on the left tend to lose electrons and form positive ions; elements towards the right tend to gain or share electrons. Group 18 already has a full outer shell.

You’ll be asked to compare properties

Physical/chemical differences are here: Metals and Non-Metals.

4. Common Mistakes

  • Saying “elements in a group are identical”. They have similar chemical properties because they share the same outer-electron pattern.
  • Writing “group number = charge” as a universal rule; it does not apply to transition elements.
  • Treating modern Group 17 as though it has 17 outer electrons; it has 7.
  • Using “atoms want to…” as an explanation. Use “atoms form ions to achieve a stable noble gas configuration”.

5. Exam Tips

Answer format that scores

When asked “why similar properties?”, write:
“same number of valence electrons → form same type of ions / same bonding behaviour”.

  • For electron configuration, count occupied shells for the period and outer electrons for the main-group position.

6. Worked Examples

Modelled example 1

Group and period from electron configuration

Core

Problem

An element has electron configuration 2,8,1. State its group and period.
Study the worked solution

  1. Read the period

    Method

    Count the occupied electron shells.

    Reason

    The period number equals the number of occupied shells.

    Working

    2 | 8 | 1 gives three occupied shells → Period 3.

  2. Read the group

    Method

    Count electrons in the outermost shell.

    Reason

    For this main-group element, one outer electron places it in Group 1.

    Working

    Outer-shell count = 1 → Group 1.

Guided practice 2

Predict the ion charge

About 5 min

Problem

An element X is in Group 17. Predict the simplest ion it forms and explain the electron change.

Use the outer-shell deficit

Electron change
Ion formed

Hints

Hint 1: translate the modern group
A Group 17 atom has seven valence electrons.
Hint 2: reach a full outer shell
It needs one additional electron, not seven, to reach eight.
View solution step by step

  1. Count the outer electrons

    Method

    Use Group 17 to infer seven valence electrons.

    Reason

    Main-group position reflects the outer-electron pattern.

    Working

    Valence electrons = 7.

  2. Predict the ion

    Method

    Gain one electron.

    Reason

    One additional electron completes the outer shell and adds one negative charge.

    Working

    X + e⁻ → X⁻; for example, Cl⁻.

Common misconception 3

Explain group similarity

Find and correct the mistake

Learner response

A student says: “Sodium and potassium have similar chemical properties because both atoms have three occupied shells.” Locate the error and give the correct Group 1 explanation.

Separate group evidence from period evidence

Relevant shared feature
Common ion pattern

View solution step by step

  1. Locate the error

    Method

    Reject the claim that sodium and potassium have the same shell count.

    Reason

    Sodium is in Period 3, while potassium is in Period 4.

    Working

    Different periods → different numbers of occupied shells.

  2. Use the group-defining feature

    Method

    State that both have one outer-shell electron.

    Reason

    Chemical reactions depend strongly on valence electrons.

    Working

    Both lose one electron and form 1 + ions.

  3. Write the correction

    Working

    Sodium and potassium show similar chemical reactions because both have one valence electron and form 1 + ions.

Examiner practice 4

Metallic character across a period

3 marks

Examination question

Describe and explain how metallic character changes from sodium to chlorine across Period 3. [3 marks]

State the trend and contrast electron behaviour

View solution step by step

  1. State the trend

    1 mark

    Method

    Move from sodium towards chlorine across Period 3.

    Reason

    Element character changes systematically across the period.

    Working

    Metallic character decreases; non-metallic character increases.

  2. Use sodium's electron behaviour

    1 mark

    Method

    State that sodium readily loses one outer electron.

    Reason

    Loss of electrons and positive-ion formation are characteristic metallic behaviour.

    Working

    Na → Na⁺ + e⁻.

  3. Contrast chlorine

    1 mark

    Method

    State that chlorine tends to gain one electron.

    Reason

    Electron gain and negative-ion formation show non-metallic behaviour.

    Working

    Cl + e⁻ → Cl⁻.

Challenge 5

Predict position from configuration

Minimal support

Position-to-behaviour transfer

An unknown element has electron configuration 2,8,7. State its period and group, classify it as metallic or non-metallic, and predict its simplest ion.

Extract every conclusion from one configuration

Position
Character
Simplest ion

Hints

Hint 1: read position first
Count shell entries for period and the last entry for the main-group outer-electron pattern.
Hint 2: predict the smallest electron change
Compare seven outer electrons with a full outer shell of eight.
View solution step by step

  1. Infer position

    Method

    Count shells and outer electrons.

    Reason

    These determine period and main-group position for the first 20 elements.

    Working

    Three shells → Period 3; seven outer electrons → Group 17.

  2. Infer character

    Method

    Place the element towards the non-metallic side of Period 3.

    Reason

    Group 17 elements are non-metals that tend to gain electrons.

    Working

    Classification: non-metal.

  3. Predict the ion

    Method

    Gain one electron.

    Reason

    This completes the outer shell.

    Working

    The simplest ion has charge 1-.

7. Mind Stretchers

Mind stretcher 1: Fix the wrong ruleExtension

Question: A student writes: “Group number always equals the charge of the ion formed.” Correct this statement in one sentence.

Show Answer

For the first 20 elements, use the number of outer electrons to predict common ion charge. Modern group number is not itself the charge: Group 17 elements have seven outer electrons and commonly form -1 ions.

Mind stretcher 2: Helium trapExtension

Question: Helium is in Group 18 but has only 2 outer electrons. Why is its outer shell still full?

Show Answer

The first electron shell can hold a maximum of 2 electrons, so helium’s outer shell is full with 2 electrons.

8. Quiz

Quiz Time!

Test group/period reading, ion charge prediction, and trend explanations (with the correct keywords).

Go to Quiz Page