(I once asked a class to predict how rubidium would react with water. No one had ever seen rubidium. No one had even heard of it. But after ten minutes of studying the trends in Group 1, they made a confident prediction: "It will react even more violently than potassium." They were right. That is the power of understanding chemical families — you can predict the behaviour of elements you have never seen, just by knowing where they sit in the periodic table.) Imagine you are a doctor. A new patient walks in with a set of symptoms. You have never met this person before, but you can make a diagnosis because you recognise the pattern. Chemical families work the same way. Once you understand how one member of a family behaves, you can predict how the others will behave — even the ones you have never seen. This is not memorisation. This is understanding. In this chapter, we are going to look at four great families of elements: the alkali metals, the alkaline earth metals, the halogens, and the noble gases. For each family, we will examine their physical properties, their chemical reactions, and the trends that run through them. And by the end, you will be able to look at any element in these families and predict — with confidence — what it will do. Let us begin.
A chemical family is a group of elements in the same vertical column of the periodic table. They share the same number of electrons in their outermost shell, which gives them similar chemical properties.
The four families we will study in this chapter are:
| Family | Group | Outer Electrons | Ion Formed |
|---|---|---|---|
| Alkali metals | Group 1 | 1 | +1 |
| Alkaline earth metals | Group 2 | 2 | +2 |
| Halogens | Group 17 | 7 | -1 |
| Noble gases | Group 18 | 8 (or 2 for He) | None |
(Why do families behave similarly? Because chemistry is all about outer electrons. The inner electrons are buried and don't get involved in reactions. Same outer shell, same behaviour. That is the whole secret.)
Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), francium (Fr). They are called "alkali" metals because they react with water to form alkaline solutions.
| Property | Description |
|---|---|
| Appearance | Silvery, shiny when freshly cut, tarnish rapidly in air. |
| Softness | Soft enough to cut with a knife. Lithium is hardest; caesium is softest. |
| Density | Low — lithium, sodium, potassium float on water. |
| Melting/boiling points | Low, and decrease down the group. |
| Conductivity | Good conductors of heat and electricity. |
Reaction with water: Produce a metal hydroxide (alkali) and hydrogen gas.
2Na + 2H₂O → 2NaOH + H₂
Observations:
Reaction with oxygen: Form metal oxides.
4Na + O₂ → 2Na₂O
Reaction with chlorine: Form white crystalline salts.
2Na + Cl₂ → 2NaCl
| Property | Trend | Reason |
|---|---|---|
| Reactivity | Increases | Outer electron further from nucleus and shielded, so easier to lose. |
| Atomic radius | Increases | A new shell is added at each step. |
| Melting point | Decreases | Metallic bond weakens as atoms get larger. |
| Ionisation energy | Decreases | Electron is easier to remove. |
Beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra). They have two outer electrons, which they lose to form +2 ions.
| Property | Description |
|---|---|
| Appearance | Silvery-white, shiny, but tarnish in air. |
| Hardness | Harder than Group 1 metals. |
| Density | Higher than Group 1 metals. |
| Melting points | Higher than Group 1, decrease down the group. |
| Conductivity | Good conductors. |
Reaction with water: React with water (or steam) to form hydroxides and hydrogen. Reactivity increases down the group.
Mg + 2H₂O → Mg(OH)₂ + H₂ (with steam)
Ca + 2H₂O → Ca(OH)₂ + H₂ (with cold water)
Reaction with oxygen: Form metal oxides.
2Mg + O₂ → 2MgO
Reaction with chlorine: Form chlorides.
Mg + Cl₂ → MgCl₂
| Property | Trend | Reason |
|---|---|---|
| Reactivity | Increases | Outer electrons further from nucleus and easier to lose. |
| Atomic radius | Increases | A new shell is added at each step. |
| Melting point | Decreases | Metallic bond weakens. |
| Ionisation energy | Decreases | Electrons easier to remove. |
Fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At). They have seven outer electrons, need one more to complete their shell, and form -1 ions.
| Property | Fluorine | Chlorine | Bromine | Iodine |
|---|---|---|---|---|
| State at room temp | Gas | Gas | Liquid | Solid |
| Colour | Pale yellow | Greenish-yellow | Red-brown | Dark grey (purple vapour) |
| Melting point | -220°C | -101°C | -7°C | 114°C |
| Boiling point | -188°C | -34°C | 59°C | 184°C |
Trend: Down the group, the state changes from gas to liquid to solid, colour darkens, and melting/boiling points increase.
Reaction with metals: Form salts.
2Na + Cl₂ → 2NaCl
Reaction with hydrogen: Form hydrogen halides.
H₂ + Cl₂ → 2HCl
Displacement reactions: A more reactive halogen displaces a less reactive one from its salt.
Cl₂ + 2KBr → 2KCl + Br₂
The solution turns orange-brown as bromine is released.
| Property | Trend | Reason |
|---|---|---|
| Reactivity | Decreases | Outer shell further from nucleus, attracts electrons less strongly. |
| Atomic radius | Increases | A new shell is added at each step. |
| Melting/boiling point | Increases | Molecules get larger, intermolecular forces stronger. |
| Colour | Darkens | Larger molecules absorb different wavelengths. |
| Electronegativity | Decreases | Nucleus has less pull on incoming electrons. |
Helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn). They have full outer shells, so they are unreactive.
| Property | Description |
|---|---|
| State at room temp | All are gases. |
| Colour | Colourless. |
| Density | Increases down the group. |
| Boiling point | Very low, increases down the group. |
| Reactivity | Extremely low — inert. |
Noble gases have full outer shells. Helium has 2 electrons (a full first shell); the others have 8 electrons in their outer shell. This is the most stable arrangement. Because they do not need to gain or lose electrons, they do not react.
| Gas | Use |
|---|---|
| Helium | Balloons, airships, cooling superconducting magnets. |
| Neon | Advertising signs (glows red-orange). |
| Argon | Light bulbs, welding. |
| Krypton | High-performance lighting. |
| Xenon | Car headlights, anaesthesia. |
| Radon | Radioactive; cancer treatment but also a health hazard. |
| Feature | Alkali Metals | Alkaline Earth Metals | Halogens | Noble Gases |
|---|---|---|---|---|
| Group | 1 | 2 | 17 | 18 |
| Outer electrons | 1 | 2 | 7 | 8 (or 2) |
| Type | Metal | Metal | Non-metal | Non-metal |
| Ion formed | +1 | +2 | -1 | None |
| Reactivity trend | Increases down | Increases down | Decreases down | Negligible |
| State at room temp | Solid | Solid | Gas/liquid/solid | Gas |
| Examples | Li, Na, K | Mg, Ca, Ba | F, Cl, Br, I | He, Ne, Ar |
The behaviour of an element is determined by its outer electrons.
(If you remember only one thing from this chapter, remember this: the outer electrons call the shots. Everything else follows from that.)
Question: Which is more reactive — calcium or barium? Explain using the trend down Group 2.
Solution: Barium is more reactive. Reactivity increases down Group 2 because the outer electrons are further from the nucleus and shielded by more inner shells. This makes them easier to lose, so barium reacts more vigorously with water than calcium.
Question: Predict what happens when bromine water is added to potassium iodide solution. Write an equation.
Solution: Bromine is more reactive than iodine, so it will displace iodine from potassium iodide. The solution will turn brown as iodine is released. Equation: Br₂ + 2KI → 2KBr + I₂.
Question: Explain why argon is used in light bulbs.
Solution: Argon is a noble gas with a full outer shell, so it is extremely unreactive. In a light bulb, the filament gets very hot. If oxygen were present, the filament would react with it and burn out quickly. Argon provides an inert atmosphere that prevents this reaction, extending the life of the bulb.
1. Name the elements in Group 1 and state one shared property.
Lithium, sodium, potassium, rubidium, caesium, francium. They are all soft, reactive metals with one outer electron.
2. Describe the trend in reactivity down Group 1 and explain why it occurs.
Reactivity increases down Group 1 because the outer electron is further from the nucleus and shielded, so it is easier to lose.
3. What is the state and colour of chlorine, bromine, and iodine at room temperature?
Chlorine — greenish-yellow gas. Bromine — red-brown liquid. Iodine — dark grey solid.
4. Describe the trend in reactivity down Group 17 and explain why it occurs.
Reactivity decreases down Group 17 because the outer shell is further from the nucleus and shielded, so the atom attracts electrons less strongly.
5. What is a displacement reaction? Give an example.
A displacement reaction is when a more reactive halogen displaces a less reactive halogen from its compound. Example: Cl₂ + 2KBr → 2KCl + Br₂.
6. Why are noble gases unreactive?
They have full outer electron shells, so they do not need to gain or lose electrons to achieve stability.
7. State one use of helium and one use of argon.
Helium — balloons or cooling superconducting magnets. Argon — light bulbs or welding.
Question 1 [10 marks]
(a) Describe the physical properties of the alkali metals. [3]
(b) Describe what you would observe when sodium reacts with water, and write the equation. [4]
(c) Explain why reactivity increases down Group 1. [3]
(a) Soft (can be cut with a knife), silvery when freshly cut but tarnish quickly, low density (lithium, sodium, potassium float on water), low melting points, good conductors. (3 marks: 1 for softness, 1 for appearance, 1 for density or melting point.)
(b) Sodium fizzes vigorously on water, melts into a silvery ball, moves rapidly across the surface, and may ignite with an orange flame. Equation: 2Na + 2H₂O → 2NaOH + H₂. (4 marks: 2 for observations, 2 for equation.)
(c) Reactivity increases down Group 1 because the outer electron is further from the nucleus and shielded by more inner shells. The attraction between the nucleus and the outer electron is weaker, so the electron is easier to lose, making the metal more reactive. (3 marks: 1 for "further from nucleus", 1 for "shielded", 1 for "easier to lose".)
Question 2 [10 marks]
(a) Describe the trend in colour and state down Group 17. [4]
(b) Describe what happens when chlorine gas is bubbled through potassium bromide solution. Write an equation. [3]
(c) Explain why fluorine is more reactive than iodine. [3]
(a) Fluorine — pale yellow gas. Chlorine — greenish-yellow gas. Bromine — red-brown liquid. Iodine — dark grey solid (purple vapour). As you go down the group, the colour darkens and the state changes from gas to liquid to solid. (4 marks: 2 for colours, 2 for states.)
(b) Chlorine displaces bromine from potassium bromide because chlorine is more reactive. The solution turns orange-brown. Equation: Cl₂ + 2KBr → 2KCl + Br₂. (3 marks: 1 for displacement, 1 for colour change, 1 for equation.)
(c) Fluorine is more reactive because its outer shell is closer to the nucleus and less shielded. The nucleus has a stronger pull on incoming electrons, so fluorine attracts an electron more easily than iodine. This makes it more reactive. (3 marks: 1 for "closer to nucleus", 1 for "less shielded", 1 for "stronger pull".)
Question 3 [10 marks]
(a) Explain why noble gases are unreactive and give two uses of noble gases. [4]
(b) Compare the reactivity of Group 1 and Group 17 elements. [3]
(c) Explain how the electronic configuration of an element determines its position in the periodic table and its chemical behaviour. [3]
(a) Noble gases have full outer electron shells, so they do not need to gain or lose electrons to achieve stability. This makes them unreactive. Uses: helium in balloons; neon in advertising signs; argon in light bulbs; xenon in car headlights. (4 marks: 2 for explanation, 2 for two uses.)
(b) Group 1 elements are metals that react by losing one electron to form +1 ions. Their reactivity increases down the group. Group 17 elements are non-metals that react by gaining one electron to form -1 ions. Their reactivity decreases down the group. Both are highly reactive, but in opposite ways. (3 marks: 1 for Group 1 behaviour, 1 for Group 17 behaviour, 1 for comparison.)
(c) The number of outer electrons determines the group; the number of shells determines the period. Elements in the same group have the same number of outer electrons, so they behave similarly. Elements with one outer electron (Group 1) lose it easily and are reactive metals. Elements with seven outer electrons (Group 17) gain one electron and are reactive non-metals. Elements with full outer shells (Group 18) do not react. The electronic configuration is the key to both position and behaviour. (3 marks: 1 for position, 1 for behaviour, 1 for link.)
Challenge 1: A student is given three unlabelled bottles containing aqueous solutions of potassium chloride, potassium bromide, and potassium iodide. Describe how they could use displacement reactions to identify each solution.
Add a few drops of chlorine water to each solution. Chlorine is more reactive than both bromine and iodine, so it will displace them from their compounds. The potassium chloride solution will remain colourless (no reaction, since chlorine cannot displace itself). The potassium bromide solution will turn orange-brown as bromine is released. The potassium iodide solution will turn brown as iodine is released. To confirm, add bromine water to each: it will displace iodine from potassium iodide (turning brown) but will not displace chlorine from potassium chloride. This establishes the order of reactivity: chlorine > bromine > iodine.
Challenge 2: Explain why the alkali metals are stored in oil, but the halogens are not.
Alkali metals are stored in oil because they react rapidly with oxygen and moisture in the air. The oil creates a barrier that prevents contact with air, keeping the metals fresh. Halogens, on the other hand, are stored in sealed containers but not in oil. They do not react with oxygen or moisture in the same way — their reactivity is directed towards gaining electrons from other substances, not towards reacting with air. Some halogens (like fluorine and chlorine) are gases and must be kept in gas-tight containers, while bromine is a volatile liquid and must be kept in a sealed bottle. Iodine is a solid and is relatively stable in air. The storage method depends on the element's reactivity and physical state.
Challenge 3: The noble gases are sometimes called "inert gases." Explain why this name is not entirely accurate, and give an example of a noble gas compound.
The name "inert gases" is not entirely accurate because noble gases are not completely unreactive. Under extreme conditions — high pressure, high temperature, or with highly reactive elements like fluorine — some noble gases can form compounds. For example, xenon tetrafluoride (XeF₄) and xenon hexafluoride (XeF₆) have been synthesised. Radon, being radioactive and heavier, also forms compounds. Helium, neon, and argon are still considered effectively inert, but the heavier noble gases (krypton, xenon, radon) can react under special conditions. So "inert" is a good approximation for everyday purposes, but not a strict rule.
"Families share more than a name — they share a personality. Learn the family, and you can predict the element. That is the quiet power of chemistry."
