The chain: structure → forces → property
“Explain why …” is one of the most common FRQ tasks. What the scoring guidelines usually want is a complete chain of reasoning:
- Structure: what are the particles like? Is the molecule polar, is H bonded directly to N, O or F, how many electrons are there; what are the ion charges and sizes; which shell holds the valence electrons?
- Forces: what forces does that structure lead to, and which side’s are stronger? (Intermolecular forces, Coulombic attraction between ions, attraction of the nucleus for electrons.)
- Property: stronger forces take more energy to overcome when changing state or removing an electron, so the boiling point, melting point or ionization energy is higher (or the vapor pressure lower).
Write every link and join them with “so.” A missing link usually means a missing point.
A template for comparisons
A … (structure), so A’s particles are held by … (forces); B … (structure) has only … (forces). … is stronger, so more energy is needed to overcome the forces when A … (vaporizes, melts), and A’s … (boiling point, melting point) is higher.
Coulomb’s-law explanations
Ionization energy, atomic and ionic radius, and lattice energy are all explained with Coulomb’s law, F ∝ q₁q₂/r².
- Mention both factors: charge (nuclear charge, effective nuclear charge or ion charge) and distance (shell, ionic radius).
- Say which factor dominates: across a period the shell is the same and the nuclear charge differs; down a group the nuclear charge increases but the extra shell (greater distance) matters more.
Answers that lose points
- Describing only one substance without comparing it with the other.
- Calling an intermolecular force a chemical bond: “the O–H bonds break when it boils” is wrong.
- Writing only “because it’s polar,” without naming the force or connecting to energy.
- Using “more stable” in place of a reason.
- Claiming hydrogen bonding when H isn’t bonded to N, O or F (as in CH₃OCH₃ or CH₃F).
Pick the right statement for each link:
Why does CH₃OH (bp 65 °C) boil at a higher temperature than CH₃SH (bp 6 °C)?
Why is Br₂ a liquid at room temperature while Cl₂ is a gas (Br₂ has the higher boiling point)?
Why is the first ionization energy of K (419 kJ/mol) smaller than that of Na (496 kJ/mol)?
Why does MgO melt at a much higher temperature (about 2800 °C) than NaF (about 1000 °C)?
Example 1: butane vs acetone
Butane, C₄H₁₀ (bp about −1 °C), and acetone, CH₃COCH₃ (bp 56 °C), both have molar masses of about 58 g/mol. Explain why their boiling points are so different.
- Structure. The two have similar molar masses, numbers of electrons and sizes. Acetone has a strongly polar C=O bond, so the molecule is polar; butane has only C–C and C–H bonds and is nonpolar. In both, H is bonded to C, so neither can hydrogen bond.
- Forces. Their dispersion forces are similar, but acetone molecules also have dipole–dipole forces, which butane lacks. So the forces between acetone molecules are stronger.
- Property. Vaporizing acetone means overcoming stronger intermolecular forces, which takes more energy, so its boiling point is higher.
- The full answer. “Butane and acetone have similar molar masses and similar dispersion forces. But acetone contains a polar C=O bond, so its molecules also attract each other through dipole–dipole forces, while nonpolar butane has only dispersion forces. The stronger intermolecular forces in acetone take more energy to overcome on vaporizing, so acetone has the higher boiling point.”
Example 2: radii of O²⁻ and F⁻
Explain why O²⁻ is larger than F⁻.
- Structure. O²⁻ and F⁻ both have 10 electrons with the same configuration (1s²2s²2p⁶), so their outer electrons are in the n = 2 shell; the nuclear charge is +8 for O and +9 for F.
- Forces. With the same electrons and the same shell, a greater nuclear charge means a stronger Coulombic attraction for the electrons. O²⁻ has one less unit of nuclear charge, so it attracts its electrons less strongly.
- Property. Weaker attraction lets the electron cloud spread farther from the nucleus, so O²⁻ is larger.
- Check. Both factors are covered, charge (+8 vs +9) and distance (same shell), and the conclusion follows from “weaker attraction” instead of simply stating that O²⁻ is bigger.
- “I₂ has stronger dispersion forces than Br₂, which take more energy to overcome, so I₂ has the higher boiling point.”
It doesn’t say why the dispersion forces are stronger (I₂ has more electrons and is more polarizable).
- “In H₂O, H is bonded to the highly electronegative O, so vaporizing H₂O takes more energy and it boils higher than H₂S.”
It never names the force: H₂O molecules hydrogen bond to each other, and H₂S molecules don’t.
- “In NH₃, H is bonded to N, so NH₃ molecules hydrogen bond and PH₃ molecules don’t. So NH₃ has the higher boiling point.”
It doesn’t connect the strength of the forces to the energy needed to vaporize.
- “Cl₂ and F₂ are both nonpolar; Cl₂ has more electrons and is more polarizable, so its dispersion forces are stronger and vaporizing it takes more energy, so Cl₂ boils higher.”
All three links are there.
- “K’s valence electron is attracted less strongly than Na’s, so less energy is needed to remove it and K’s ionization energy is smaller.”
It doesn’t say why the attraction is weaker: K’s valence electron is in a higher shell, farther from the nucleus.
- “The ions in CaO have charges of +2 and −2 and those in KF +1 and −1, with similar sizes, so the Coulombic attraction between ions is stronger in CaO.”
It stops before saying stronger attraction → more energy to melt → higher melting point.
- “Na⁺ and Mg²⁺ have the same number of electrons, but Mg²⁺ has a greater nuclear charge and attracts its electrons more strongly, so Mg²⁺ is smaller.”
Charge, distance (same electrons) and the conclusion are all there.
- “CH₃OCH₃ is polar and CH₃CH₂CH₃ is nonpolar, with similar molar masses, so vaporizing CH₃OCH₃ takes more energy.”
It never names the force: CH₃OCH₃ molecules have dipole–dipole forces.
Common mistakes
Mistake 1: “Covalent bonds inside the molecules break when a liquid boils.” Melting and boiling a molecular substance overcome the forces between molecules; the molecules stay intact. Writing “the O–H bonds break” loses the explanation point outright.
Mistake 2: “Any molecule with H and O hydrogen bonds.” Molecules hydrogen bond to each other only when H is bonded directly to N, O or F. CH₃OCH₃ contains O and H, but every H is bonded to C, so its molecules don’t hydrogen bond to each other.
Mistake 3: “In a comparison, describing one substance is enough.” If the question asks why A is higher than B, describe the structure and forces of both A and B and compare them. Writing only “A has hydrogen bonding” is usually scored as an incomplete comparison.
Quick check
Which explanation best accounts for CCl₄ boiling higher than CH₄?
Both are nonpolar; the chain is electrons → dispersion forces → energy.
What is overcome when a molecular substance boils?
The molecules themselves stay intact.
Which of these can hydrogen bond between its own molecules?
CH₃NH₂ has H bonded directly to N; in the others every H is bonded to C.
Using Coulomb’s law, what is the key reason Na⁺ is smaller than a Na atom?
The nuclear charge is unchanged; one fewer shell means a smaller distance.
“HF has a high boiling point because HF is polar.” What is missing?
Polarity is only structure; the answer also needs hydrogen bonding and the energy needed to vaporize.
What is the main reason LiF has a larger lattice energy than CsF?
The ion charges are the same; the difference is the distance.
AP-style practice
Explain each observation in terms of structure and forces, giving the full chain of reasoning each time.
(a) H₂S (bp −60 °C) boils far lower than H₂O (bp 100 °C).
(b) Ar (bp −186 °C) boils higher than Ne (bp −246 °C).
(c) The first ionization energy of Mg (738 kJ/mol) is larger than that of Na (496 kJ/mol).
(d) NaCl dissolves readily in water, but I₂ barely dissolves.
(e) A student writes, “H₂O has a high boiling point because O–H bonds are stronger than S–H bonds.” Identify the error.
Model answer and scoring
(a) In H₂O, H is bonded to the highly electronegative O, so the molecules hydrogen bond to each other; S in H₂S is much less electronegative, so its molecules have mainly dipole–dipole and dispersion forces. Hydrogen bonds are stronger, so vaporizing H₂O takes more energy and it boils higher, while H₂S, with weaker forces, boils lower.
(b) Ar and Ne are monatomic, with only dispersion forces between atoms. Ar has more electrons, a larger and more polarizable electron cloud, and therefore stronger dispersion forces, so vaporizing it takes more energy and it boils higher.
(c) The valence electrons of Na and Mg are both in the n = 3 shell, at similar distances; Mg has one more unit of nuclear charge, so its valence electrons feel a stronger Coulombic attraction and take more energy to remove, giving Mg the larger first ionization energy.
(d) When NaCl dissolves, strong ion–dipole attractions form between the ions and water molecules, enough to make up for the energy needed to break up the ionic lattice and the hydrogen bonds between water molecules. I₂ is nonpolar and interacts only weakly with water (dispersion and induced-dipole forces), which can’t make up for breaking water’s hydrogen bonds, so it barely dissolves.
(e) Boiling doesn’t break H₂O molecules apart; the O–H bonds stay intact, and what is overcome is the hydrogen bonding between molecules. Boiling points are explained by intermolecular forces, not by covalent bonds within molecules.
Scoring: (a) hydrogen bonding vs weaker forces, linked to energy; (b) electrons, dispersion forces and energy; (c) same shell, greater nuclear charge, stronger attraction; (d) ion–dipole forces compensate, while I₂–water forces are too weak; (e) boiling doesn’t break covalent bonds.
Summary
- The chain: structure → forces → property, with every link written out and joined by “so.”
- In comparisons, cover both substances and finish with “stronger → more energy needed → higher boiling (melting) point.”
- Coulomb’s-law explanations mention both charge and distance and say which one dominates.
AP tip: Before writing an explanation, jot the key words for each link on scratch paper (for example “H on O / hydrogen bonding / more energy”), then turn them into sentences. Afterward, check: did you cover both substances, and did you avoid calling an intermolecular force a bond?