(I once held up three test tubes — one containing lithium, one sodium, one potassium — and asked my class, "If these were people, what would they be like?" One student said, "They'd be the kind of people who react to everything." Exactly. That's what a chemical family is — a group of elements with the same personality. And once you know the family, you can predict the behaviour. That is the power of this chapter.) Think about your own family. You share certain traits — maybe a love of music, a temper, or a talent for cooking. You're not identical, but you're recognisably related. Chemical families are the same. The alkali metals — lithium, sodium, potassium, rubidium, caesium — all have one electron in their outer shell. That single shared feature gives them a shared personality: they are soft, reactive, and desperate to lose that electron. The halogens — fluorine, chlorine, bromine, iodine — all need one electron to complete their outer shell. That shared need makes them reactive in a different way: they are hungry for electrons. And the noble gases — helium, neon, argon — have full outer shells. They don't need anything. They are the calm, unreactive members of the family. In this chapter, we're going to look at each of these families in depth. You'll learn not just what they do, but why they do it. Let's meet the relatives.
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 three families we'll study in depth are:
(Why do elements in the same group behave similarly? Because chemistry is all about the outer electrons. The inner electrons are buried and don't get involved in reactions. The outer electrons — the ones in the outermost shell — are the ones that do all the work. Same outer shell, same behaviour. That's the whole secret.)
The alkali metals are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). They are called "alkali" metals because they react with water to form alkaline (basic) solutions.
| Property | Description |
|---|---|
| Appearance | Silvery, shiny when freshly cut, but tarnish rapidly in air. |
| Softness | Soft enough to cut with a knife. Lithium is the hardest; caesium is the softest. |
| Density | Low density. Lithium, sodium, and potassium float on water. |
| Melting and boiling points | Low, and decrease down the group. |
| Conductivity | Good conductors of heat and electricity. |
Alkali metals are the most reactive metals. They have one electron in their outer shell, which they lose easily to form a +1 ion. This makes them highly reactive.
Reaction with water: Alkali metals react with water to form a metal hydroxide (an alkali) and hydrogen gas.
2Na + 2H₂O → 2NaOH + H₂
Observations:
(I have demonstrated sodium reacting with water many times, and I still hold my breath every time. It is one of the most dramatic reactions in all of chemistry. But never, ever do this without proper safety equipment and supervision. The alkali metals are not toys.)
Reaction with oxygen: Alkali metals react with oxygen in the air to form metal oxides. They are stored in oil to prevent this.
4Na + O₂ → 2Na₂O
Reaction with chlorine: Alkali metals react with chlorine to form white crystalline salts called chlorides.
2Na + Cl₂ → 2NaCl
| Property | Trend Down the Group | Reason |
|---|---|---|
| Reactivity | Increases | Outer electron is further from the nucleus and shielded by more inner shells, so it is easier to lose. |
| Atomic radius | Increases | A new shell is added at each step. |
| Melting point | Decreases | The metallic bond weakens as the atoms get larger. |
| Ionisation energy | Decreases | Electron is easier to remove. |
| Density | Generally increases | Atoms get heavier, but the size increase is not enough to offset the mass increase. |
The halogens are fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). They are the most reactive non-metals. They have seven electrons in their outer shell, so they need one more electron to complete it. They form -1 ions.
| Property | Fluorine (F₂) | Chlorine (Cl₂) | Bromine (Br₂) | Iodine (I₂) |
|---|---|---|---|---|
| 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 |
| Density | Low | Low | Moderate | High |
Trend: As you go down the group, the halogens change from gas to liquid to solid, their colour darkens, and their melting and boiling points increase.
Halogens are very reactive non-metals. They react with metals to form salts (hence the name "halogen," meaning "salt-former"). They also react with hydrogen to form hydrogen halides.
Reaction with metals: 2Na + Cl₂ → 2NaCl
Reaction with hydrogen: H₂ + Cl₂ → 2HCl
Displacement reactions: A more reactive halogen can displace a less reactive halogen from its salt solution. This is a key way to establish the order of reactivity.
Example: Chlorine displaces bromine from potassium bromide:
Cl₂ + 2KBr → 2KCl + Br₂
The solution turns from colourless to orange-brown as bromine is released.
(Displacement reactions are the halogens' way of showing who's boss. The more reactive halogen pushes the less reactive one out of its compound. If you want to know the order of reactivity, just watch who displaces whom. It's like a playground hierarchy — the strongest one wins.)
| Property | Trend Down the Group | Reason |
|---|---|---|
| Reactivity | Decreases | The outer shell is further from the nucleus and shielded, so the atom attracts electrons less strongly. |
| Atomic radius | Increases | A new shell is added at each step. |
| Melting/boiling point | Increases | The molecules get larger, so the intermolecular forces are stronger. |
| Colour | Darkens | Larger molecules absorb different wavelengths of light. |
| Electronegativity | Decreases | The nucleus has less pull on incoming electrons. |
The noble gases are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). They are colourless, odourless, and extremely unreactive. They have full outer electron shells, so they have no need to gain or lose electrons.
| Property | Description |
|---|---|
| State at room temp | All are gases. |
| Colour | Colourless. |
| Density | Increases down the group. Helium is the lightest; radon is the densest. |
| Boiling point | Very low, increases down the group. |
| Reactivity | Extremely low — they are 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 don't need to gain or lose electrons, they don't react.
(Imagine a person who is completely content with what they have. They don't need to borrow or lend anything. That's a noble gas. They are the calm, self-sufficient members of the periodic table family.)
| Gas | Use |
|---|---|
| Helium | Balloons, airships, cooling superconducting magnets in MRI machines. |
| Neon | Advertising signs (glows red-orange when electricity passes through it). |
| Argon | Light bulbs (prevents the filament from burning), welding (provides an inert atmosphere). |
| Krypton | High-performance lighting, photographic flashes. |
| Xenon | Car headlights, anaesthesia, ion thrusters in spacecraft. |
| Radon | Radioactive; used in some cancer treatments, but also a health hazard in buildings. |
| Feature | Alkali Metals (Group 1) | Halogens (Group 17) | Noble Gases (Group 18) |
|---|---|---|---|
| Outer electrons | 1 | 7 | 8 (or 2 for helium) |
| Type of element | Metals | Non-metals | Non-metals |
| Reactivity | Very high | Very high | Extremely low |
| Ion formed | +1 | -1 | None (no ions formed) |
| Trend down group | Reactivity increases | Reactivity decreases | Reactivity stays negligible |
| State at room temp | Solid | Gas, liquid, or solid | Gas |
| Examples | Li, Na, K | F, Cl, Br, I | He, Ne, Ar |
Here's the deep insight that ties everything together: 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 — lithium or potassium? Explain using the trend down Group 1.
Solution: Potassium is more reactive. Reactivity increases down Group 1 because the outer electron is further from the nucleus and shielded by more inner shells. This makes it easier to lose, so potassium reacts more vigorously with water than lithium.
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 by more inner shells, 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 for the reaction. [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."
