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IGCSE Chemistry 0620 · Chapter 8 notes · ~12 min read

The Periodic Table

The map of all elements: why groups behave alike, why Group I gets more reactive down and Group VII gets less — plus transition elements and the noble gases.

Groups & periodsGroup IGroup VIITransition elementsNoble gasesTrends
RSby Riyaz Sir
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Elements are arranged in order of proton number. Columns (groups) have the same number of outer electrons, so similar properties. Rows (periods) show the number of shells.
  • Group I metals are more reactive as you go down. Group VII halogens are less reactive as you go down.
  • Transition elements are hard, dense metals with high melting points, coloured compounds and good catalysts.
  • Noble gases have a full outer shell, so they do not react.

By the end you can

  • Describe how the table is arranged and link group and period numbers to electrons.
  • Describe the trends in Group I and Group VII and predict the properties of other members.
  • Describe halogen displacement reactions.
  • Describe the properties of transition elements and the noble gases.
  • Explain trends using the electronic configuration of atoms. EXT

1How the table is arranged

transition elementsIIIIIIIVVVIVII0HHe1LiBeBCNOFNe2NaMgAlSiPSClAr3KCaScTiVCrMnFeCoNiCuZnGaGeAsSeBrKr4RbSrYZrNbMoTcRuRhPdAgCdInSnSbTeIXe5alkali metalsalkaline earthtransitionother metalsmetalloidsnon-metalshalogensnoble gasesgroup number = outer electrons · period number = number of shells
The first four periods of the Periodic Table with the families colour-coded. Group number = outer electrons; period number = number of shells.

Reading the table

  • Elements are in order of increasing proton number, from left to right across each period.
  • A group is a vertical column (I to VII and VIII/0). Elements in the same group have the same number of outer-shell electrons and so similar chemical properties.
  • A period is a horizontal row. The period number is the number of electron shells.
  • Group number = outer electrons (for Groups I–VII). Chlorine (2,8,7) → Group VII, Period 3.
  • Group number and ion charge: Group I → +1, Group II → +2, Group III → +3. Group VI → 2−, Group VII → 1−. Groups IV and V mostly form covalent compounds.

Metals to non-metals

  • Across a period the elements change from metals (left) to non-metals (right). A "staircase" line separates them. Metalloids (such as silicon) lie along the line and have properties of both.
  • The type of oxide changes too: Na₂O and MgO are basic, Al₂O₃ is amphoteric, SiO₂, P₄O₁₀ and SO₂ are acidic. See oxides.

Using position to predict

  • To predict, find the element in its group and use the trend. If you know K and Cs, rubidium (Rb) lies between them.
  • EXT You may be given data for some group members and asked to describe the trend or predict a missing value.
Worked example

An element has the electronic configuration 2,8,6. Find its group, period and likely ion.

  1. 6 outer electrons → Group VI
  2. 3 shells → Period 3
  3. it needs 2 electrons to fill its shell → ion X²⁻
  4. the element is sulfur

2Group I — the alkali metals

What they are like

reactivity increases down the grouplithiumsodiumpotassiumfizzes steadilyfloatsfizzes quicklymelts into a ballmoves aboutvery vigorouslilac flame2Na + 2H₂O → 2NaOH + H₂alkaline solution: universal indicator turns purple
Lithium, sodium and potassium in water. Reactivity increases down the group. All make hydrogen and an alkaline solution.
  • Soft metals that can be cut with a knife; shiny when freshly cut but they tarnish quickly in air.
  • Low density — lithium, sodium and potassium float on water.
  • Low melting points for metals. They decrease down the group.
  • They form +1 ions and white ionic compounds that dissolve in water to give colourless solutions.
  • Stored under oil to keep air and water away.
ElementMelting point (°C)Density (g/cm³)With water
lithium1810.53fizzes steadily
sodium980.97fizzes quickly, melts into a ball
potassium630.86very vigorous, lilac flame
rubidium391.5violent, explodes
caesium291.9explodes
  • Melting points go down the group. Density generally goes up (potassium is an exception). Reactivity goes up.
  • Reaction with water: metal + water → metal hydroxide + hydrogen. For example 2Na + 2H₂O → 2NaOH + H₂. The solution is alkaline — universal indicator turns purple.

Why does reactivity increase down the group?EXTENDED

  • A Group I atom reacts by losing its one outer electron.
  • Down the group the atoms get bigger: the outer electron is further from the nucleus and more shielded by inner shells, so it is less strongly attracted and easier to lose.
Picture itThink of a magnet pulling on a paper clip: when the clip is further away, the pull is weaker and it is easier to remove. The outer electron of caesium is further from the "magnet" than that of lithium.
Worked example

Predict the melting point and the reaction with water of rubidium, using the table above.

  1. The melting points go down the group: K 63 °C and Cs 29 °C, so Rb is between 29 and 63 °C (about 39 °C)
  2. reactivity increases down the group, so rubidium reacts more violently than potassium with water
  3. 2Rb + 2H₂O → 2RbOH + H₂ and the solution is alkaline
Common mistakeMixing up the trend with Group VII. In Group I, reactivity increases down the group.

3Group VII — the halogens

Diatomic non-metals

chlorine Cl₂bromine Br₂iodine I₂pale yellow-greengasred-brown liquidorange vapourgrey-black solidpurple vapourdown the group: darker colour · higher melting/boiling point · less reactiveF₂: pale yellow gas(very reactive)At₂: black solid(radioactive)a more reactive halogen displaces a less reactive one from a salt solutionCl₂ + 2KBr → 2KCl + Br₂ (turns orange-brown)Cl₂ + 2KI → 2KCl + I₂ (turns brown)Br₂ + 2KI → 2KBr + I₂ (turns brown)Br₂ + KCl → no reaction (Br₂ is less reactive than Cl₂)
The halogens: chlorine, bromine and iodine. Down the group the colour gets darker and the state changes from gas to liquid to solid. A more reactive halogen displaces a less reactive one.
  • The halogens are non-metals with diatomic molecules: F₂, Cl₂, Br₂, I₂. They have 7 outer electrons and form 1− ions (halides: Cl⁻, Br⁻, I⁻).
HalogenColour and state at room temperatureMelting and boiling point
chlorine Cl₂pale yellow-green gaslowest
bromine Br₂red-brown liquid (gives orange-brown vapour)middle
iodine I₂grey-black solid (gives purple vapour)highest
  • Down the group: the colour gets darker, the melting and boiling points and density increase, and the reactivity decreases.
  • Fluorine is a very reactive pale yellow gas at the top. Astatine is a black, radioactive solid at the bottom.
  • Predicting astatine: a black solid, with a higher melting point than iodine, less reactive than iodine, and forming At⁻ ions.

Why does reactivity decrease down the group?EXTENDED

  • A halogen atom reacts by gaining one electron into its outer shell.
  • Down the group the outer shell is further from the nucleus and more shielded, so the nucleus attracts the incoming electron less strongly, and the atom is less reactive.
Memory trickGroup I: the bigger the atom, the easier to lose an electron, so more reactive. Group VII: the smaller the atom, the easier to gain an electron, so more reactive. Opposite trends!

Displacement reactions

Displacement — a more reactive element takes the place of a less reactive element in a compound.

  • A more reactive halogen displaces a less reactive halogen from a solution of its halide salt.
KCl(aq)KBr(aq)KI(aq)
chlorine waterno reactionorange-brown (bromine)brown (iodine)
bromine waterno reactionno reactionbrown (iodine)
iodine solutionno reactionno reactionno reaction
  • Cl₂ + 2KBr → 2KCl + Br₂ (Cl is more reactive than Br).
  • Cl₂ + 2KI → 2KCl + I₂ and Br₂ + 2KI → 2KBr + I₂.
  • EXT Ionic equation: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂. Chlorine gains electrons (it is reduced) so it is the oxidising agent.
Worked example

Will a reaction happen if bromine water is added to potassium chloride solution? What if fluorine is added to sodium chloride solution?

  1. bromine is less reactive than chlorine, so it cannot displace chlorine: no reaction
  2. fluorine is more reactive than chlorine, so it will displace it: F₂ + 2NaCl → 2NaF + Cl₂
Common mistakeWriting that a halogen "displaces" a halogen from the element. It displaces it from the halide in solution.

4Transition elements

The middle block

Cu²⁺blueFe²⁺palegreenFe³⁺orangebrownMnO₄⁻purpletransition elements:• high melting points, dense• compounds are coloured• act as catalysts (Fe in the Haber process)• ions with different charges (Fe²⁺, Fe³⁺)coloured solutions
Transition elements form coloured compounds. Copper(II) is blue, iron(II) is pale green, iron(III) is orange-brown and manganate(VII) is purple.
  • Hard, strong and dense metals with high melting points (mercury is an exception — a liquid).
  • Coloured compounds: Cu²⁺ blue, Fe²⁺ pale green, Fe³⁺ orange-brown, MnO₄⁻ purple.
  • Good catalysts: iron in the Haber process, vanadium(V) oxide in the Contact process, nickel in making margarine.
  • EXT Variable oxidation numbers: they form ions with different charges, for example Fe²⁺ and Fe³⁺ (iron(II) and iron(III)); copper forms Cu⁺ and Cu²⁺.
Group ITransition elements
hardnesssofthard
densitylowhigh
melting pointlowhigh
reactivityvery reactivemuch less reactive
compoundswhite or colourlessusually coloured
ionsone charge onlyoften more than one charge
catalysts?noyes
Exam tipCompare Group I with transition elements using properties — "soft, low density, low melting point" versus "hard, high density, high melting point".

5Noble gases

Group VIII (0) — the unreactive gases

full outer shell → very stable, unreactive, monatomichelium2balloons, airshipsneon2,8bright advertising signsargon2,8,8filament lamps, weldingall are colourless gases · melting and boiling points rise down the group
The noble gases helium, neon and argon. Each has a full outer shell, so the atoms do not bond. They exist as single atoms.
  • The noble gases are monatomic (single atoms). They have a full outer shell (He 2; others 8), so they do not need to gain, lose or share electrons — they are very unreactive.
  • All are colourless gases. Down the group the density and boiling point increase.
GasUseWhy
heliumballoons and airshipsmuch less dense than air, and not flammable
neonadvertising signsglows red-orange when electricity passes through
argonfilament lamps; shielding gas for weldingdoes not react with the hot metal filament
Common mistake"The noble gases are unreactive because they are gases." They are unreactive because of their full outer shell.

6Compare the trends

Group I and Group VII together

Group IGroup VII
typemetalsnon-metals
outer electrons17
ion formed+1−1
reactivity down the groupincreasesdecreases
melting point down the groupdecreasesincreases
appearancesoft grey metalscoloured gases, liquid, solids
why the trend?outer electron further away, easier to loseouter shell further away, harder to gain an electron

7Test yourself

Test yourself — cover the answers first

Quick check

  1. What is the link between the group number and the electrons in an atom?
  2. What does the period number tell you?
  3. Name two properties of Group I metals.
  4. What are the products when sodium reacts with water?
  5. What is the colour and state of bromine at room temperature?
  6. Name two properties of transition elements.
  7. Why are noble gases unreactive?
  8. EXT Why is potassium more reactive than lithium?

Exam-style

  1. Describe what you would see when a small piece of potassium is put on water, and write the equation. [3]
  2. Chlorine water is added to potassium iodide solution. Describe the observation and explain it. [3]
  3. EXT Explain why the reactivity of the halogens decreases down the group. [3]
  4. Predict two properties of astatine. [2]
Show answers
  1. group number = number of outer-shell electrons
  2. the number of electron shells
  3. soft, low density, low melting point, react with water
  4. sodium hydroxide and hydrogen
  5. red-brown liquid
  6. any two from: high melting point, high density, coloured compounds, catalysts
  7. they have a full outer shell of electrons
  8. the outer electron is further from the nucleus and more shielded, so it is lost more easily
  9. floats and moves around, fizzing very vigorously
    burns with a lilac flame
    2K + 2H₂O → 2KOH + H₂
  10. the colourless solution turns brown
    chlorine is more reactive than iodine
    so chlorine displaces iodine from potassium iodide: Cl₂ + 2KI → 2KCl + I₂
  11. a halogen reacts by gaining an electron into its outer shell
    down the group the outer shell is further from the nucleus and more shielded
    so the attraction for an incoming electron is weaker
  12. black solid
    less reactive than iodine / higher melting point than iodine
Practise this chapter
Arrangement of the elementsGroup IGroup VIITransition elementsNoble gases
Next chapterMetals →
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