- How ionic bonds form
- Covalent bonds
Same group, very different behaviour
Drop a small piece of lithium into a bowl of water, and it floats and fizzes steadily. Drop in a piece of sodium, the next metal down Group 1, and it melts into a silvery ball that races across the surface, fizzing hard. Drop in potassium, the next one down, and the hydrogen it gives off catches fire with a lilac flame. Each step down the group, the reaction is more violent.
Now look at the halogens in Group 17. Fluorine is the most reactive of them, and reacts with almost every other element. Chlorine is very reactive. Bromine is less so, and iodine less again. Each step down the group, the reaction is less violent.
Two groups, two trends in opposite directions. The last lecture promised that the same tug-of-war that explains atomic radius, ionisation energy and electronegativity would also explain these. This lecture keeps that promise.
What "reactive" means for metals and for non-metals
The key is to be exact about what each kind of element does when it reacts.
- A metal reacts by losing electrons. A Group 1 atom gives away its single outer electron and becomes a 1+ ion, such as Na⁺. A reactive metal is one that loses its outer electron easily.
- A non-metal such as a halogen reacts by gaining electrons. A Group 17 atom takes one electron to fill its outer shell and becomes a 1− ion, such as Cl⁻. A reactive halogen is one that gains an electron easily.
Losing and gaining are opposite processes. The same change down a group, a weaker pull from the nucleus on the outer shell, makes losing easier and gaining harder. That is why the two trends run in opposite directions.
Group 1: easier to lose, more reactive
Going down Group 1, each atom has one more shell than the one above. The outer electron is further from the nucleus and better shielded by the inner electrons. In the lecture on atomic radius, the rough effective nuclear charge on the outer electron stayed at about 1 all the way down the group, while the distance grew. So the outer electron is held less and less tightly.
Ionisation energy measures exactly this: the energy needed to remove that outer electron. It falls down the group.
Every Group 1 metal reacts with water in the same way, giving the metal hydroxide and hydrogen gas. For lithium and potassium:
The equations have the same form. The difference is how fast and how violently the reaction goes, and that comes from how easily each atom gives up its outer electron. Potassium's outer electron, in shell 4, is lost far more easily than lithium's, in shell 2. Because these metals react with water and with the oxygen in air, they are stored under oil.
Potassium has 19 protons and lithium has only 3. More protons pull harder. So shouldn't potassium hold its outer electron more tightly, and be less reactive?
Show the answer
No. Most of potassium's extra protons are cancelled by its extra inner electrons: 18 inner electrons shield 19 protons, leaving a rough effective charge of about 1, the same as for lithium (3 protons, 2 inner electrons). What does change is the distance: potassium's outer electron is two shells further out. The data settle it. Potassium's first ionisation energy is 419 kJ/mol, against 520 kJ/mol for lithium. So potassium loses its electron more easily, and it is more reactive.
Group 17: harder to gain, less reactive
A halogen reacts by pulling an extra electron into its outer shell. How strongly it can pull depends on the same three things as before: the nuclear charge, the distance and the shielding.
Going down Group 17, each atom has one more shell. An incoming electron would sit further from the nucleus and be better shielded from it. So the pull on it is weaker, and the halogen gains an electron less readily. Fluorine, the smallest, pulls hardest. Iodine, the largest of the four, pulls least.
The last lecture's electronegativity values show the same weakening pull. Electronegativity measures the pull on a shared pair rather than on a captured electron, but the cause is the same: a nucleus pulling on electrons in its outer shell.
That is it. That is the whole idea. One cause, a weaker pull from the nucleus on the outer shell as the atoms get bigger, gives two opposite results. For metals, which must lose electrons, a weaker pull helps, so reactivity rises down the group. For halogens, which must gain electrons, a weaker pull hurts, so reactivity falls down the group.
Displacement: watching the halogens compete
The order of reactivity of the halogens can be seen directly in a test tube. A more reactive halogen takes electrons from the ions of a less reactive one. This is called a displacement reaction.
Add chlorine water to a colourless solution of potassium bromide. The solution turns orange, because bromine has been formed:
The potassium ions take no part. They start and finish in solution unchanged. So the reaction can be written more simply, showing only the particles that change. This is an ionic equation:
Read it as a tug-of-war over electrons. Each chlorine atom pulls an electron from a bromide ion. Chlorine wins because it pulls harder. Now try the reverse: add bromine water to potassium chloride. Nothing happens, and the solution stays the colour of the bromine water. Bromine cannot take electrons from chloride ions, because chlorine holds on to them more strongly.
Try every combination below.
Choose a halogen solution and a halide solution to mix. See whether a reaction happens, what you would observe, and why.
Add:
To:
What happens when each halogen solution is added to each potassium halide solution. "Reacts" means the halogen added is more reactive and displaces the halogen in the salt.
| Added | to KCl(aq) | to KBr(aq) | to KI(aq) |
|---|---|---|---|
| Cl₂ | — | reacts: orange (Br₂) | reacts: brown (I₂) |
| Br₂ | no reaction | — | reacts: brown (I₂) |
| I₂ | no reaction | no reaction | — |
Chlorine displaces both others; bromine displaces only iodine; iodine displaces neither. So the order of reactivity is Cl₂ > Br₂ > I₂.
Did you build the full order? Chlorine displaces both bromine and iodine. Bromine displaces only iodine. Iodine displaces neither. So the order is chlorine, then bromine, then iodine, exactly the order down the group. Fluorine is left out of these test-tube experiments, because it reacts with the water itself.
Worked examples
Explain why potassium reacts much more vigorously with water than lithium does.
Both react by losing their single outer electron. Lithium's outer electron is in shell 2. Potassium's is in shell 4, much further from the nucleus and shielded by 18 inner electrons. The rough effective charge on the outer electron is about 1 for both, so the extra distance decides it: potassium's outer electron is held much less tightly. Its first ionisation energy is 419 kJ/mol, against 520 kJ/mol for lithium. Potassium loses its electron more easily, so it reacts more vigorously.
Chlorine water is added to sodium iodide solution. Write the full equation and the ionic equation, and state what you would see.
Chlorine is more reactive than iodine, so it displaces it.
The colourless sodium iodide solution turns brown as iodine forms. The sodium ions do not change, so they are left out of the ionic equation.
Predict how caesium, near the bottom of Group 1, and astatine, below iodine in Group 17, would compare with the other members of their groups.
Caesium has six shells. Its outer electron is the furthest from the nucleus of any element in the lecture's data, and its first ionisation energy, 376 kJ/mol, is the lowest. So it should be even more reactive than potassium and rubidium.
Astatine has one more shell than iodine. It should pull on an incoming electron even more weakly, so it should be the least reactive halogen, unable to displace iodine from iodide ions. Astatine is extremely rare and radioactive, so few experiments have been done with it. This is a prediction from the trend, which is exactly what the periodic table is for.
The answer below gets the right order of reactivity for the wrong reason. Find the wrong line.
A student explains why chlorine is more reactive than bromine. One line is wrong. Tap it.
The student's answer, with the mistake marked:
- Line 1. Chlorine and bromine are both in Group 17, with seven electrons in their outer shells. (Correct.)
- Line 2. Bromine has one more shell than chlorine, so its outer shell is further from the nucleus. (Correct.)
- Line 3. Bromine's outer shell is also better shielded by inner electrons. (Correct.)
- Line 4. So bromine loses its outer electrons more easily than chlorine, which is why bromine is less reactive. The mistake: halogens react by gaining an electron, not by losing them. The weaker pull makes it harder for bromine to gain an electron.
- Line 5. Chlorine therefore displaces bromine: Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq). (Correct.)
Three classic mistakes
Metals react by losing electrons, so a weaker pull makes them more reactive. Halogens react by gaining electrons, so a weaker pull makes them less reactive. Always ask first: does this element lose or gain electrons when it reacts?
It increases down Group 1 and decreases down Group 17. A single rule for every group will be wrong half the time. State which group, and give the reason.
A more reactive halogen displaces a less reactive one from its ions. Chlorine water added to potassium bromide reacts. Bromine water added to potassium chloride does not. The halogen added must be the more reactive one for anything to happen.
Work each one on paper first. Open a hint only when you are stuck.
-
Arrange K, Li, Rb and Na in order of increasing reactivity with water, and give the reason.
Hint 1
All four are in Group 1. Which way does reactivity change down the group?
Hint 2
Their order down the group is Li, Na, K, Rb.
Full answer
Li < Na < K < Rb. Down Group 1, the outer electron is further from the nucleus and better shielded, so it is lost more easily. The first ionisation energies confirm it: 520, 496, 419 and 403 kJ/mol.
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Bromine water is added to potassium iodide solution. Write the ionic equation, with state symbols, and describe what you would see.
Hint 1
Is bromine more or less reactive than iodine?
Hint 2
Each bromine molecule takes one electron from each of two iodide ions.
Full answer
. The orange bromine colour is replaced by the brown of iodine. Bromine is more reactive than iodine, so it displaces it.
-
A solution of halogen X turns potassium iodide solution brown, but has no effect on potassium bromide solution. Which of chlorine, bromine and iodine is X? Explain.
Hint 1
X displaces iodine, so X is more reactive than iodine.
Hint 2
X does not displace bromine. What would chlorine have done?
Full answer
X is more reactive than iodine but not more reactive than bromine. Chlorine would have displaced bromine too, so X is not chlorine. Iodine cannot displace itself. So X is bromine. (Bromine added to potassium bromide shows no change, because a halogen cannot displace its own ions.)
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The most reactive metals are at the bottom left of the periodic table, and the most reactive non-metals are at the top right. Use one idea to explain both, and say how this connects to the last three lectures.
Hint 1
Where are the largest atoms? Where are the smallest?
Hint 2
Large atoms hold their outer electrons weakly. Is that good or bad for losing electrons? For gaining them?
Full answer
The one idea is how strongly the nucleus pulls on the outer shell. At the bottom left, the atoms are largest, with the lowest ionisation energies, so they lose electrons most easily: the most reactive metals. At the top right (leaving out the noble gases), the atoms are smallest, with the highest electronegativities, so they pull hardest on extra electrons: the most reactive non-metals. These are the same two corners as the largest and smallest atoms (atomic radius), the lowest and highest ionisation energies, and the lowest and highest electronegativities. Four lectures, one tug-of-war.
This topic began with a question about size and ends with a question about chemical behaviour. Atomic radius, ionisation energy, electronegativity and reactivity are four views of one contest: the pull of the nucleus on the outer electrons, against distance and shielding. Once you can see that contest, the periodic table stops being a list of facts and becomes a map you can reason with.
Displacement is also useful outside the laboratory. Seawater contains bromide ions, and bromine is extracted from it using chlorine, the same reaction you have just explored. In the next topic, on chemical bonding, the electrons that atoms lose, gain and share become the bonds that hold all substances together.
- Metals react by losing electrons, while halogens react by gaining electrons, so the same change in an atom can affect their reactivity in opposite ways.
- Down Group 1, the outer electron is further from the nucleus and better shielded, so it is lost more easily and reactivity increases.
- Down Group 17, an incoming electron is held less strongly by a more distant, better shielded nucleus, so reactivity decreases.
- A more reactive halogen displaces a less reactive one from a solution of its ions, which shows the order Cl₂ > Br₂ > I₂.
- An ionic equation shows only the particles that change, such as Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq).