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K326 / K328 / 2027
Periodicity and Metals

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Periodicity and Metals

Find patterns in the Periodic Table and use evidence to predict reactions.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

01

Read position from electron arrangement

Proton number orders the table; outer electrons explain recurring behaviour.

The Periodic Table orders elements by increasing proton number. A period is a horizontal row; a group is a vertical column. For the first twenty elements, occupied electron shells give the period. Main-group elements in one group have the same number of outer electrons, which explains similar chemical reactions.

Group number and period number come straight from the electron arrangement. The number of occupied shells gives the period. The number of outer electrons gives the group, for groups 1, 2 and 13 to 17 (group 13 has 3 outer electrons, group 14 has 4, and so on; group 18 has a full outer shell). So an atom with arrangement 2,8,2 has three shells and two outer electrons: period 3, group 2, which is magnesium. Elements in the same group react in similar ways because they have the same number of outer electrons, and outer electrons are the ones lost, gained or shared in reactions.

Worked exampleLocate an element

An atom has 17 protons and electron arrangement 2,8,7. Locate it and predict its character.

Try it first. Then show the worked stepsHide the worked steps
  1. Seventeen protons identify chlorine.
  2. Three occupied shells put it in period 3.
  3. Seven outer electrons place it in group 17; it is a non-metal.
Answer

Period 3, group 17. It behaves similarly to other halogens because their outer shells have the same electron count.

Check your understanding

Why do sodium and potassium have similar chemistry, even though their proton numbers differ?

  1. Both have the same number of shells, so their atoms are the same size.
  2. Both have eleven protons in a nucleus that sits inside similar shells.
  3. Both have one outer-shell electron, which they lose in reactions.
Show the answer

Both have one outer-shell electron, which they lose in reactions.

Elements in the same group have the same number of outer electrons, so they react in similar ways. Potassium has one more occupied shell, which helps explain why it is more reactive.

Check your understanding

An atom has the electron arrangement 2.8.8.2. Where is it in the Periodic Table?

  1. Period 4, Group 2
  2. Period 2, Group 8
  3. Period 2, Group 4
  4. Period 4, Group 8
Show the answer

Period 4, Group 2

Four occupied shells put it in Period 4. Two outer electrons put it in Group 2. It is calcium, proton number 20.

02

Metal to non-metal across a period

The number of outer electrons decides whether an element is a metal or a non-metal.

Across a period, character generally changes from metallic on the left to non-metallic on the right. Metals with few outer electrons tend to lose them; non-metals with nearly full outer shells tend to gain or share electrons. Group 18 has full outer shells, including helium with two rather than eight.

Use Period 3 as the model. Sodium, magnesium and aluminium have 1, 2 and 3 outer electrons. They lose these electrons easily to form positive ions, so they are metals: shiny, good conductors, with high melting points. Silicon, with 4 outer electrons, is in between: it has a giant covalent structure and is a semiconductor (a metalloid). Phosphorus, sulfur and chlorine have 5, 6 and 7 outer electrons. They tend to gain electrons to form negative ions or share electrons in covalent bonds, so they are non-metals: dull, poor conductors, and mostly with low melting points. Argon has a full outer shell and is unreactive.

Outer electrons and character across Period 3
ElementOuter electronsCharacterUsual behaviour in reactions
Na, Mg, Al1, 2, 3MetalLose outer electrons to form Na+, Mg2+, Al3+
Si4Metalloid (between metal and non-metal)Shares electrons in a giant covalent structure
P, S, Cl5, 6, 7Non-metalGain electrons to form negative ions, or share electrons
Ar8 (full)Noble gasDoes not react

Check your understanding

Element Q is in Period 3 and has 6 outer electrons. Which statement describes Q?

  1. A noble gas, because its outer shell is nearly full
  2. A metal that conducts electricity as a solid
  3. A metal that loses 6 electrons to form Q6+ ions
  4. A non-metal that gains 2 electrons to form Q2- ions
Show the answer

A non-metal that gains 2 electrons to form Q2- ions

Six outer electrons is close to a full shell of eight, so Q gains 2 electrons rather than losing 6. Q is sulfur, which forms S2- ions.

03

Group 1: lithium, sodium and potassium

Similar products, increasingly vigorous reaction with water.

Make a guess. You are not marked.

A small piece of sodium reacts quickly with cold water. What does the same size piece of potassium do?

  1. It reacts less vigorously, as its atoms are bigger
  2. It does not react, because it has a full outer shell
  3. It reacts more vigorously, losing its electron more easily
Show the answer

It reacts more vigorously, losing its electron more easily

Potassium is further down Group 1. Its outer electron is further from the nucleus and lost more easily, so it reacts faster.

Lithium, sodium and potassium are relatively soft, low-density metals. Their melting points decrease down this sequence, while their reactions with cold water become more vigorous. Low density is a shared property; do not assume density follows a perfectly steady trend.

Reaction with cold water
MetalTypical observationProducts
LithiumFloats and fizzes steadilyLithium hydroxide and hydrogen
SodiumFloats, moves and may melt into a ball as heat is releasedSodium hydroxide and hydrogen
PotassiumReacts very vigorously; hydrogen may ignitePotassium hydroxide and hydrogen

For sodium: 2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g). The resulting solution is alkaline because it contains hydroxide ions. The shared outer electron explains the common product pattern. Use observed trends to predict an unfamiliar group-1 element, while treating predicted observations as predictions rather than measured facts.

Check your understanding

Using the lithium to potassium trend, how should the Group 1 element below potassium compare with potassium?

  1. It should have a higher melting point, because it has more protons and a bigger atom.
  2. It should react more vigorously with water and have a lower melting point.
  3. It should react less vigorously, because its outer electron is further from the nucleus.
Show the answer

It should react more vigorously with water and have a lower melting point.

Down Group 1, reactivity rises and melting point falls, so the next element continues both trends. It still forms a hydroxide and hydrogen. A trend cannot give exact rates or temperatures.

Check your understanding

Rubidium is below potassium in Group 1. Which prediction is most likely to be correct?

  1. It reacts with water to form an acidic solution of rubidium ions and hydrogen
  2. It does not react with water, because its outer electron is far from the nucleus
  3. It has a higher melting point than potassium, because its atoms are heavier
  4. It reacts more vigorously with cold water than potassium does, forming RbOH and hydrogen
Show the answer

It reacts more vigorously with cold water than potassium does, forming RbOH and hydrogen

Reactivity with water increases down Group 1, so rubidium reacts even more vigorously: 2Rb + 2H2O -> 2RbOH + H2.

Exam-style question

The melting points of three Group 1 metals are: lithium 181 °C, sodium 98 °C, potassium 63 °C. Rubidium is the next element down the group.
(a) Predict the melting point of rubidium. [1]
(b) Predict how rubidium reacts with cold water, and name both products. [2]
(c) Explain why lithium, sodium and potassium react with water in a similar way. [1] [4]

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04

Group 17: identify displacement from the evidence

A more reactive halogen oxidises a less reactive halide.

Make a guess. You are not marked.

Chlorine water is added to colourless aqueous potassium bromide. What do you see?

  1. The solution turns orange-brown as bromine forms
  2. No change, because both of them are halogens
  3. A white precipitate of potassium chloride forms
Show the answer

The solution turns orange-brown as bromine forms

Chlorine is more reactive than bromine, so it displaces bromine from the bromide. The orange/brown colour is the bromine formed.

Halogens at room temperature
ElementForm, state and appearanceRelative reactivity
ChlorineCl2, pale green gasMost reactive of these three
BromineBr2, red-brown liquidIntermediate
IodineI2, grey-black solid; purple vapour on heatingLeast reactive of these three

Down the group, colour becomes darker and melting/boiling points rise, giving the gas-liquid-solid sequence. All three are diatomic non-metals with seven outer electrons. Their displacement reactivity decreases down the group.

Halogen added to a halide solution
Added halogenChloride solutionBromide solutionIodide solution
ChlorineNo displacementBromine formsIodine forms
BromineNo displacementNo displacementIodine forms
IodineNo displacementNo displacementNo displacement

Cl2(aq) + 2Br-(aq) -> 2Cl-(aq) + Br2(aq). The orange/brown bromine colour is evidence of displacement. Chlorine or bromine added to iodide can produce brown aqueous iodine; distinguish solution colours from the colours of pure elements.

Check your understanding

Halogen X displaces bromine from a bromide, but bromine cannot displace X from its halide. What follows?

  1. X is less reactive than bromine, so of Cl, Br and I it fits iodine.
  2. X and bromine are equally reactive, so neither reaction should take place.
  3. X is more reactive than bromine, so of Cl, Br and I it fits chlorine.
Show the answer

X is more reactive than bromine, so of Cl, Br and I it fits chlorine.

A more reactive halogen displaces a less reactive one from its halide. X displaces bromine, and not the other way round, so X is above bromine in Group 17.

Check your understanding

Astatine is below iodine in Group 17. Which prediction is most likely to be correct?

  1. A pale yellow gas that displaces iodine from potassium iodide solution
  2. A dark solid that does not displace iodine from potassium iodide solution
  3. A red-brown liquid that is more reactive than chlorine in displacement
  4. A dark solid made of single At atoms, since it is below iodine
Show the answer

A dark solid that does not displace iodine from potassium iodide solution

Down Group 17, colour darkens and melting and boiling points rise, so astatine should be a dark solid. Reactivity falls, so it cannot displace iodine.

05

Recognise noble gases and transition metals

Electron arrangement explains one family; characteristic properties identify another.

Group 18 elements have filled outer shells, making them chemically unreactive under ordinary conditions. Helium has two outer electrons; neon and argon have eight. They do not need to gain, lose or share electrons to reach this stable arrangement.

Check your understanding

Helium has only two electrons. Why is it unreactive?

  1. Its two electrons are shared, so it already has one bond.
  2. Two electrons fill its first shell, so its outer shell is full.
  3. It is too small for other atoms to collide with it often.
Show the answer

Two electrons fill its first shell, so its outer shell is full.

The first shell holds a maximum of two electrons. Like the other noble gases, helium has a full outer shell, so it does not tend to lose, gain or share electrons.

Check your understanding

Neon is 2.8 and argon is 2.8.8. Why do they exist as single atoms and not form compounds?

  1. Their atoms are joined by strong covalent bonds, so they cannot react with anything else.
  2. Their outer shells are nearly empty, so they have no electrons to share with other atoms.
  3. Each atom already has a full outer shell, so it does not lose, gain or share electrons.
Show the answer

Each atom already has a full outer shell, so it does not lose, gain or share electrons.

A full outer shell is a stable arrangement. With no tendency to bond, neon and argon atoms do not join to each other (they are monoatomic) or react to form compounds.

06

Build the metal reactivity series from experiments

More reactive metals form positive ions more readily.

Make a guess. You are not marked.

An iron nail is left in blue copper(II) sulfate solution. What happens?

  1. A brown coating forms on the nail and the blue colour fades
  2. The nail gets a silver coating and the solution turns colourless at once
  3. Nothing happens, because the nail is solid
Show the answer

A brown coating forms on the nail and the blue colour fades

Iron is more reactive than copper, so it displaces copper from the solution. Copper coats the nail, and fewer blue copper(II) ions remain.

The prescribed order is K, Na, Ca, Mg, Zn, Fe, Pb, H, Cu, Ag, from most to least reactive. Hydrogen is a reference point, not a metal. Compare reactions under the same conditions: a larger surface area or higher temperature can make a reaction look faster without changing the underlying reactivity order.

Evidence from water and dilute hydrochloric acid
MetalsWater or steamDilute HCl
K, Na, CaReact with cold water; hydroxide and hydrogen formVery vigorous reaction; acid is not a suitable routine comparison
MgVery slow with cold water; heated magnesium reacts with steam to give MgO and H2Reacts readily
Zn, FeReact with steam when strongly heatedReact to give a chloride and hydrogen
PbNo appreciable reaction with water or steam in the usual school comparisonReaction may be slow or stop as insoluble PbCl2 coats the surface
Cu, AgNo reaction with water or steamDo not liberate hydrogen

Worked exampleDeduce an order without memorising it

Under comparable conditions, A reacts rapidly with dilute HCl, B reacts slowly, and C does not react.

Try it first. Then show the worked stepsHide the worked steps
  1. The evidence places A and B above hydrogen and C below it, assuming no passivating coating.
  2. The rates suggest A is more reactive than B, but comparable surface area and temperature matter.
  3. A displacement experiment can give additional evidence.
Answer

A > B > H > C is supported under the stated conditions. Surface coatings, such as on lead, can complicate rate comparisons.

Check your understanding

Why does copper not produce hydrogen with dilute hydrochloric acid?

  1. Copper is below hydrogen in the reactivity series, so it cannot displace it.
  2. Copper has an oxide coating, like aluminium, which stops the acid reaching it.
  3. Hydrochloric acid is too weak an acid to react with a metal as hard as copper.
Show the answer

Copper is below hydrogen in the reactivity series, so it cannot displace it.

Only metals above hydrogen displace it from dilute acids. Copper is less reactive than hydrogen, so no hydrogen forms.

07

Use reactivity to explain extraction and heating

Strongly bound compounds of reactive metals are harder to break down.

An ore contains a metal compound in a form worth extracting. More reactive metals generally need more demanding extraction. Carbon can reduce oxides of metals below it in the relevant series, such as zinc, iron, lead and copper, when heated appropriately. Very reactive metals such as potassium, sodium, calcium and magnesium require other methods, commonly electrolysis of suitable molten compounds. This is a principle, not a requirement to memorise every industrial furnace.

Check your understanding

Why is heating magnesium oxide with carbon not the usual way to extract magnesium?

  1. Magnesium is more reactive than carbon, so carbon cannot remove its oxygen.
  2. Magnesium is less reactive than carbon, so it would bond to the carbon instead.
  3. Carbon would react with the magnesium oxide too violently for the furnace.
Show the answer

Magnesium is more reactive than carbon, so carbon cannot remove its oxygen.

Carbon reduces oxides of metals below it in the reactivity series, such as iron and zinc. Magnesium is above carbon, so it is extracted by electrolysis of a molten compound.

08

Prevent rust by removing a condition or supplying protection

Rusting needs both oxygen and water.

Make a guess. You are not marked.

An iron nail is placed in water that was boiled to remove dissolved air, with a layer of oil on top. Does it rust?

  1. Yes, because the nail is surrounded by water
  2. Yes, faster than usual, as the water was hot
  3. No, as there is little oxygen in the water
Show the answer

No, as there is little oxygen in the water

Rusting needs both oxygen and water. Boiling removes the dissolved oxygen and the oil stops more air dissolving.

Iron rusts when oxygen and water are both available. In a fair comparison, iron exposed to dry air lacks water; iron in boiled water protected by an oil layer lacks fresh oxygen; iron in ordinary water exposed to air has both. The last sample rusts. The controls isolate the conditions rather than showing that water alone is sufficient.

Painting, greasing and plastic coating provide a barrier to oxygen and water. Protection fails at a scratch if the exposed iron can contact both. The coating must remain intact and suit the object: grease can protect moving metal parts where paint would be unsuitable.

Check your understanding

Iron kept in a tube of dry air does not rust. Does this prove that oxygen is not needed for rusting?

  1. No. Oxygen is needed, but rusting in dry air takes weeks rather than days to appear.
  2. No. Oxygen is present, so it shows oxygen alone is not enough; water is needed too.
  3. Yes. The iron had plenty of air, so the result shows rusting does not depend on oxygen.
Show the answer

No. Oxygen is present, so it shows oxygen alone is not enough; water is needed too.

Dry air contains oxygen but no water. The result shows water is necessary. Compare with wet iron in air, and wet iron without oxygen, to show both are needed.

Quick revision

Revisit the essentials, then return to an explanation when you need it.

Exam definitions: learn these in mark-scheme words
TermDefinition to write
GroupA vertical column of the Periodic Table. Its elements have the same number of outer (valence) electrons.
PeriodA horizontal row of the Periodic Table. Its elements have the same number of occupied electron shells.
RustingThe corrosion of iron. It needs both oxygen and water, and forms hydrated iron(III) oxide.

Groups share valence-electron patterns. Group 1 becomes more reactive down the group; group 17 becomes less reactive. Noble gases have full outer shells.

Metal order: K, Na, Ca, Mg, Zn, Fe, Pb, H, Cu, Ag. Use comparable experiments. Rusting needs oxygen and water; barriers remove contact with them.

Scope

Learning outcomes

8. Periodicity and Metals (5086 / 5088 / K326 / K328). Use the outcome map to find the explanation for a particular syllabus requirement.

See the learning outcome map

  1. 8.1(a) Order elements by proton number

    • Increasing atomic number

    Read position from electron arrangement

  2. 8.1(b) Relate position and electron arrangement

    • Periods and occupied shells
    • Proton number

    Read position from electron arrangement

  3. 8.1(c) Explain group similarity

    • Same valence electron count

    Read position from electron arrangement

  4. 8.1(d) Describe metallic-to-non-metallic trend

    • Across a period

    Metal to non-metal across a period

  5. 8.1(e) Relate outer electrons to character

    • Electron loss, gain and sharing

    Metal to non-metal across a period

  6. 8.1(f) Predict group properties

    • Group 1 and group 17

    Group 1: lithium, sodium and potassiumGroup 17: identify displacement from the evidence

  7. 8.2(a) Compare alkali metals

    • Li, Na, K
    • Softness and low density
    • Melting-point and water-reaction trends

    Group 1: lithium, sodium and potassium

  8. 8.2(b) Compare halogens

    • Cl2, Br2, I2
    • Diatomic forms, colours and states
    • Halide displacement

    Group 17: identify displacement from the evidence

  9. 8.2(c) Explain noble-gas unreactivity

    • Full outer shells

    Recognise noble gases and transition metals

  10. 8.3(a) Order prescribed metals

    • K, Na, Ca, Mg, Zn, Fe, Pb, H, Cu, Ag
    • Water, steam, dilute HCl

    Build the metal reactivity series from experiments

  11. 8.3(b) Deduce reactivity from data

    • Comparable experiments
    • Confounding factors

    Build the metal reactivity series from experiments

  12. 8.3(c) Relate extraction to reactivity

    • Ease of obtaining metals from ores

    Use reactivity to explain extraction and heating

  13. 8.3(d) Explain rusting and barriers

    • Oxygen plus water
    • Painting, greasing, plastic coating

    Prevent rust by removing a condition or supplying protection