Rucete ✏ AP Chemistry In a Nutshell
4. Covalent Compounds, Formulas, and Structures — Practice Questions 3
This chapter introduces the octet rule, Lewis structures, resonance, formal charges, molecular geometry, hybrid orbitals, and bonding theories such as sigma and pi bonding.
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(Multiple Choice — Click to Reveal Answer)
1. Which molecule follows the octet rule strictly?
(A) BCl3
(B) NO2
(C) CH4
(D) SF6
Answer
(C) — CH4 has 8 electrons around carbon, satisfying the octet rule fully.
2. What is the correct molecular geometry for XeF2?
(A) T-shaped
(B) Linear
(C) Bent
(D) Trigonal planar
Answer
(B) — XeF2 has 3 lone pairs and 2 bonding pairs, resulting in a linear shape.
3. Which molecule exhibits a square planar geometry?
(A) BF3
(B) CH4
(C) XeF4
(D) PF5
Answer
(C) — XeF4 has 2 lone pairs and 4 bonding pairs, leading to a square planar structure.
4. Which species violates the octet rule by having fewer than 8 electrons?
(A) CO2
(B) BeCl2
(C) H2O
(D) NH3
Answer
(B) — Beryllium in BeCl2 has only 4 valence electrons.
5. Which type of hybridization corresponds to 5 electron domains around a central atom?
(A) sp
(B) sp2
(C) sp3
(D) sp3d
Answer
(D) — Five electron domains indicate sp3d hybridization.
6. What causes the equal bond lengths in the carbonate ion?
(A) Hybridization
(B) Lone pairs
(C) Ionic character
(D) Resonance
Answer
(D) — Resonance structures distribute electron density equally among the three bonds.
7. What is the molecular shape of PF5?
(A) Octahedral
(B) Tetrahedral
(C) Trigonal bipyramidal
(D) Trigonal planar
Answer
(C) — PF5 has five bonding pairs, forming a trigonal bipyramidal geometry.
8. What hybridization is found in a molecule with 3 bonding domains and no lone pairs?
(A) sp
(B) sp2
(C) sp3
(D) sp3d
Answer
(B) — 3 bonding regions and no lone pairs suggest sp2 hybridization.
9. Which of the following is a polar molecule despite having symmetrical bond polarities?
(A) CO2
(B) CH4
(C) NH3
(D) BF3
Answer
(C) — NH3 is trigonal pyramidal with a lone pair, making it polar.
10. What is the approximate bond angle in H2O?
(A) 90°
(B) 104.5°
(C) 120°
(D) 109.5°
Answer
(B) — The bond angle in water is reduced to ~104.5° due to two lone pairs.
11. Which of the following molecules has a bent molecular geometry?
(A) CO2
(B) CH4
(C) H2O
(D) BF3
Answer
(C) — H2O has two lone pairs on oxygen, giving it a bent shape.
12. What is the electron geometry of NH3?
(A) Trigonal planar
(B) Linear
(C) Trigonal pyramidal
(D) Tetrahedral
Answer
(D) — NH3 has four regions of electron density, making the electron geometry tetrahedral, even though the molecular geometry is trigonal pyramidal.
13. What is the correct Lewis structure for CH2O?
(A) One double bond between C and O, and two single bonds to H
(B) All single bonds
(C) Triple bond between C and O
(D) One lone pair on each hydrogen
Answer
(A) — CH2O has a double bond between carbon and oxygen, and single bonds from carbon to each hydrogen.
14. Which molecule is linear in shape?
(A) H2O
(B) CO2
(C) NH3
(D) CHCl3
Answer
(B) — CO2 has two bonding pairs and no lone pairs, making it linear.
15. What type of bonding involves the sharing of electron pairs?
(A) Ionic
(B) Metallic
(C) Covalent
(D) Hydrogen
Answer
(C) — Covalent bonding involves the sharing of electrons between atoms.
16. In which molecule does the central atom have one lone pair?
(A) CH4
(B) NH3
(C) CO2
(D) BF3
Answer
(B) — NH3 has one lone pair on the nitrogen atom.
17. What is the hybridization of the central atom in BCl3?
(A) sp
(B) sp2
(C) sp3
(D) sp3d
Answer
(B) — BCl3 has three bonding domains and no lone pairs, so it is sp2 hybridized.
18. Which of the following best explains why SF6 is stable despite having 12 valence electrons around sulfur?
(A) Ionic bonding
(B) Hydrogen bonding
(C) d-orbital expansion
(D) Resonance
Answer
(C) — Sulfur uses empty d-orbitals to expand its valence shell and accommodate more than 8 electrons.
19. Which of the following molecules has a trigonal planar molecular geometry?
(A) SO3
(B) NH3
(C) CH4
(D) H2O
Answer
(A) — SO3 has three bonding regions and no lone pairs on the central atom, giving it a trigonal planar shape.
20. Which of the following has the most polar bond?
(A) H–H
(B) H–C
(C) H–N
(D) H–F
Answer
(D) — H–F has the largest difference in electronegativity, making it the most polar bond.
21. What is the purpose of drawing resonance structures?
(A) To predict magnetic properties
(B) To represent electron delocalization
(C) To determine hybridization
(D) To assign formal charges
Answer
(B) — Resonance structures show the delocalization of electrons across multiple bonds or atoms.
22. What is the formal charge on nitrogen in NH4⁺?
(A) 0
(B) –1
(C) +1
(D) +2
Answer
(A) — In NH4⁺, nitrogen has a formal charge of 0, while the overall charge is from the extra proton.
23. Which molecular shape has bond angles of approximately 120°?
(A) Linear
(B) Tetrahedral
(C) Trigonal planar
(D) Bent
Answer
(C) — Trigonal planar molecules have bond angles of approximately 120°.
24. What causes a molecule to be polar overall?
(A) Equal bond polarities
(B) Symmetrical molecular shape
(C) Asymmetrical charge distribution
(D) Octet rule violation
Answer
(C) — A molecule is polar if it has an asymmetrical shape and uneven distribution of charge.
25. Which of the following molecules is nonpolar overall despite having polar bonds?
(A) NH3
(B) H2O
(C) CH3Cl
(D) CCl4
Answer
(D) — CCl4 is tetrahedral and symmetrical, so its polar bonds cancel out, making the molecule nonpolar.
26. Which of the following molecules is predicted to have a bond angle less than 109.5°?
(A) CH4
(B) NH3
(C) CO2
(D) BF3
Answer
(B) — NH3 has a lone pair on nitrogen, which compresses the bond angle to about 107°.
27. Which species has a central atom that is sp3d2 hybridized?
(A) PCl5
(B) SF6
(C) CO2
(D) NO3⁻
Answer
(B) — SF6 has six bonding pairs around sulfur, requiring sp3d2 hybridization.
28. Which of the following species has a trigonal pyramidal molecular geometry?
(A) BH3
(B) CH4
(C) NH3
(D) NO3⁻
Answer
(C) — NH3 has three bonding pairs and one lone pair, resulting in a trigonal pyramidal shape.
29. Which compound has at least one coordinate covalent bond?
(A) O2
(B) NH4⁺
(C) H2O
(D) N2
Answer
(B) — In NH4⁺, the nitrogen donates a lone pair to a proton, forming a coordinate covalent bond.
30. Which molecule violates the octet rule by having an incomplete octet?
(A) BCl3
(B) PCl5
(C) SF6
(D) H2O
Answer
(A) — BCl3 has only six electrons around boron, violating the octet rule by having too few electrons.
31. Which of the following has delocalized electrons and resonance stabilization?
(A) CH4
(B) NH3
(C) NO3⁻
(D) CCl4
Answer
(C) — NO3⁻ has resonance structures where the double bond shifts among the three oxygen atoms.
32. Which molecule has a net dipole moment due to molecular asymmetry?
(A) CO2
(B) CCl4
(C) CH3Cl
(D) BF3
Answer
(C) — CH3Cl has a tetrahedral shape but one Cl atom causes an asymmetrical distribution of charge, resulting in a net dipole.
33. What is the electron geometry and molecular geometry of SF4?
(A) Trigonal bipyramidal; see-saw
(B) Octahedral; square planar
(C) Tetrahedral; bent
(D) Trigonal planar; linear
Answer
(A) — SF4 has five regions of electron density (4 bonding, 1 lone pair), leading to a trigonal bipyramidal electron geometry and a see-saw molecular shape.
34. Which of the following combinations of atoms is most likely to form a polar covalent bond?
(A) O and O
(B) Na and Cl
(C) C and H
(D) H and F
Answer
(D) — The electronegativity difference between H and F is large but not enough to transfer electrons completely, so the bond is polar covalent.
35. Which of the following molecules would be expected to have a molecular dipole of zero?
(A) H2O
(B) SO2
(C) CO2
(D) CH3Cl
Answer
(C) — CO2 is linear and symmetrical, so the bond dipoles cancel each other out, resulting in no net dipole moment.
36. Explain why BF3 is an exception to the octet rule.
Answer
BF3 has only six valence electrons around boron — Boron in BF3 forms three bonds and lacks a full octet, making it an incomplete octet exception.
37. Describe the bonding and hybridization in the ammonium ion (NH4⁺).
Answer
Four N–H sigma bonds; sp3 hybridization — The nitrogen uses sp3 hybrid orbitals to form four equivalent bonds, including one coordinate bond to H⁺.
38. Why does water (H2O) have a bent shape instead of a linear one?
Answer
Due to two lone pairs on oxygen — Lone pairs repel bonding pairs, forcing a bent geometry with an angle of ~104.5°.
39. What is the difference between a sigma (σ) bond and a pi (π) bond?
Answer
σ-bonds are head-on overlaps; π-bonds are side-on overlaps — Sigma bonds form along the axis between nuclei; pi bonds form above and below this axis.
40. Explain how formal charge helps identify the best Lewis structure.
Answer
Lowest formal charges and negative charges on electronegative atoms are favored — Structures with formal charges close to zero are typically more stable.
41. Why is CO2 nonpolar even though it contains polar bonds?
Answer
Bond dipoles cancel due to linear shape — The molecule is symmetrical, so the effects of the polar bonds cancel out.
42. How many sigma and pi bonds are present in the molecule C2H4?
Answer
5 sigma, 1 pi — Each C–H bond and the C–C single bond are sigma; the C=C also contains one pi bond.
43. Describe the geometry and hybridization of SF6.
Answer
Octahedral geometry; sp3d2 hybridization — SF6 has six bonding pairs and no lone pairs around sulfur.
44. What is the molecular geometry of BrF3 and what causes its shape?
Answer
T-shaped due to two lone pairs — BrF3 has five electron domains; the two lone pairs cause repulsion, giving a T-shape.
45. Why can elements in Period 3 or beyond form expanded octets?
Answer
They have available d orbitals — Atoms in Period 3 or higher can use d orbitals to hold more than 8 electrons.
46. What are the bond angles in a trigonal bipyramidal structure?
Answer
90°, 120°, and 180° — The geometry includes three different bond angles due to the axial and equatorial positions.
47. Explain the polarity of H2S.
Answer
Polar due to bent geometry — Although S is less electronegative than O, the molecule is bent, causing an uneven distribution of charge.
48. How do lone pairs affect molecular shape and bond angle?
Answer
Lone pairs repel more strongly than bonding pairs — They distort the shape and decrease bond angles compared to ideal geometries.
49. What makes a bond "polar covalent" rather than "ionic"?
Answer
Unequal sharing of electrons — A polar covalent bond shares electrons unequally between atoms of differing electronegativity, unlike complete transfer in ionic bonds.
50. Why does O3 (ozone) exhibit resonance, and what is its effect?
Answer
Electron delocalization across O–O bonds — O3 cannot be represented by a single Lewis structure; resonance makes all bonds equivalent in length and strength.