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Patterns in the Periodic Table

Learn periodic arrangement, Groups 1, 17 and 18, reactivity, extraction and rust prevention.

  • SEC G2 Science Chemistry component 2027
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Learning objectives

Show all 13 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
  • 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
  • describe the relationship between number of outer (valence) electrons and metallic/non-metallic character
  • predict the properties of elements in Group 1 and Group 17 using the Periodic Table.
  • describe lithium, sodium and potassium in Group 1 (the alkali metals) as a collection of relatively soft, low density metals showing a trend in melting point and in their reaction with water
  • describe chlorine, bromine and iodine in Group 17 (the halogens) as a collection of diatomic non- metals showing a trend in colour, state and their displacement reactions with solutions of other halide ions
  • describe the lack of reactivity of the elements in Group 18 (the noble gases) in terms of their electronic configurations.
  • place in order of reactivity calcium, copper, (hydrogen), iron, lead, magnesium, potassium, silver, sodium and zinc by reference to the reactions, if any, of the metals with water, steam and dilute hydrochloric acid
  • deduce the order of reactivity from a given set of experimental results
  • describe the ease of obtaining metals from their ores by relating the elements to their positions in the reactivity series
  • describe the essential conditions for the corrosion (rusting) of iron as the presence of oxygen and water; prevention of rusting can be achieved by placing a barrier around the metal, e.g. painting; greasing; plastic coating.

This lesson covers the periodic, group and metal-reactivity patterns required for G2 Science (Chemistry).

1. Definition

The Periodic Table arranges elements in increasing proton number so that elements with related chemical properties appear in the same vertical group.

  • A group is a vertical column. Main-group elements in one group have the same number of outer electrons.
  • A period is a horizontal row. Its number gives the number of occupied electron shells.
  • The reactivity series orders metals by their tendency to form positive ions.
Course boundary

Transition-element properties and uses are not required for G2. Use the prescribed Groups 1, 17 and 18 plus the metal-reactivity, extraction and rusting patterns in this lesson.

Study this lesson in three passes

Begin with position and electron arrangement. Then compare Groups 1, 17 and 18. Finish with the metal reactivity series, obtaining metals and rust prevention. Each pass uses a different kind of evidence.

2. Key Ideas

Evidence or positionPrediction
same groupsimilar chemical properties
down Group 1reactivity increases
down Group 17reactivity decreases
Group 18very unreactive because the outer shell is full
metal P displaces metal QP is more reactive than Q
metal high in the seriesits compounds are generally harder to reduce

Across a period, elements generally change from metallic to non-metallic character.

3. Detailed Explanations

Position and electron arrangement

An atom with electron configuration 2,8,1 is in Period 3 because it has three occupied shells, and Group 1 because it has one outer electron. It forms a positive ion by losing that electron:

Na → Na⁺ + e⁻

The table is ordered by increasing proton number. Across a period, the number of outer electrons generally increases. Elements on the left tend to lose electrons and are metallic; elements on the right tend to gain or share electrons and are non-metallic.

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 an electron configuration in order: occupied shells give the period, outer electrons give the main-group pattern, and the outer-shell change predicts a common ion.

Lithium, sodium and potassium are relatively soft, low-density Group 1 metals. Their melting points decrease down the group, while their reactions with water become more vigorous. Each produces hydrogen and an alkaline metal-hydroxide solution:

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Group 1 metals react by losing one outer electron. Down the group, that electron is farther from the nucleus and is lost more readily, so reactivity increases.

Group 1 reaction with water becomes more vigorous down the groupLithium floats and fizzes steadily, sodium melts into a ball and moves rapidly, and potassium reacts very vigorously and may ignite. All three form an alkaline metal hydroxide solution and hydrogen gas. Comparing equal pieces under the same conditions shows that reaction vigour and reactivity increase down the group.Reaction with water: increasing vigour down Group 1Lithium, Lifloats and fizzes steadilymoves slowly on the surfaceSodium, Namelts into a ball and moves rapidlyfizzes more vigorouslyPotassium, Kvery vigorous; may ignitea lilac flame may be seenreactivity and reaction vigour increase2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)Fair comparison: use equal amounts under the same conditions, then extend the observed trend qualitatively.
Compare the same small amount of each metal under the same conditions. Increasing vigour is evidence that reactivity increases from lithium to potassium.

Chlorine, bromine and iodine are diatomic Group 17 non-metals: Cl₂ is a pale green gas, Br₂ is a red-brown liquid and I₂ is a grey-black solid at room temperature. Down the group, colour darkens and the state changes from gas to liquid to solid. Reactivity decreases because an incoming electron is farther from the nucleus and more shielded. A more reactive halogen displaces a less reactive halide:

Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)
Group 17 reactivity and halogen displacement outcomesChlorine is more reactive than bromine, which is more reactive than iodine. Chlorine displaces bromide to form orange bromine and iodide to form brown iodine. Bromine displaces iodide to form brown iodine. Bromine cannot displace chloride, and iodine cannot displace chloride or bromide.Group 17 displacement: compare reactivity firstMore reactiveCl₂Br₂I₂Less reactivereactivity decreases down the groupCl₂(aq) + Br⁻(aq)Yes → Br₂(aq) formsorange solutionCl₂(aq) + I⁻(aq)Yes → I₂(aq) formsbrown solutionBr₂(aq) + I⁻(aq)Yes → I₂(aq) formsbrown solutionBr₂ cannot displace Cl⁻No reaction: bromine is less reactive than chlorine.I₂ cannot displace Cl⁻ or Br⁻No reaction: iodine is the least reactive of the three.Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)Symbolic check: atoms and total charge are balanced; chlorine gains electrons and bromide loses electrons.
A displacement occurs only when the added halogen is more reactive than the halogen in the halide ion. Name the halogen formed before stating its aqueous colour.

Group 18 atoms have full outer electron shells: helium has a full first shell with two electrons, while the other noble gases have eight outer electrons at this depth. They therefore have little tendency to gain, lose or share electrons, which explains their very low reactivity.

Metals, extraction and corrosion

Learn the required order:

potassium > sodium > calcium > magnesium > zinc > iron > lead > (hydrogen) > copper > silver

Hydrogen is a reference point, not a metal. Metals above hydrogen can react with dilute hydrochloric acid to produce hydrogen, although rate and surface coatings can affect what is observed. Potassium, sodium and calcium react with cold water. Magnesium reacts very slowly with cold water but more readily with steam; zinc and iron react with steam. Copper and silver do not react with water, steam or dilute hydrochloric acid.

Use more than one test when interpreting an unfamiliar metal:

Position or metalCold waterSteamDilute hydrochloric acid
potassium, sodium, calciumreactnot needed to establish the orderreact too vigorously for a routine comparison
magnesiumvery slow reactionreactsreacts
zinc, ironno useful reactionreactreact
leadno useful reactionno useful reaction at this depthcan react slowly, but a coating may stop the visible reaction
copper, silverno reactionno reactionno reaction

This table is a reasoning guide, not a replacement for supplied observations. Reaction rate, oxide coatings and experimental conditions can affect what you see, so use the complete data set before deducing an order.

A more reactive metal displaces a less reactive metal from its compound. Use experimental results to build an order instead of forcing every unknown into a memorised position. The higher a metal is in the reactivity series, the more difficult it generally is to obtain from its ore, so less reactive metals are easier to obtain. At G2, compare ease of obtaining metals; specific carbon-reduction and electrolysis rules are not required.

Iron rusts only when both oxygen and water are present. Paint, grease and plastic coating prevent rusting by placing a barrier between iron and oxygen or water.

4. Common Mistakes

  • Swapping group and period when reading an electron configuration.
  • Saying all groups become more reactive down the group.
  • Predicting that a less reactive metal displaces a more reactive metal.
  • Treating hydrogen as a metal rather than a reference point in the series.
  • Saying rusting needs oxygen but not water.
  • Assuming every rust-prevention method works by the same mechanism.
  • Forgetting that chlorine, bromine and iodine are diatomic, or reversing their colour, state and reactivity trends.

5. Exam Tips

Make a prediction from evidence

State the relevant position or observation, name the correct trend or rule, then apply it to the named element or metal.

Exam question 1: Infer a reactivity orderCore

Metal P displaces Q from its salt solution, while Q does not displace R. What can you conclude?

Show Answer

P is more reactive than Q. Q is less reactive than R. The evidence does not decide whether P is more or less reactive than R without another comparison.

Exam question 2: Predict a Group 1 propertyCore

A Group 1 metal lies below potassium. Predict its melting point and reaction with water compared with potassium, and explain the reactivity prediction.

Show Answer

It should have a lower melting point and react more vigorously with water. Its outer electron is farther from the nucleus and more shielded, so it is lost more easily.

6. Worked Examples

Modelled example 1

Locate an Element from Electron Configuration

Core

Problem

An element has electron configuration 2,8,7. State its period, group and likely ion.
Study the worked solution

  1. Count occupied shells

    Method

    Identify three occupied shells.

    Reason

    The number of occupied shells gives the period.

    Working

    Period 3.

  2. Count outer electrons

    Method

    Identify seven outer-shell electrons.

    Reason

    Main-group position follows the outer-electron count.

    Working

    Group 17.

  3. Predict the ion

    Method

    Gain one electron to form a 1− ion.

    Reason

    One additional electron completes the outer shell.

    Working

    Likely ion charge: 1-.

Guided practice 2

Deduce a Metal Order from Results

About 6 min

Problem

Metal A reacts with cold water. Metal B does not react with cold water but reacts with dilute hydrochloric acid. Metal C reacts with neither, and B displaces C from its salt solution. Place A, B, hydrogen and C in order of decreasing reactivity.

Use each result once

Most reactive
Metal above hydrogen
Least reactive

Hints

Hint 1: water result
Reaction with cold water places A above a metal that reacts only with acid.
Hint 2: hydrogen reference
A metal that reacts with dilute hydrochloric acid is above hydrogen; one that does not is below it.
View solution step by step

  1. Use water and acid

    Method

    Place A above B, and B above hydrogen.

    Reason

    A reacts with cold water, while B reacts only with acid; B can displace hydrogen from acid.

    Working

    A > B > hydrogen

  2. Place C

    Method

    Put C below hydrogen.

    Reason

    C does not react with acid and is displaced by B, so both results support a lower position.

    Working

    A > B > hydrogen > C

Common misconception 3

Predict a Halogen Displacement

Find and correct the mistake

Learner claim

Chlorine water is added to potassium bromide. A learner says bromide displaces chlorine because bromide is lower in Group 17. Correct the prediction and explain it.

Compare the halogens, not the halide position

More reactive halogen
Halogen formed

View solution step by step

  1. Apply the group trend

    Method

    Rank chlorine above bromine in reactivity.

    Reason

    Group 17 reactivity decreases down the group.

    Working

    Cl₂ more reactive than Br₂.

  2. Predict displacement

    Method

    Form bromine and an orange-brown colour.

    Reason

    The more reactive chlorine displaces bromide ions.

    Working

    Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂.

Challenge 4

Explain Barrier Protection

Minimal support

Damage transfer

Explain why painting an iron gate prevents rusting and why a scratch must be repaired.

Compare intact and damaged barriers

Intact paint excludes
At a scratch

Hints

Hint 1: requirements
Rusting needs both oxygen and water.
Hint 2: barrier integrity
A barrier works only where it separates iron from those substances.
View solution step by step

  1. Explain intact protection

    Method

    Use paint as a physical barrier.

    Reason

    It prevents oxygen and water reaching the iron surface.

    Working

    Intact paint → rusting conditions excluded.

  2. Explain the scratch

    Method

    Cover exposed iron promptly.

    Reason

    A scratch breaks the barrier, allowing oxygen and water to contact iron and initiate rusting.

    Working

    Scratch → exposed iron → rusting can begin.

7. Mind Stretchers

Mind stretcher 1: Compare two opposite trendsExtension

Explain why Group 1 and Group 17 show opposite reactivity trends down their groups.

Show Answer

Group 1 reacts by losing an outer electron, which becomes easier down the group. Group 17 reacts by gaining an electron, which becomes harder down the group because the incoming electron is farther from the nucleus and more shielded.

Mind stretcher 2: Choose a barrier for an exposed surfaceExtension

Compare paint with grease for an iron bicycle chain that moves continuously.

Show Answer

Both can block oxygen and water. Paint may crack or wear away as the chain moves, whereas grease can coat the moving surfaces but must be reapplied when it is removed.

Mind stretcher 3: Connect reactivity to obtaining a metalExtension

Metal X is above magnesium in the reactivity series, while metal Y is below copper. Which metal is generally easier to obtain from its ore, and why?

Show Answer

Y is generally easier to obtain. A less reactive metal forms compounds that are easier to reduce or break down, while the more reactive X forms more stable compounds. G2 questions require this comparison, not a detailed extraction method.

8. Quiz

Try this first: for one unfamiliar element, write position → electron arrangement → predicted behaviour. For one unfamiliar metal, write observation → relative reactivity → consequence for displacement or ease of obtaining it.