Formal charge, resonance and the octet (HL)
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Question
What is formal charge?
Answer
A bookkeeping label comparing the electrons an atom 'owns' in a Lewis structure (lone pairs + half of each bond) with its normal valence count. It is **not** a real charge.
Question
State the formal-charge formula.
Answer
$\text{FC} = V - N - \tfrac{1}{2}B$ — valence electrons − non-bonding electrons − ½(bonding electrons).
Question
What is V in FC = V − N − ½B?
Answer
The atom's **valence electrons as a free atom** — equal to its **group number** (C → 4, N → 5, O → 6).
Question
How do you count B (bonding electrons)?
Answer
Total electrons in the atom's bonds: **2 per single, 4 per double, 6 per triple**, summed over all its bonds.
Question
FC of N in NH_{4}^{+}?
Answer
V=5, N=0, B=8 → FC = 5 − 0 − 4 = **+1** (matches the ion's +1 charge).
Question
Which is the best Lewis structure?
Answer
The one with formal charges **closest to zero**, with any **negative** formal charge on the **most electronegative** atom.
Question
What is resonance?
Answer
When ≥2 valid Lewis structures can be drawn; the real species is a **hybrid** with electrons **delocalized** over the positions (drawn with a ↔ arrow).
Question
Give three resonance examples.
Answer
Carbonate CO_{3}^{2-}, nitrate NO_{3}^{-} (three forms each) and ozone O_{3} (two forms).
Question
What is the evidence for resonance?
Answer
**Equal bond lengths** — e.g. all three C–O bonds in CO_{3}^{2-} are identical, between a single and a double bond.
Question
What is an electron-deficient species?
Answer
A molecule whose central atom has **fewer than 8** outer electrons, e.g. BeCl_{2} (4 on Be) and BF_{3} (6 on B).
Question
What is an expanded octet?
Answer
A central atom holding **more than 8** outer electrons — only **period-3-or-below** atoms can, e.g. PCl_{5} (10) and SF_{6} (12).
Question
Why can carbon never expand its octet?
Answer
Carbon is **period 2** — it has no available extra valence space, so it is limited to 8 outer electrons.
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Topic 2.2 hub
The covalent model
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