This mini-lesson walks you through the whole of Cambridge IGCSE Chemistry (0620) Topic 8: how the table is arranged, the Group I alkali metals, the Group VII halogens, the transition elements and the noble gases.
Make a guess: why do alkali metals get more reactive further down?
Mission 1Why are Group 1 metals not equally reactive?
Work through each screen, answer the questions as you go and collect ⭐ stars. Supplement (extended-only) points are clearly flagged. Press Start when you're ready.
8.1 · Arrangement
How the table is built
The elements are laid out in order of increasing proton number (atomic number) — each element has one more proton than the one before it. Two rules let you read off an atom's electron arrangement straight from its position:
The group number (the vertical column) tells you the number of electrons in the outer shell for Groups I–VII; the Group VIII noble gases have a full outer shell.
The period number (the horizontal row, 1–7) tells you the number of occupied electron shells.
Across a period, elements change from metallic to non-metallic character (left to right).
The group number links to the charge on the ions: Group I → 1+, II → 2+, III → 3+, V → 3−, VI → 2−, VII → 1−.
Elements in the same group have the same number of outer-shell electrons, so they have similar chemical properties — which is why an element's position lets you predict its properties.
Metals sit to the left of the stepped line, non-metals to the right. Na is in Group I, Period 3 → 1 outer electron, 3 shells.
⬥ Supplement (extended only)
Given data about the elements in a group (e.g. melting points or densities), identify the trend and use it to predict a value for another element in that group.
Quick check
Reading a position
?An element sits in Group II, Period 3. How many electrons are in its outer shell, and how many occupied shells does it have?
8.2 · Group I — the alkali metals
The alkali metals
Lithium, sodium and potassium are the alkali metals you must know. Their physical properties stand out for metals:
Soft — they can be cut with a knife.
Low density — Li, Na and K all float on water.
Low melting points (low for metals).
Down the group: melting point decreases, density increases (a general trend — potassium is slightly less dense than sodium) and reactivity increases.
They react with water to give a metal hydroxide (an alkaline solution) plus hydrogen gas:
Reactivity with water increases down Group I: K reacts far more violently than Li.
Watch out: Group I reactivity increases down the group. This is the opposite of Group VII (the halogens), where reactivity decreases down the group — a classic exam trap.
⬥ Going further (not required by 0620)
The trend is explained by the single outer electron: further down the group the outer electron is in a shell further from the nucleus, so it is held less tightly and is lost more easily — making the atom more reactive.
Quick check
Predict the Group I trend
?Rubidium (Rb) sits below potassium in Group I. Predict how it reacts with water compared with potassium.
Mission 1 check
Why are Group 1 metals not equally reactive?
?A teacher drops small pieces of lithium, sodium and potassium into separate troughs of water. Which statement is correct?
8.3 · Group VII — the halogens
The halogens
The halogens are coloured, diatomic non-metals. Their colours and physical states at room temperature are worth memorising:
Going down: chlorine (gas) → bromine (liquid) → iodine (solid). Colour darkens, melting point and density rise, reactivity falls.
A more reactive halogen will displace a less reactive halogen from a solution of its salt (a halide). For example, chlorine displaces bromine:
Displacement: the more reactive halogen ends up as the element; the displaced halogen's colour appears in solution.
Watch out: displacement only happens when the added halogen is more reactive than the one in the salt. Chlorine displaces bromine and iodine; bromine displaces only iodine; iodine displaces neither.
⬥ Going further (not required by 0620)
Reactivity decreases down Group VII because a halogen reacts by gaining one electron. Lower down, the outer shell is further from the nucleus, so the atom attracts and gains an electron less easily.
Match-up
Halogen colours & states
Tap a halogen on the left, then tap its correct colour and state on the right.
Quick check
Will it displace?
?Bromine water is added to a colourless solution of potassium iodide (KI). What happens?
Quick check
Identify the halogen
?A Group VII element is a red-brown liquid at room temperature. Which halogen is it?
8.4 · The transition elements
The transition elements
The transition elements sit in the central block of the table (between Groups II and III). Compared with the Group I metals, they are typically:
Coloured compounds (e.g. blue Cu²⁺, pale green Fe²⁺, yellow-brown Fe³⁺) and catalysis (e.g. iron in the Haber process) are hallmark transition-metal behaviour.
Watch out: don't confuse transition metals with Group I. Group I metals are soft, low-density with low melting points and form white/colourless compounds; transition metals are hard, dense with high melting points and coloured compounds.
⬥ Supplement (extended only)
Transition elements have ions with variable oxidation numbers (variable charges) — for example iron forms Fe²⁺ (iron(II)) and Fe³⁺ (iron(III)). Group I metals only ever form a fixed +1 ion.
Quick check
Transition or Group I?
?Which property is typical of a transition element but not of a Group I metal?
8.5 · The noble gases (Group VIII / 0)
The noble gases
The noble gases — helium, neon, argon and the rest — form Group VIII (also written Group 0). They are unreactive (inert) and exist as single atoms (monatomic gases).
A full outer shell means a noble gas has no tendency to gain, lose or share electrons — so it stays inert.
Why so unreactive? Noble gases already have a full outer electron shell. They have no tendency to gain, lose or share electrons, so they hardly react with anything. When other elements react, their atoms often end up with this same full-outer-shell arrangement.
Quick check
Why are they unreactive?
?The noble gases are extremely unreactive. What is the reason?
Sort it
Which way does reactivity go?
Tap a statement, then tap the box that matches the trend going down the group.
⬆ Reactivity INCREASES down
⬇ Reactivity DECREASES down
Recap
The points to know
Arrangement: increasing proton number; group number = outer electrons (and sets the ion charge); period number = occupied shells; metallic → non-metallic across a period; same group → similar properties, so position predicts properties. ⬥ identify trends in a group from given data.
Group I: soft, low density, low m.p.; react with water → hydroxide + hydrogen; down the group m.p. decreases, density and reactivity increase. (Going further: the outer electron is lost more easily lower down.)
Group VII: Cl₂ pale yellow-green gas, Br₂ red-brown liquid, I₂ grey-black solid; density increases and reactivity decreases down; more reactive halogen displaces less reactive. (Going further: harder to gain an electron lower down.)
Transition elements: high density, high m.p., coloured compounds, catalysts. ⬥ variable oxidation numbers (Fe²⁺/Fe³⁺).