This mini-lesson walks you through the whole of CCEA Unit 1.6 — The Periodic Table: how it is arranged, how Mendeleev built it, and the families you must know — Group 1 alkali metals, Group 7 halogens, Group 0 noble gases and the transition metals.
Make a guess: why did Mendeleev break his own rule?
Mission 1How did breaking his own rule help Mendeleev?
Mission 2Why does displacement only go one way?
Work through each screen, answer the questions as you go (some are wordy, some spot trends) and collect ⭐ stars. Press Start when you're ready.
Arrangement
How the table is built
The Periodic Table organises every known element. CCEA wants you to know its layout:
An element is a substance made of only one type of atom, and cannot be broken down into simpler substances by chemical means.
Elements are placed in order of increasing atomic (proton) number.
A group is a vertical column; a period is a horizontal row.
Metals sit to the left of the "staircase" line; non-metals to the right.
Metals (left) are malleable, ductile, sonorous and good conductors, usually with high melting points; non-metals (right) are generally the opposite.
States: at room temperature, just two elements are liquids — mercury and bromine; most are solids; 11 are gases (including the noble gases, fluorine, chlorine, hydrogen, oxygen and nitrogen).
Quick check
Group or period?
?Sodium (Na) and potassium (K) sit one above the other on the far left. What does sharing a vertical column tell you about them?
Electrons & position
Position tells you the electrons
An element's place in the table is set by its electron arrangement. CCEA wants you to link the two:
The group number = the number of electrons in the outer shell (Group 1 → 1 outer electron; Group 7 → 7; Group 0 → a full shell).
The period number = the number of occupied shells the atom has.
Same group → same outer electrons → similar chemical properties.
Sodium (2, 8, 1): 1 outer electron places it in Group 1; 3 shells place it in Period 3.
Watch out: the group number is the count of outer-shell electrons, not the total electrons or the number of shells. The number of shells gives you the period.
Quick check
Read the configuration
1An atom has the electronic configuration 2, 8, 7. Which group number is it in?
(group)
Hint: the group number = the number of electrons in the outer shell.
Historical development
Mendeleev's table
In 1869 the Russian scientist Dmitri Mendeleev published the arrangement that became the basis of today's table:
He listed elements in order of increasing atomic mass, but would swap the order if an element's properties fitted a different position better.
He left gaps for undiscovered elements and predicted their properties — when germanium was found, it matched his predictions closely.
Modern vs Mendeleev: today's table is ordered by atomic number (not mass), includes the noble gases, has no gaps and more elements, and puts the transition metals in their own central block (plus lanthanides & actinides).
Quick check
What did the gaps do?
?Why is Mendeleev given so much credit even though many elements were still undiscovered in 1869?
Mission 1 check
How did breaking his own rule help Mendeleev?
?Tellurium atoms are heavier than iodine atoms. Iodine's properties are like those of chlorine and bromine. Yet Mendeleev put tellurium before iodine. Why did he break his mass order?
Group 1 (I)
The alkali metals
Group 1 — Li, Na, K… — are very reactive metals, stored under oil to keep air and water away. They are:
soft and easily cut with a knife; shiny when freshly cut but tarnish rapidly in air;
low density — the first three (Li, Na, K) are less dense than water and float;
grey solids with relatively low melting points (which decrease down the group).
They react with water to give a metal hydroxide + hydrogen:
Group 1 metal + water → metal hydroxide + hydrogene.g. 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
Observations (potassium): floats, fizzes, heat released, lilac flame, moves on the surface and disappears with a crackle/explosion. Sodium is similar but with no flame or crackle. The H₂ gives a squeaky pop with a lighted splint.
Group 1 trend
Reactivity increases down Group 1
Every Group 1 atom has one outer electron, which it loses when it reacts to form a 1+ ion with a stable, full-shell configuration:
Na → Na⁺ + e⁻half-equation: a Group 1 atom loses its single outer electron
Down the group the outer electron is further from the nucleus, so it is lost more easily — reactivity increases.Predict the trend
Lithium, sodium or potassium?
?Rubidium (Rb) sits below potassium in Group 1. Predict how vigorously it reacts with water compared with potassium.
Mission 2 of 2
Why does displacement only go one way?
Make a guess: why does halogen displacement only go one way?
Group 7 (VII)
The halogens
The halogens are reactive, toxic non-metals that exist as diatomic molecules (F₂, Cl₂, Br₂, I₂). You must recall their colours and states:
Down the group the colour darkens and the state shifts gas → liquid → solid.Match it
Colour & state match
Tap a halogen on the left, then tap its correct colour & state on the right.
Group 7 trend
Reactivity decreases down Group 7
Every halogen atom has seven outer electrons and reacts by gaining one electron to form a 1− halide ion with a full outer shell:
Cl₂ + 2e⁻ → 2Cl⁻half-equation: a halogen molecule gains electrons to form halide ions
Down the group the incoming electron is added to a shell that is further from the nucleus and more shielded by inner shells, so the attraction is weaker:
Opposite to Group 1: gaining an electron gets harder down the group, so reactivity decreases.
Watch out: Group 1 reactivity increases going down (an electron is lost); Group 7 reactivity decreases going down (an electron is gained). Don't mix the two trends up.
Displacement reactions
A more reactive halogen wins
The trend can be shown by displacement: a more reactive halogen displaces a less reactive one from a solution of its halide.
Chlorine is more reactive than iodine, so chlorine displaces iodine from potassium iodide:
Cl₂ + 2KI → I₂ + 2KClionic: Cl₂ + 2I⁻ → I₂ + 2Cl⁻
The colourless KI solution turns brown as iodine is displaced — proof chlorine is the more reactive halogen.
Watch out: displacement only happens when the added halogen is more reactive than the one in the salt. Iodine cannot displace chlorine from KCl — iodine is less reactive, so nothing happens.
Predict the reaction
Will it react?
?Bromine water is added to a solution of potassium chloride (KCl). What happens?
Group 0
The noble gases
Group 0 are unreactive non-metals — the noble gases (He, Ne, Ar…). CCEA wants you to explain why they are so inert:
Each atom has a full outer shell of electrons — a stable arrangement, so it has no tendency to gain, lose or share electrons.
They are colourless gases at room temperature and exist as individual atoms (monatomic).
Going down the group, boiling points increase because the van der Waals' forces between atoms get stronger.
Neon (2, 8): a full outer shell is a stable arrangement — this explains why the noble gases are so unreactive.Quick check
Why so unreactive?
?Argon is used to fill light bulbs because it will not react with the hot filament. What makes argon so unreactive?
Mission 2 check
Why does displacement only go one way?
?Bromine water is added to potassium iodide solution, and iodine forms. Iodine is added to potassium bromide solution, and nothing happens. Why does this displacement only go one way?
Transition metals
The transition metals
The transition metals form the central block. Compared with the Group 1 alkali metals, they differ in several ways:
The transition metals sit between Groups 2 and 3 — a wide central block.
Higher melting points than Group 1 metals.
Higher density than Group 1 metals.
Much less reactive with water (Group 1 react vigorously).
Form ions with different charges — e.g. iron(II) Fe²⁺ and iron(III) Fe³⁺ — whereas Group 1 metals only ever form 1+ ions.
Form coloured compounds: copper(II) oxide is black, copper(II) carbonate is green, hydrated copper(II) sulfate is blue, and copper(II) salts are usually blue in solution.
Watch out: "transition metal vs Group 1" is a classic CCEA compare question — remember transition metals are denser, higher-melting, less reactive and form variable-charge, coloured compounds.
Quick check
Transition metal or Group 1?
?A metal forms a green carbonate and can make ions of two different charges. Which statement is correct?
Sort it
Which trend goes which way?
Tap a statement, then tap the group whose trend it describes.
⬇ Group 1 — reactivity INCREASES
⬇ Group 7 — reactivity DECREASES
Recap
The big ideas to know
Arrangement: ordered by atomic number; group = column, period = row; metals left of the staircase, non-metals right.
Electrons: group number = outer-shell electrons; period number = number of shells.
Mendeleev: ordered by mass, left gaps, predicted undiscovered elements (e.g. germanium).
Group 1: soft, reactive metals; react with water → hydroxide + H₂; reactivity increases down (lose 1 e⁻).
Group 7: toxic, diatomic; colour darkens / gas→solid down; reactivity decreases down (gain 1 e⁻); displacement = more reactive displaces less.
Group 0: inert — full outer shells; colourless gases; boiling point rises down.