Alkali metals

Drop a tiny piece of lithium into a bowl of water and it fizzes politely as it scoots around the surface. Swap it for a same-sized piece of sodium and the scooting turns frantic. Swap the sodium for potassium and the hydrogen that bubbles off actually catches fire. Three metals, one family, one clear message: these elements behave alike, and they behave increasingly violently as you descend their group. That double observation — same behaviour, graded intensity — is the entire subject of this chapter. The elements of Group 1, lithium to caesium plus the ghost at the bottom, are the alkali metals, and they are the first family of the Periodic Table you will study as a family rather than as individuals.

Every objective of record for this lesson, so you can see the whole territory before we start:

• understand how the similarities in the reactions of these elements with water provide evidence for their recognition as a family of elements (spec 2.1);
• understand how the differences between the reactions with air and water provide evidence for the trend in reactivity down Group 1 (spec 2.2);
• use knowledge of trends in Group 1 to predict the properties of other alkali metals — francium is the exam's favourite target (spec 2.3);
• explain the trend in reactivity in terms of electronic configurations (spec 2.4C).

One routing note before you start, because this lesson is different from the last three you have met. Specifications 2.1, 2.2 and 2.3 are Paper 1 (1C) and Paper 2 (2C) statements, shared with Science Double Award. Specification 2.4C — the deep explanation of the reactivity trend — is chemistry-only: Paper 2C only, and Double Award candidates do not need it. So most of this chapter is core territory, with one marked Paper-2-only summit near the top.

The lesson runs in five movements. First, the physical profile: a family of low-density, low-melting, knife-cuttable metals, with a trend running down the group. Second, storage and handling — why these metals live under oil, and what the name alkali actually records. Third, the family evidence: the shared electronic configuration and the shared reactions, from water to air to the white compounds they form. Fourth, the explanation summit: why the reactivity increases down the group, told in terms of distance and attraction. Fifth, the payoff: predicting every property of an element nobody has ever seen.

Ask a simple question: what does a metal that reacts vigorously with both of the two most common substances on Earth — air and water — need in order to survive? The answer is isolation. All the alkali metals are extremely reactive, and they get more reactive as you go down the group. They react quickly with the oxygen in air to form oxide, and they react rapidly with water to form strongly alkaline solutions of the metal hydroxide. That second reaction is where the family's name comes from: an alkali is a soluble base, and these metals literally make alkalis when they meet water. The name alkali metals is a behaviour report, not an accident.

The isolation is real and physical. Lithium, sodium and potassium are stored under oil — you will see them in a school lab as dull lumps suspended in a jar of colourless liquid. Oil keeps out both the oxygen and the water vapour of the air, so the metal inside stays as unreacted as the day it was cut. Rubidium and caesium are so reactive that oil is not enough: they are stored in sealed glass tubes to remove any possibility of oxygen reaching them. The storage regime is the reactivity trend made visible — the harsher the containment, the more violent the metal.

Handling rules follow the same logic, and they are strict. Great care must be taken never to touch these metals with bare fingers. There could be enough sweat on your skin — sweat is mostly water, with dissolved salts — to start a reaction on your hand, producing a lot of heat and a very corrosive metal hydroxide. The metals are cut, when needed, under oil or with dry tools, and only lithium, sodium and potassium are ever handled at all; rubidium and caesium are far too reactive for school demonstration.

One notation convention runs through this whole chapter and the exam questions built on it, so set it now: 'M' stands for any one of the alkali metals (Li, Na, K, Rb, Cs), and 'X' stands for any one of the halogens (Group 7). Equations written with M are family equations — they are true for every member of the group at once. You will meet them constantly: MOH for the hydroxides, M₂O for the oxides, MX for the salts formed with halogens. When an examiner writes "an element M in Group 1…", they are handing you the whole family and asking you to use its pattern.

Why is Group 1 a family at all? Two reasons, and the second follows from the first.

Reason 1: they all have one electron in their outer shell. The electronic-configuration read like a chant, each member adding one filled shell:

Rubidium adds one more shell (2,8,8,18,1) and caesium one more again — the outer shell is always a lone pair-free single electron sitting on top of a complete kernel. That shared outer-shell count is the structural fact behind everything else in this chapter.

Reason 2: they have similar chemical properties — and reason 2 is really just a consequence of reason 1. Chemistry happens at the outer shell, so atoms with identical outer shells react identically. Set the family reactions side by side and the pattern is total:

• they all react with water in the same way, forming a hydroxide with the formula MOH (LiOH, NaOH, KOH…) and hydrogen gas;
• they react with oxygen to form an oxide with the formula M₂O (Na₂O, K₂O…);
• they react with halogen to form salts with the formula MX (LiCl, KBr, CsI…);
• they form ion containing an M⁺ ion (Na⁺, K⁺…).

In every case the metal atom sheds its single outer electron onto something else — water, oxygen, a halogen — and every case produces the same 1+ ion. Even the charges are predictable: the hydroxide ion carries a 1− charge, so M⁺ pairs with OH⁻ one-to-one to give MOH (that is the ionic-charge bookkeeping from Chapter 7, u1:c07, at work). The family equations are not approximations; they are the outer-shell fact written in symbols.

This is the evidence structure the exam wants you to articulate for spec 2.1: the similarities in the reactions with water — same products (hydroxide + hydrogen), same ionic shape (M⁺ and OH⁻), same equations with only the metal's letter changing — are what justify treating these six elements as one family. A family is a claim about shared behaviour, and shared behaviour traces back to the shared outer shell.

Here is the reaction that defines the family. All the alkali metals react in the same way with water, producing a metal hydroxide and hydrogen gas. Written as the family equation, with M standing for any member of the group:

2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)

Read what the products are telling you. The MOH(aq) is the metal hydroxide dissolved in the water — and a solution of any metal hydroxide is strongly alkaline, which is why the family carries the name alkali metals. The H₂(g) is hydrogen, and it is the visible star of the show: hydrogen gas bubbling off is what makes the reaction look alive. The hydroxide is the quiet product — colourless, dissolved — but it is the one that turns the water into an alkali, and it is the one the equations must never omit.

The reaction is also exothermic — a lot of heat is produced — and that fact does quiet explanatory work in every observation you are about to meet in the next section. Melting a ball of sodium into shape and igniting potassium's hydrogen both run on the reaction's own released heat.

So if the reactions are all the same kind, what makes the family interesting? The main difference between the reactions is how quickly they happen. As you go down the group — lithium, then sodium, then potassium — the metals become more reactive and the reactions occur more rapidly. Lithium fizzes briskly but needs a few patient seconds; sodium skates around its dish in an urgent hurry; potassium ignites its own hydrogen on the spot. Same equation, same products, steadily escalating speed. That escalation is the trend in reactivity, and evidencing it with these water reactions is exactly what specification 2.2 asks you to understand: the differences between the reactions — speed, violence, heat output — provide the evidence for the trend.

Keep the logic of the trend-evidence sharp, because it is a favourite examination framing. A trend is not demonstrated by one reaction; it is demonstrated by a comparison across the family. Sodium versus lithium: sodium's reaction is faster. Potassium versus sodium: potassium's is faster again. The comparison is the evidence — the reactions with water are the standard measuring stick the family gets judged against.

The sodium reaction is the family's standard demonstration, and you need to be able to write it and narrate it. The equation:

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

One subtlety in that equation is worth a whole mark of care. Strictly speaking, most of the time the sodium is reacting it is present as molten sodium, not solid sodium — the reaction's own heat has melted it into a ball. It is still correct, and expected, to write (s) in the equation: writing (l) for sodium has the potential to confuse an examiner and is best avoided. Use the state symbol for the metal as it started, not as the reaction temporarily reshaped it.

Now the observations — the list the exam keeps asking for, each with its physical cause:

• The sodium floats, because it is less dense than water.
• It melts into a small ball, because its melting point is low and the reaction produces a lot of heat — enough to take the metal past its melting point almost instantly.
• There is fizzing, because hydrogen gas is produced.
• The sodium moves around on the water's surface, because the hydrogen is not given off symmetrically — the escaping gas pushes the ball in random directions like a jet.
• A white trail forms behind it: that is the sodium hydroxide, which dissolves in the water to form a strongly alkaline solution (add universal indicator and the water turns purple).

Lithium runs the same reaction with the same equation shape — 2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g) — but slower. Lithium's melting point is higher and the heat is produced more slowly, so the metal does not melt; it simply fizzes and slides around as a solid piece, taking noticeably longer to disappear.

Potassium runs faster than sodium — 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) — and there is one headline difference: enough heat is produced to ignite the hydrogen, which burns with a lilac flame. The reaction often ends with the potassium spitting around and exploding. With sodium the hydrogen usually does not light; with potassium it nearly always does. That flame is the single most examinable sodium-versus-potassium contrast.

At the very top of the family the reaction leaves the classroom: rubidium and caesium react so violently that the reactions are too hazardous to attempt in school — they would explode.

This is the summit of the chapter — and the specification marks it explicitly: explain the trend in reactivity in Group 1 in terms of electronic configurations (spec 2.4C). Understand it as a chain, because the exam awards the links.

The fact to explain: as you go down the group, the metals become more reactive.

Step 1 — what the reaction actually is. In all these reactions the metal atoms are losing their outer electron and forming metal ions. For sodium going into water:

Na(s) → Na⁺(aq) + e⁻

Everything else — the hydroxide forming, the hydrogen bubbling off — follows from that one loss. So the reactivity question is really a question about how easily the outer electron is lost.

Step 2 — what makes the electron easy or hard to lose. That depends on how strongly the outer electron is attracted to the nucleus. The nucleus is positive (it contains the protons), so it attracts the negative electrons — and the closer an electron sits to that positive nucleus, the harder it is to pull away.

Step 3 — follow the atoms down the group. As you move down Group 1, the atoms have more shells of electrons and therefore get bigger: a sodium atom is bigger than a lithium atom, a potassium atom bigger than a sodium atom. In the bigger atom, the outer electron is further from the nucleus.

Step 4 — the conclusion, stated cleanly. Because the outer electron is further from the positive nucleus, the attraction on it is weaker — so it is lost more easily — so the reaction happens faster. Bigger atom down the group → weaker pull on the outer electron → more reactive metal. That chain, in that order, is the 2.4C answer.

Notice what the explanation is not. It is not about mass, and it is not about eagerness. Potassium is not more reactive than lithium because it is heavier, and atoms do not want anything; reactivity is a geometry-and-charge story — distance between the outer electron and the nucleus, and the strength of the electrostatic attraction that distance allows. The electronic configurations from earlier in this chapter are the map: each extra shell pushes the outer electron one layer further out.

Water is the family's loud reaction; air is its quiet one — and it is the quiet one you can watch happening on a bench in seconds. Lithium, sodium and potassium are all stored in oil because they react with the air; take a piece of sodium out of its oil and you can catch the reaction mid-run.

A piece of sodium fresh from the oil usually has a crust on the outside — the residue of its last conversation with the air. It is not shiny unless it has been freshly cut. So cut it, and watch:

• the fresh surface is shiny — that is what unreacted sodium actually looks like, silvery like any metal;
• within seconds the surface tarnishes rapidly as the freshly exposed sodium reacts with oxygen in the air, growing a dull coating of metal oxide;
• do the same with lithium and the fresh surface tarnishes more slowly, because lithium reacts more slowly than sodium;
• do the same with potassium and the fresh surface tarnishes extremely rapidly — faster than sodium's.

Read the comparison carefully, because it is a second, independent measurement of the reactivity trend. Potassium is more reactive than sodium, which is more reactive than lithium — the same ordering the water reactions gave, now delivered by a different reaction on a different timescale. That convergence is what makes the trend evidence rather than anecdote: specification 2.2 asks you to see the trend in the differences between the reactions with both air and water, and this section is the air half of that evidence.

In each case the chemistry is the same simple story: metal + oxygen in the air → metal oxide coating. No flame is needed for the tarnishing reaction — oxygen attacks the fresh surface on contact. The oxide layer is why lab samples look dull and crusted: a piece of "sodium" under oil is really a shiny metal wearing an oxide overcoat it grew the last time it met the air.

The tarnishing test is also the fastest qualitative demonstration of the trend you can perform with no water, no indicator and no flame — just a knife, three small samples and a stopwatch. Fresh cut, fresh surface, and the family orders itself in front of you: lithium sluggish, sodium swift, potassium nearly instant.

The metals themselves are dramatic; their compounds are almost invisible — and that contrast is itself a family property worth knowing. All Group 1 metal ion are colourless. That single fact settles the colour of essentially every alkali-metal compound you will ever meet: their compounds are colourless or white — unless they are combined with a coloured negative ion.

The logic runs one way only, and it is worth getting exactly right. The metal's own ion contributes no colour, ever. So:

• sodium chloride, potassium bromide, lithium sulfate — white or colourless solids, colourless in solution. The positive ion is colourless and the negative ion (Cl⁻, Br⁻, SO₄²⁻) is colourless too.
• potassium dichromate(VI) is orange — not because of potassium, which contributes nothing, but because the dichromate(VI) ion, Cr₂O₇²⁻, is orange;
• potassium manganate(VII) is purple — again, not the potassium, but the manganate(VII) ion, MnO₄⁻.

The diagnostic habit this builds is genuinely useful across the course: when a Group 1 compound shows any colour, the colour belongs to the negative ion, never to the metal. Potassium is a passenger in both of the famous coloured compounds above; the chromium and the manganese are doing all the showing.

Beyond colour, the compounds behave as a family too: Group 1 compounds are typical ionic solids and are mostly soluble in water. The strong ionic bonding (Chapter 7, u1:c07 — positive metal ion to negative ion) is there in the solid, but put the solid in water and the lattice gives way: the ions separate and the solid dissolves. That solubility is what made the water reactions of earlier sections produce solutions — the MOH product does not sit as a solid, it disperses as aqueous ions, which is why the hydroxide solutions are strongly alkaline throughout.

So the compound portrait completes the family: soft reactive metals that build white, mostly-soluble, colourless-ion compounds. If an exam question hands you a white solid that dissolves to give a colourless solution and asks what the metal ion could be, Group 1 is always on the candidate list — and if the solid is orange or purple with potassium inside, you now know exactly which ion is wearing the colour.

Every fact of this chapter now fits on one card. Here is the family portrait, assembled from the sections above — this list is the exam's checklist for "state the main features of the Group 1 elements":

• they are metals;
• they are soft, with melting points and densities that are very low for metals;
• they have to be stored out of contact with air or water (under oil for lithium, sodium and potassium; sealed glass tubes for rubidium and caesium);
• they react rapidly with air to form coatings of the metal oxide;
• they react with water to produce an alkaline solution of the metal hydroxide and hydrogen gas;
• they increase in reactivity as you go down the group;
• they form compounds in which the metal is a 1+ ion;
• their compounds are mainly white or colourless, dissolving to give colourless solutions.

Check each line against where you met it. Softness, low melting points and low densities came from the physical profile; storage from the handling regime; the oxide coatings from the air reaction; the alkaline hydroxide + hydrogen from the water reaction; the trend from both — tarnishing rate and fizzing rate agreeing; the M⁺ ion and the white/colourless compounds from the ionic chemistry. Nothing on the card is new; the card is the chapter compressed.

Two lines of that card deserve a second look because exams like to probe them by contrast. "Very low for metals" is a comparative claim — these metals are soft enough to cut with a knife and light enough to float, which no everyday metal is, and that is precisely what makes them remarkable. And "increase in reactivity down the group" is the line that converts the whole list into a prediction engine: if every property slides smoothly down the group, then a property you cannot measure can be read off the slide.

Which brings the chapter to its natural question. The summary card describes five elements you can put on a bench — but the family has a sixth member, at the very bottom of Group 1, of which nobody has ever seen a single piece. You cannot observe its softness, melt it, cut it or drop it into water. Does the prediction engine still run? The next — final — section runs it, and the run is the payoff of everything above.

Meet the family's ghost. Francium, at the bottom of Group 1, is extremely radioactive: one of its isotope is produced during the radioactive decay of uranium-235, and it is extremely short-lived. At any one moment scientists estimate that only about 20–30 g of francium exist in the whole of the Earth's crust, and nobody has ever seen a piece of it. There is no jar in any lab on Earth labelled "francium".

And yet, by the end of this section you will be able to describe it in detail — because as you move down a group in the Periodic Table, the properties of the elements change gradually, so if you know most of a group, you can predict the rest. This is specification 2.3 in its purest form: use knowledge of trends in Group 1 to predict the properties of other alkali metals. The summary card from the last section is the machine; francium is the demonstration. Take the trend through caesium and the predictions fall out:

• it is very soft;
• its melting point is around room temperature — careful prediction work puts it between 21 °C and 27 °C, continuing the slide (181 → 98 → 63 → 39 → 29 → …);
• its density is probably just over 2 g/cm³, continuing the density slide (0.53 → 0.97 → 0.86 → 1.53 → 1.88 → …);
• it will be a silvery metal — but will tarnish almost instantly in air, because tarnishing rate slides up the group;
• it will react violently with water to give francium hydroxide and hydrogen — more violently than any member above it;
• it will be more reactive than caesium, the current record-holder;
• its hydroxide, FrOH, will be a soluble, strongly alkaline hydroxide — the MOH pattern does not break at the bottom of the group.

Study the method, because the method is the transferable skill. Each prediction is a single move: find the property's slide, place the unknown at the bottom of it, read off the next value. The examiner can run the same trick with a fictional element — call it edexcelium, placed below potassium — and the machinery works identically: its reaction with water gives an alkaline solution (hydroxide + hydrogen, family pattern); its compound with air has the formula of an oxide; its chloride has the formula MCl; its ion carries 1+. You were never taught edexcelium. You were taught the slide, and the slide answers for it.

That is the deepest lesson of Group 1, and the reason this family opens Unit 2: a trend is a prediction engine. Learn the family portrait and its direction of travel, and you own every member of the group — seen and unseen alike.