Unit 2 · 7–9% of the exam Beta

Unit 2 practice test: Compound Structure and Properties

12 exam-style questions from the unit, free, with an explanation for every option. Four answer choices each, and data sets that share one table, graph or particle diagram, as on the real exam.

Covers: Types of Chemical Bonds; Intramolecular Force and Potential Energy; Structure of Ionic Solids; Structure of Metals and Alloys; Lewis Diagrams; Resonance and Formal Charge; VSEPR and Hybridization.

Data table

Bond lengths and bond energies

Typical values for bonds between carbon, nitrogen and oxygen atoms.

Typical bond lengths and bond energies
BondBond length (pm)Bond energy (kJ/mol)
C–C154347
C=C134614
C≡C120839
N–N145160
N=N125418
N≡N110945
C–O143358
C=O123745

1. Which statement describes the trend for carbon-carbon bonds?

  1. From single to triple, the bond gets shorter and weaker
  2. From single to triple, the bond gets longer and stronger
  3. From single to triple, the bond gets shorter and stronger
  4. The length changes, but the bond energy stays about the same
Show the answer

C–C 154 pm and 347 kJ/mol; C=C 134 pm and 614; C≡C 120 pm and 839. More shared pairs: shorter and stronger.

  • From single to triple, the bond gets shorter and weaker: The energies rise from 347 to 839 kJ/mol, so the bonds get stronger.
  • From single to triple, the bond gets longer and stronger: The lengths fall from 154 to 120 pm, so the bonds get shorter, not longer.
  • Correct: From single to triple, the bond gets shorter and stronger: Right: both trends from the table.
  • The length changes, but the bond energy stays about the same: The energy more than doubles from single to triple.

2. Why is a C=O double bond shorter than a C–O single bond?

  1. Four shared electrons attract both nuclei more strongly than two, pulling them closer
  2. The oxygen atom shrinks when it forms a double bond, so the two nuclei can come closer
  3. A double bond is two separate single bonds, each half the length of a single bond
  4. The double bond has less energy, so it takes up less room between the two atoms
Show the answer

A double bond places two shared pairs between the nuclei. More negative charge between them means a stronger attraction to both nuclei, so the minimum of the potential energy curve sits at a shorter distance (123 pm against 143 pm) and is deeper.

  • Correct: Four shared electrons attract both nuclei more strongly than two, pulling them closer: Right: more shared electrons, stronger attraction, shorter bond.
  • The oxygen atom shrinks when it forms a double bond, so the two nuclei can come closer: The atoms' cores are unchanged; what changes is how much electron density sits between the nuclei.
  • A double bond is two separate single bonds, each half the length of a single bond: If it were two bonds each half as long, the double bond would be 72 pm. The table shows 123 pm.
  • The double bond has less energy, so it takes up less room between the two atoms: The double bond is stronger (745 against 358 kJ/mol); its potential energy minimum is lower, which means more energy is needed to break it, not less room.

3. The potential energy curves for C–C and C≡C are drawn on the same axes. How does the C≡C curve compare?

  1. Its minimum is at the same place, but the curve is narrower
  2. Its minimum is deeper and farther to the right
  3. Its minimum is shallower and farther to the left
  4. Its minimum is deeper and farther to the left
Show the answer

Deeper means a larger bond energy (839 against 347 kJ/mol); farther left means a shorter bond length (120 against 154 pm).

  • Its minimum is at the same place, but the curve is narrower: Both the length and the energy differ, so the minimum moves.
  • Its minimum is deeper and farther to the right: Farther right would mean a longer bond, but C≡C is shorter.
  • Its minimum is shallower and farther to the left: Shallower would mean a weaker bond, but C≡C is stronger.
  • Correct: Its minimum is deeper and farther to the left: Right: stronger and shorter.

Data table

Measured bond angles

Measured H–X–H bond angles in four molecules with a central atom from period 2.

Bond angles around the central atom
MoleculeCentral atomBonding pairs on the central atomLone pairs on the central atomMeasured H–X–H angle (°)
CH₄C40109.5
NH₃N31107
H₂OO22104.5
BH₃B30120

4. Which statement best explains why the angle falls from CH₄ to NH₃ to H₂O?

  1. The bonds get shorter from C to O, so the H atoms are pushed closer together
  2. The central atom gets more electronegative, so it pulls the H atoms closer together
  3. The molecules have fewer H atoms, so the H atoms move closer to fill the space
  4. Each has four domains, and each added lone pair, repelling more strongly, squeezes the angle
Show the answer

All three have four electron domains (tetrahedral arrangement). A lone pair is held by one nucleus only, so it spreads out closer to the central atom and pushes the bonding pairs together: 109.5° with none, 107° with one, 104.5° with two.

  • The bonds get shorter from C to O, so the H atoms are pushed closer together: Bond length does not set the angle between the bonds here; repulsion between domains does.
  • The central atom gets more electronegative, so it pulls the H atoms closer together: Electronegativity is not what sets these angles; the number of lone pairs is.
  • The molecules have fewer H atoms, so the H atoms move closer to fill the space: Fewer atoms do not fill space; lone pairs take the place of the missing bonds and push the others together.
  • Correct: Each has four domains, and each added lone pair, repelling more strongly, squeezes the angle: Right: lone pairs take up more room than bonding pairs.

5. BH₃ and NH₃ both have three bonding pairs on the central atom. Why is the BH₃ angle larger?

  1. B is less electronegative than N, so the B–H bonds repel each other more strongly
  2. B has three domains, 120° apart in a plane; N has four, tetrahedral
  3. BH₃ has fewer electrons in total, so its H atoms have more room to spread out
  4. B is a larger atom than N, so its bonds are longer and their angles are wider
Show the answer

BH₃ has no lone pair, so its three domains spread out to 120° in a plane (trigonal planar). NH₃'s lone pair is a fourth domain: the arrangement is tetrahedral and the H–N–H angle closes to about 107°.

  • B is less electronegative than N, so the B–H bonds repel each other more strongly: Bond repulsion due to electronegativity is a small effect; the lone pair is the main difference.
  • Correct: B has three domains, 120° apart in a plane; N has four, tetrahedral: Right: three domains against four.
  • BH₃ has fewer electrons in total, so its H atoms have more room to spread out: Total electron count does not set the shape; the number of domains on the central atom does.
  • B is a larger atom than N, so its bonds are longer and their angles are wider: Bond length does not set the angle; the number of domains does.

6. What is the hybridization of the central atom in each of CH₄, NH₃ and H₂O?

  1. sp³, sp² and sp
  2. sp³ in all three
  3. sp³ in CH₄; sp² in NH₃ and H₂O
  4. sp in each of the three
Show the answer

Each central atom has four electron domains (bonds plus lone pairs: 4 + 0, 3 + 1, 2 + 2), so each is sp³. Hybridization counts domains, including lone pairs.

  • sp³, sp² and sp: That counts only the bonds. Lone pairs are domains too.
  • Correct: sp³ in all three: Right: four domains each.
  • sp³ in CH₄; sp² in NH₃ and H₂O: NH₃ and H₂O have lone pairs that make up their four domains.
  • sp in each of the three: sp needs two domains, as in CO₂.

7. When sodium reacts with chlorine, an electron moves from Na to Cl. Which explanation is best?

  1. Na wants a full outer shell, so it chooses to give its single valence electron away to the nearest atom it meets
  2. Na's valence electron is far from its nucleus and shielded, so it is held weakly; Cl's nucleus attracts one strongly
  3. Cl already has more electrons than Na, so Cl has more electrons of its own with which to attract the extra one
  4. Na is the larger atom, so it has more room to lose an electron than a small Cl atom has to lose one of its own
Show the answer

Coulomb's law: Na's 3s electron sits in the third shell, shielded by 10 core electrons, so Na has a low ionization energy. Cl has a high effective nuclear charge for its valence shell, so it attracts an added electron strongly. Moving the electron gives Na⁺ and Cl⁻, which then attract each other.

  • Na wants a full outer shell, so it chooses to give its single valence electron away to the nearest atom it meets: Atoms do not want or choose anything. The cause is the strength of the attractions, which this option leaves out.
  • Correct: Na's valence electron is far from its nucleus and shielded, so it is held weakly; Cl's nucleus attracts one strongly: Right: the explanation compares the attraction in both atoms.
  • Cl already has more electrons than Na, so Cl has more electrons of its own with which to attract the extra one: Electrons repel each other; Cl attracts the added electron because of its nuclear charge and short distance, not because it already has many electrons.
  • Na is the larger atom, so it has more room to lose an electron than a small Cl atom has to lose one of its own: Size matters because of distance from the nucleus, and this option skips that reasoning; 'room' is not a cause.

8. Why does an ionic solid such as NaCl have a high melting temperature?

  1. Ionic solids are dense, and a denser solid needs more heat to melt
  2. Each Na–Cl molecule is held together by a very strong covalent bond
  3. The electrons moving through the solid hold the ions firmly in place
  4. Each ion is held by many oppositely charged neighbors at once
Show the answer

To melt, ions must leave their fixed places. Each ion is held by strong Coulombic attraction to several neighbors throughout the lattice, so a lot of energy is needed.

  • Ionic solids are dense, and a denser solid needs more heat to melt: Density does not explain melting temperature; the strength of the attraction between the particles does.
  • Each Na–Cl molecule is held together by a very strong covalent bond: NaCl has ionic bonds between ions in a lattice, not covalent bonds within molecules.
  • The electrons moving through the solid hold the ions firmly in place: Free-moving electrons describe a metal; the electrons in NaCl stay on their ions.
  • Correct: Each ion is held by many oppositely charged neighbors at once: Right: many strong attractions in every direction.

9. Sterling silver is 92.5% silver (atomic radius 144 pm) and 7.5% copper (128 pm). Which classification and reasoning is best?

  1. Substitutional: Cu atoms are close enough in size to Ag atoms to take their places
  2. Interstitial: Cu atoms are smaller than Ag atoms, so they go into the gaps between them
  3. Interstitial: there is much less copper than silver, so the copper fills the gaps
  4. Substitutional: Cu and Ag are in the same group, so their alloys are substitutional
Show the answer

Cu is about 11% smaller than Ag; atoms this close in size replace host atoms. Only much smaller atoms (such as C, N or B) fit in the gaps between metal atoms.

  • Correct: Substitutional: Cu atoms are close enough in size to Ag atoms to take their places: Right: the size match decides.
  • Interstitial: Cu atoms are smaller than Ag atoms, so they go into the gaps between them: Being somewhat smaller is not enough to fit in the gaps between atoms; the gaps are far smaller than a Cu atom.
  • Interstitial: there is much less copper than silver, so the copper fills the gaps: The amount of an element does not decide where its atoms go; their size does.
  • Substitutional: Cu and Ag are in the same group, so their alloys are substitutional: Group membership is not the reason; the similar atomic radii are.

10. Draw the Lewis diagram of nitrogen trichloride, NCl₃. How many lone pairs does the whole molecule have?

Type a number in lone pairs.

Show the answer

Valence electrons: 5 + 3 × 7 = 26. Three N–Cl bonds use 6, leaving 20 electrons = 10 lone pairs: three on each Cl (9) and one on N.

  • Answer: 10 lone pairs

11. The best Lewis diagram of carbon monoxide is C≡O with one lone pair on each atom. Which statement about its formal charges is correct?

  1. C has −1 and O has +1, even though O is more electronegative
  2. C has 0 and O has 0, because CO is a neutral molecule
  3. C has +1 and O has −1, because O is more electronegative
  4. C has −2 and O has +2, because the triple bond is shared unequally
Show the answer

C: 4 − 2 − 3 = −1. O: 6 − 2 − 3 = +1. The diagram is still the best one because it is the only one that gives both atoms an octet; formal charge is a bookkeeping tool, not the real charge.

  • Correct: C has −1 and O has +1, even though O is more electronegative: Right: computed from the diagram, with both octets kept.
  • C has 0 and O has 0, because CO is a neutral molecule: A neutral molecule needs formal charges that add to zero, not formal charges that are each zero: −1 + 1 = 0.
  • C has +1 and O has −1, because O is more electronegative: That is what electronegativity would suggest, but the formula gives the opposite for this diagram.
  • C has −2 and O has +2, because the triple bond is shared unequally: Each atom has one lone pair (2) and half of six bonding electrons (3): the values are −1 and +1.

12. Magnesium oxide and copper are both solids. A student says both are held together by ionic bonds because both contain positive ions. Which response is correct?

  1. Copper is metallic: delocalized electrons hold its cations
  2. The student is right: any solid that contains positive ions is ionic
  3. Magnesium oxide is metallic, since magnesium is itself a metal
  4. Both are covalent, since the atoms in each of them share electrons
Show the answer

MgO is ionic (Mg²⁺ and O²⁻ attract each other). Copper contains Cu cations too, but the negative charge holding them together is a sea of delocalized electrons, not anions: metallic bonding.

  • Correct: Copper is metallic: delocalized electrons hold its cations: Right: what balances the cations decides the bond type.
  • The student is right: any solid that contains positive ions is ionic: Positive ions appear in both ionic and metallic bonding; the difference is whether anions or delocalized electrons hold them.
  • Magnesium oxide is metallic, since magnesium is itself a metal: Mg is a metal, but in MgO it has given its electrons to O, forming O²⁻ anions; that is ionic bonding.
  • Both are covalent, since the atoms in each of them share electrons: In MgO electrons are transferred, and in copper they are delocalized; neither is a shared pair between two atoms.

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Practice has every question in the unit, with feedback after each one; this unit's lessons are free. Other units: Unit 1 · Unit 2 · Unit 3 · Unit 4 · Unit 5 · Unit 6 · Unit 7 · Unit 8 · Unit 9.