8 The Periodic Table | IGCSE Chemistry 0620

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🎯What you need to be able to do

Describe the Periodic Table as elements in order of proton number, in periods and groups; describe the change from metal to non-metal across a period.
Relate group number to ion charge and explain similar chemical properties in a group by electronic configuration.
Describe the trends in Group I (lithium, sodium, potassium) and Group VII (chlorine, bromine, iodine), and predict the properties of other members.
Describe and explain halogen displacement reactions.
Describe the properties of the transition elements, and their variable oxidation numbers EXTENDED.
Explain why the noble gases are unreactive; identify trends from given data EXTENDED.

📚The chemistry

Arrangement

Elements are arranged in order of increasing proton number. A row is a period; a column is a group. Across a period the elements change from metals on the left to non-metals on the right.

The first four periods of the Periodic Table, hydrogen to krypton, with proton numbers. Groups I to VIII are labelled across the top and periods 1 to 4 down the side. Colour coding shows Group I alkali metals, other metals, the transition elements from scandium to zinc, non-metals, the Group VII halogens and the Group VIII noble gases. A thick stepped line separates metals from non-metals, running between beryllium and boron, aluminium and silicon, and germanium and arsenic. — The group number is the number of outer-shell electrons (Group VIII: a full outer shell); the period number is the number of occupied shells.

Elements in the same group have the same number of outer electrons, so they react in similar ways. Metals lose their outer electrons: Group I forms 1+ ions, Group II 2+ ions, Group III 3+. Non-metals gain electrons to fill the shell: Group VI forms 2− ions and Group VII 1− ions. So you can predict an unfamiliar element’s properties from its position.

Group I: the alkali metals

Lithium, sodium and potassium are relatively soft metals that are stored under oil. Going down the group:

melting point decreases

density increases (in general)

reactivity increases

They all react with cold water to make an alkaline hydroxide solution and hydrogen:

\[ \mathrm{2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)} \]

All three float (they are less dense than water) and fizz, giving hydrogen. The reaction gets more vigorous down the group.

Reactivity increases because the outer electron is further from the nucleus, so it is lost more easily.

Group VII: the halogens

The halogens are diatomic non-metals (Cl₂, Br₂, I₂). At room temperature and pressure: chlorine is a pale yellow-green gas, bromine a red-brown liquid, iodine a grey-black solid. Going down the group, density increases and reactivity decreases (it gets harder to gain an electron as the outer shell gets further from the nucleus).

A more reactive halogen displaces a less reactive one from a solution of its halide ions:

\[ \mathrm{Cl_2(aq) + 2KI(aq) \rightarrow 2KCl(aq) + I_2(aq)} \]

A grid of results. Chlorine water added to potassium bromide turns orange as bromine forms, and added to potassium iodide turns brown as iodine forms. Bromine water added to potassium iodide turns brown. Every other combination is no reaction; a halogen is not added to its own halide. — Three displacements, three no-reactions. The colour of the product halogen in solution is the evidence: bromine orange, iodine brown.

EXTENDED Displacement is redox: in \( \mathrm{Cl_2 + 2I^{-} \rightarrow 2Cl^{-} + I_2} \) chlorine gains electrons (reduced) and the iodide ions lose them (oxidised).

Transition elements

The block between Groups II and III. Compared with Group I metals, transition elements have high densities and high melting points, they form coloured compounds (copper(II) sulfate is blue, iron(III) compounds are orange-brown), and they and their compounds often act as catalysts (iron in the Haber process, vanadium(V) oxide in the Contact process).

EXTENDED Their ions have variable oxidation numbers: iron forms iron(II), Fe²⁺, and iron(III), Fe³⁺, so it has two chlorides, FeCl₂ and FeCl₃. The Roman numeral in the name tells you which.

Noble gases

Group VIII elements are unreactive, monatomic gases. Their atoms already have a full outer shell (a stable electronic configuration), so they have no tendency to gain, lose or share electrons.

Shell diagrams of helium with 2 electrons, neon with 2,8 and argon with 2,8,8. Each outer shell is full. — Helium (2), neon (2,8) and argon (2,8,8): every outer shell is full, which is why they do not form compounds.

✏️Worked example

Aqueous bromine is added to aqueous potassium iodide. (a) Describe the colour change and write the word equation. [2] (b) Explain the result in terms of reactivity. [1] (c) EXTENDED Write the ionic equation, and state which species is oxidised. [2] (d) Astatine is below iodine in Group VII. Predict its state at room temperature and whether aqueous iodine would react with potassium astatide. [2]

(a) Orange to brown. bromine + potassium iodide → potassium bromide + iodine.

(b) Bromine is more reactive than iodine, so it displaces iodine from its compound.

(c) \( \mathrm{Br_2(aq) + 2I^{-}(aq) \rightarrow 2Br^{-}(aq) + I_2(aq)} \). The iodide ions are oxidised: each loses an electron.

(d) A solid (the trend runs gas → liquid → solid down the group, and astatine is below solid iodine). Yes: iodine is more reactive than astatine, so it would displace astatine.

Check it. Potassium is in both the reactants and the products as K⁺, so it is left out of the ionic equation; charge balances at −2 on each side ✓.

Bromine and bromide. The element (bromine, Br₂) is coloured and reactive; the ion (bromide, Br⁻) is colourless and is what gets displaced. Swapping the two names loses the mark.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which element is in Period 3 and Group II? A: beryllium. B: calcium. C: magnesium. D: aluminium.

C. Magnesium is 2,8,2: three shells (Period 3), two outer electrons (Group II).

2. (Theory.) A small piece of potassium is added to water containing universal indicator. Give two observations and name both products. [4]

Any two: floats; moves about; fizzes; lilac flame; the metal disappears; the indicator turns blue/purple. Products: potassium hydroxide and hydrogen.

3. (Theory.) Explain why sodium and potassium have similar chemical properties. [2]

They are both in Group I, so both have one electron in their outer shell, which they lose in reactions to form a 1+ ion.

4. (Theory.) EXTENDED The boiling points of the halogens are: fluorine −188 °C, chlorine −34 °C, bromine 59 °C, iodine 184 °C. (a) Describe the trend, and predict a boiling point for astatine. (b) Chlorine melts at −101 °C. State its physical state at −50 °C. [3]

(a) Boiling point increases down the group; astatine higher than 184 °C (any value clearly above, e.g. about 300 °C). (b) Liquid: −50 °C is above its melting point (−101 °C) but below its boiling point (−34 °C).

5. (Theory.) Name one transition element used as a catalyst, and state the process it is used in. Give one other property that distinguishes transition elements. [2]

Iron in the Haber process (or vanadium(V) oxide in the Contact process). Any: coloured compounds; high density; high melting point.

6. (Theory.) Explain, in terms of electrons, why argon is unreactive, and state one use that depends on this. [2]

Its outer shell is full (2,8,8), so it does not gain, lose or share electrons. Use: filling light bulbs, or as an inert atmosphere for welding.

7. (Theory.) EXTENDED Iron reacts with chlorine to give an orange-brown solid, but with hydrochloric acid to give a pale green solution. What does this show about the ions of iron? Give the formula of each iron compound formed. [3]

Iron has variable oxidation numbers: iron(III) with chlorine, FeCl₃; iron(II) with the acid, FeCl₂.

8. (Theory.) Predict one observation when aqueous chlorine is added to aqueous sodium bromide, and explain it. [2]

The solution turns orange (bromine forms). Chlorine is more reactive than bromine, so it displaces bromine from sodium bromide.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

Royal Society of Chemistry — the interactive Periodic Table, with data for every element
Royal Society of Chemistry — halogen displacement microscale practical