IB Chemistry SLTopic 6 — Electron SharingPaper 1 & 2Core idea~10 min read
Free Radicals
Proton transfer moved a whole proton. Electron transfer moved whole electrons. This last family is the awkward middle case: a bond breaks and the pair is split, leaving each fragment holding a single, unpaired electron.
📚 What you need to know
A radical is a species with an unpaired electron. That is the only requirement.
It is shown by a dot written on the atom carrying that electron, e.g. Cl• and •CH3.
Radicals can be atomic, molecular, anionic or cationic — the charge is a separate matter.
A quick test: count the valence electrons. An odd total means one is unpaired.
Radicals are highly reactive and short-lived, because an unpaired electron is a high-energy arrangement.
They react either by taking an electron from something else, which creates a new radical, or by pairing up with another radical.
Equations tell you what reacts; mechanisms tell you how, by tracking the electrons.
Spotting one
Almost every species you have met so far keeps its electrons in pairs — bonding pairs and lone pairs. A radical is the exception, and drawing the valence electrons makes it obvious.
The chloride ion is the useful comparison. It is one electron away from the radical and behaves completely differently, because that extra electron gives every other one a partner.
🧩 Is it a radical?
Add up the valence electrons of every atom in the species.
Subtract one for each positive charge, add one for each negative charge.
If the total is odd, one electron cannot be paired — it is a radical.
If it is even, draw the Lewis structure to be certain, but it is almost always not a radical.
Four kinds, one definition
Textbooks list radicals under four headings, and the list is longer than it needs to be. All four say the same thing about the electron and differ only in the charge, which has nothing to do with whether something is a radical.
Do not read “radical” as “charged”. An ion has gained or lost an electron overall; a radical has one that is unpartnered. A species can be either, both, or neither.
Where the dot goes matters. In a species made of several atoms, write it on the atom that actually holds the unpaired electron — •CH3 puts it on the carbon, OH• puts it on the oxygen. In a displayed formula it is unmistakable, because you can see the atom with only three bonds where it should have four.
Why they never last
An unpaired electron is an unstable arrangement: the species sits at a high enthalpy and there is an obvious way down. So a radical does not sit around waiting. It has two ways to lose the problem, and the difference between them will matter enormously two pages from now.
The left-hand route is why a single radical can consume thousands of molecules. It never disappears — it simply relocates.
WORKED EXAMPLE
Which of these species are radicals? Justify each answer. (a) Cl– (b) F (c) NH3 (d) NO2
(a) chloride ion7 + 1 = 8 valence electronsnot a radical — four lone pairs(b) fluorine atom7 valence electronsa radical — three pairs and one alone(c) ammonia5 + 3(1) = 8 valence electronsnot a radical — three bonds and a lone pair(d) nitrogen dioxide5 + 2(6) = 17 valence electronsa radical — an odd total cannot pair upNotice that a lone atom of a halogen is a radical, while the ion is not. Writing Cl and Cl⁻ carelessly changes the answer completely.
WORKED EXAMPLE
Explain why radicals are described as highly reactive and short-lived, and state the two ways one can be removed from a reaction mixture.
Why reactiveThe unpaired electron is a high-energy arrangement, so there is a strong driving force to pair it up.it reacts with almost anything it meetsRoute 1It takes an electron, or an atom, from another species.a new radical is created, so one still existsRoute 2It meets a second radical and the two unpaired electrons pair into a covalent bond.both radicals are destroyedRoute 2 is rare in practice, simply because radicals are so scarce that two seldom meet.
💡 Exam tip
Define a radical as a species with an unpaired electron — not as “a very reactive species”.
Count the valence electrons to check. Odd means radical.
Put the dot on the correct atom, and remember it is one electron, not a charge.
Do not confuse a dot with a charge sign: Cl• and Cl– are completely different species.
If asked why radicals are reactive, mention the high enthalpy of an unpaired electron.
⚠️ Common mix-up
Assuming every radical is neutral. Anionic and cationic radicals exist.
Assuming every ion is a radical. Most ions have all their electrons paired.
Writing Cl– when you mean Cl• in a mechanism. The whole reaction changes.
Putting the dot on the wrong atom of a polyatomic radical.
Saying a radical is “unstable” and stopping there. Say why: the unpaired electron.
Up next: Homolytic Fission — radicals have to come from somewhere. The next page is about the one kind of bond-breaking that produces them, and the curly arrows used to show it.
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