Structure, Bonding and Introduction to Organic Chemistry

A lump of sodium metal and a flask of chlorine gas sit a whole periodic table apart. Unit 1 explains what makes them different — shells, bonding, structure — and then makes them react: the arithmetic of the mole tells you exactly how much salt you get. This is the unit where chemistry's two languages, the equation and the diagram, are both learned properly.

WCH11 Structure, Bonding and Introduction to Organic Chemistry is the first IAS paper: 1 hour 30 minutes, 80 marks, all compulsory. Its Section A is 20 one-mark multiple-choice questions — the fastest marks on any paper if the definitions are exact — followed by structured questions on Topics 1-5.

One mole is $6.02 \times 10^{23}$ particles; one molar mass in grams contains that many. Every amount-of-substance question is three moves:

Worked check. 0.12 mol dm⁻³ sulfur dioxide, 50 cm³: $n = 0.12 \times 0.050 = 6.0 \times 10^{-3}$ mol. Balanced equations then act as exchange rates between substances: multiply by the coefficient ratio, never by the mass. Atom economy and percentage yield complete the story — atom economy 原子经济 asks how much of the reactant mass ends up in the WANTED product:

Empirical formulae come from dividing each mass by its $A_r$ and ratio-ing the results; scale to the molecular formula with the measured $M_r$.

Strip electrons one at a time and record each energy: a big jump 大跳跃 marks the start of a new shell. Across Period 3 the first ionisation energy rises — nuclear charge grows, shells shrink — with two famous dips:

• Mg → Al: the outer electron moves from 3s to 3p — a slightly higher subshell, slightly further out, easier to remove.
• P → S: sulfur's fourth 3p electron is the first PAIRED one; the pairing repulsion makes it easier to remove.

The mass spectrometer weighs atoms: vaporise, ionise, accelerate, deflect (light and low-charge bend most), detect. Chlorine's two isotopes give peaks at m/z 35 and 37 with the 3:1 ratio, plus molecular peaks at 70, 72 and 74 in the 9:6:1 pattern.

Shapes by VSEPR: count bonding pairs AND lone pairs, arrange to minimise repulsion (lone pairs push harder). Water's 104.5° from two lone pairs, ammonia's 107° from one. Polarity 分子极性 needs BOTH polar bonds AND dipoles that do not cancel: CO₂ is linear and non-polar; H₂O is bent and polar. A small, highly charged cation polarises an anion and gives ionic bonds covalent character — that is why MgCl₂ has more covalent character than NaCl.

A homologous series 同系列 shares a formula and a chemistry; structural isomers share a molecular formula only. Learn the prefixes (meth-, eth-, prop-, but-, pent-, hex-) and the suffix map (-ane, -ene, -ol, -al, -one, -oic acid). Count from the end giving the LOWEST locants.

Alkanes: unreactive skeletons. Radical substitution with chlorine — initiation 始发 (Cl₂ → 2Cl·, UV light), propagation 传递 (Cl· + alkane → HCl + alkyl·; alkyl· + Cl₂ → chloroalkane + Cl·), termination 终止 (any two radicals combine). The mechanism explains the mixture of products: any radical can meet any radical.

Alkenes: the π bond is an exposed electron cloud above and below the σ bond, so it attracts electrophiles. Electrophilic addition with HBr: Markovnikov's rule — H goes where more hydrogens already are — because that route passes through the MORE STABLE secondary carbocation. Bromine water decolourises: the test for unsaturation 不饱和. E/Z isomerism needs two DIFFERENT groups on each double-bond carbon; apply CIP precedence to name them.

Reactions happen when collisions carry at least the activation energy $E_a$. Heat the mixture and the Maxwell-Boltzmann distribution flattens rightward — the shaded tail beyond $E_a$ grows dramatically:

A catalyst offers a lower $E_a$ without being consumed. At equilibrium 平衡, forward and backward rates are equal in a closed system; Le Chatelier's principle predicts the response to any change. Concentration, pressure and temperature shift position; only TEMPERATURE changes the equilibrium constant itself.

1. Calculate the mass of sodium carbonate needed to make 250 cm³ of 0.100 mol dm⁻³ solution. ($M_r$ = 106)
2. Why is the second ionisation energy of sodium nearly ten times its first?
3. Draw the shape of and bond angle in NH₄⁺ and in BF₃.
4. Why does SF₆ have zero dipole moment while SF₄ is polar?
5. Write the two propagation steps for the chlorination of methane.
6. Why does HBr add to propene the "wrong way round" (2-bromopropane major)?
7. What does a catalyst change on the Maxwell-Boltzmann diagram, and what does it not change?
8. State two changes that shift an equilibrium but leave K unchanged.

Answers: 1 $0.100 \times 0.250 \times 106 = 2.65$ g; 2 the second electron comes from the inner 2p shell, much closer to the nucleus; 3 NH₄⁺ tetrahedral 109.5°, BF₃ trigonal planar 120°; 4 SF₆ is octahedral - the six bond dipoles cancel; SF₄ has a lone pair giving a see-saw shape whose dipoles do not cancel; 5 Cl· + CH₄ → HCl + CH₃·; CH₃· + Cl₂ → CH₃Cl + Cl·; 6 the secondary carbocation intermediate is more stable than the primary, so the route through it dominates; 7 lowers the position of Eₐ (more of the distribution exceeds it) - it does not change the distribution itself; 8 concentration changes and pressure changes (temperature changes K).