IB Chemistry SLTopic 3 — Classifying the ElementsPaper 1 & 2Trends~11 min read
Group 1 Metals Reacting with Water
Lithium fizzes politely. Sodium melts into a ball and skates about. Potassium bursts into lilac flame. Same reaction, same products, same group — the only thing that changes is how easily that single outer electron leaves. This is the periodic trend you can actually watch happen.
📘 What you need to know
Group 1 elements are the alkali metals: lithium, sodium, potassium, rubidium, caesium and francium.
All have an outer configuration of ns1 — one valence electron, which they lose to form a 1+ ion.
They are soft, shiny when freshly cut, good conductors, and have low melting points and low densities for metals.
They react with water to give a metal hydroxide solution and hydrogen gas. The solution is alkaline, which is where the name comes from.
Reactivity increases down the group, because the outer electron is further from the nucleus and better shielded, so it is lost more easily.
They also react vigorously with the halogens to form ionic halide salts.
They are stored under oil because they react with oxygen and water vapour in the air.
Physical properties first
Alkali metals do not behave like the metals you meet in everyday life. You can cut sodium with a knife, and lithium floats on water. Both facts come from the same source: each atom contributes only one delocalised electron to the metallic bonding, so the attraction between the cations and the sea of electrons is comparatively weak.
Soft and easy to cut — weak metallic bonding, so layers slide with almost no effort.
Low melting points, falling down the group — the cations get larger, so the attraction to the delocalised electrons weakens further.
Low densities — lithium, sodium and potassium all float on water, which is why you can watch the reaction from above.
Shiny when cut, then dull within seconds — oxygen in the air attacks the fresh surface immediately.
The reaction with water
The general pattern
group 1 metal + water → metal hydroxide + hydrogen
Three specific equations you should be able to write from memory:
2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g)
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
The 2:2:2:1 pattern is identical every time. If you can write one of these, you can write all six — including rubidium and caesium, which you will never see done in a school lab for very good reasons.
Sodium melting is itself an observation worth stating in an exam — it proves the reaction is exothermic and that sodium’s melting point is low, in one word.
Why reactivity increases down the group
Every one of these reactions starts with the same step: the metal atom loses its single outer electron. Anything that makes that easier makes the metal more reactive, so the question becomes “how tightly is that electron held?” — which is the three-factor argument from the previous page.
This is why a trend question about group 1 reactivity is really an ionisation energy question wearing different clothes.
Metallic character is the general name for this: how readily an atom loses electrons to form a positive ion. It increases down a group and decreases across a period, which is why the most reactive metals sit at the bottom left of the table.
Reaction with the halogens
The same electron transfer happens with group 17 elements, but here the halogen takes the electron directly and an ionic salt forms.
A typical example
2Na(s) + Cl2(g) → 2NaCl(s)
The trend is the same: the reaction gets more vigorous down group 1, for the same reason. The most violent combination of all is the lowest alkali metal with the highest halogen.
Worked examples
WORKED EXAMPLE
Write a balanced equation for rubidium reacting with water, and predict two observations.
Step 1: use the general pattern
Metal + water gives metal hydroxide + hydrogen. Rubidium forms Rb+, so the hydroxide is RbOH.
Step 2: balance2Rb(s) + 2H₂O(l) → 2RbOH(aq) + H₂(g)Step 3: predict from the trend
Rubidium is below potassium, so the reaction is even more vigorous.
Expect an explosive reaction, rapid gas evolution, and the metal disappearing almost instantlya resulting alkaline solution is a third observation — universal indicator would turn purple
WORKED EXAMPLE
0.230 g of sodium reacts completely with excess water. Calculate the volume of hydrogen produced at STP.
Use M(Na) = 22.99 g mol−1 and a molar volume of 22.7 dm3 mol−1 at STP.
Step 1: moles of sodiumn = 0.230 ÷ 22.99 = 0.0100 molStep 2: use the equation ratio
2Na gives 1H2, so halve it.
n(H₂) = 0.0100 ÷ 2 = 0.00500 molStep 3: convert to volumeV = 0.00500 × 22.7 = 0.1135 dm³0.114 dm3, or about 114 cm3 (3 s.f.)the 2:1 ratio is the step people forget — read it off your own balanced equation
WORKED EXAMPLE
Explain why caesium reacts more violently with water than lithium does. [3]
Mark 1: identify the key step
Both react by losing one outer electron to form a 1+ ion.
Mark 2: compare the atoms
Caesium’s outer electron is in a much higher shell, so it is further from the nucleus and shielded by more inner shells.
Mark 3: link to the observation
Less energy is needed to remove it, so caesium loses its electron far more readily.
Lower first ionisation energy means a faster, more vigorous reactiondo not say caesium is “bigger so more reactive” without explaining why size matters
💡 Exam tip
Learn one equation properly and generate the rest. The 2 metal : 2 water : 2 hydroxide : 1 hydrogen pattern never changes.
Include state symbols. They are frequently worth a mark in group 1 questions.
Observations must be things you can see or hear: fizzing, floating, melting, a flame, the metal disappearing. “Hydrogen is produced” is a deduction, not an observation.
Mention the alkaline solution — it is the reason the group is called the alkali metals and is often the easiest mark.
For any trend question, name distance and shielding, then say they outweigh the increased nuclear charge.
Group 1 metals are stored under oil, not water. Saying “under water” is an instant loss.
⚠ Common mix-up
Writing the metal oxide as the product. With water you get the hydroxide; the oxide comes from reaction with oxygen.
Forgetting to balance. Na + H2O → NaOH + H2 does not balance for hydrogen.
Saying the reaction is more reactive because there are more protons. More protons would make it harder, not easier. Distance and shielding are what change the outcome.
Claiming the flame colour proves hydrogen burns lilac. Hydrogen burns almost colourless; the lilac comes from potassium ions in the flame.
Confusing reactivity with the strength of the alkali produced. All group 1 hydroxides are strong bases.
Saying group 1 metals are dense and hard. They are unusually soft and light for metals.
Up next: Group 17 Elements and Halide Ions — the mirror image of this page. Same electron transfer, opposite direction, and a trend that runs the other way down the group.
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