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Reading the Periodic Table

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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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 position from neutral atoms, then predicting ionsSodium's neutral atom is two eight one: three occupied shells give Period 3, and one outer electron gives Group 1. It loses one electron to form Na plus with arrangement two eight. Chlorine's neutral atom is two eight seven: three occupied shells give Period 3, and seven outer electrons give Group 17. It gains one electron to form Cl minus with arrangement two eight eight. Forming an ion does not change either element's position.Sodium atom2,8,13 occupied shells → Period 31 outer electron → Group 1Loses 1 electronNa⁺: arrangement 2,8Na → Na⁺ + e⁻Position stays the sameChlorine atom2,8,73 occupied shells → Period 37 outer electrons → Group 17Gains 1 electronCl⁻: arrangement 2,8,8Cl + e⁻ → Cl⁻Position stays the same
Read the period from occupied shells in a neutral atom. For these first-20 examples, the outer-electron count gives the main group and predicts a common ion charge. Group numbers use the modern 1–18 system.

Proton number puts each element in order

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

Read a neutral atom’s electron arrangement

  • Proton number increases by 1 from one element to the next, including when a new period begins. The table is ordered by proton number, not relative atomic mass.
  • Group is a vertical column. For neutral atoms of 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.
  • Main-group elements usually have similar chemical properties because their neutral atoms have the same number of outer (valence) electrons. Helium has a full shell of two; other Group 18 atoms have eight.
  • Across a period, elements change from metallic → non-metallic character as the number of outer electrons increases.

For example, a neutral atom with proton number 15 has 15 electrons. Fill the first shell with 2 and the second with 8; the remaining 5 go into the third: 2,8,5. Three occupied shells give Period 3; five outer electrons give Group 15. The element is phosphorus.

For the first 20 elements, use the shell pattern 2,8,8,2. This is a school-level filling pattern for these elements, not a claim that the third shell can never hold more than eight electrons.

Use the atom, not its ion, to find the position

A sodium atom is 2,8,1, while its ion is 2,8. Losing an electron does not change the 11 protons or sodium’s Period 3, Group 1 position. The number of occupied shells in an ion need not equal the element’s period.

A. Group and period (what they mean)

Periodic Table wordWhat it meansWhy it matters
groupvertical column; main-group atoms have the same outer-electron count (helium is the Group 18 exception)similar chemical properties
periodhorizontal row; neutral atoms 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.

Many simple ions of the first 20 elements have a full outer shell. Electron loss gives a positive charge; electron gain gives a negative charge. Use the named element and its common compounds as well as the outer-electron count.

Elements / group patternOuter electrons in the neutral atomCommon simple ion chargeExample
Li, Na, K (Group 1 metals)11 +Na⁺
Be, Mg, Ca (Group 2)22 +Mg²⁺
Aluminium (Group 13)33 +Al³⁺
Nitrogen (Group 15) in metal nitrides53-N³⁻ in Mg₃N₂
Oxygen and sulfur (Group 16)62-O²⁻, S²⁻
Fluorine and chlorine (Group 17)71-F⁻, Cl⁻
He, Ne, Ar (Group 18)full: 2 for He, 8 for Ne/Arno common simple ionsAr

Boron and carbon usually form covalent compounds; do not predict a simple boron ion of 3 + or a carbon ion of 4 + just from the group number. Hydrogen is a non-metal despite its Group 1 position, and its chemistry is not described by the alkali-metal rule.

Overclaim detector

Do not write “group number = ion charge” as a universal rule.
For the first 20 elements, combine outer-electron count with the common ion patterns shown above. 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.
  • Saying atoms “want” electrons. State the actual electron gain or loss and the resulting charge and electron arrangement.

5. Exam Tips

Answer format that scores

When asked “why similar properties?”, write:
“same number of outer electrons → similar ion formation or 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

A neutral atom of 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

A neutral atom of 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

Practise and check

Practise reading position from a neutral atom, explaining group similarities, and predicting common ion charges.

Open the periodic-table topic check
Syllabus and review details

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