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AP Chemistry practice questions

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Question 1Atomic Structure and Properties

A neutral atom of iron-56 (⁵⁶Fe) has an atomic number of 26. How many neutrons does this atom contain?

  1. 26
  2. 56
  3. 30 Correct answer.
  4. 82

Explanation

The mass number equals the number of protons plus the number of neutrons, while the atomic number gives the proton count. Subtracting 26 protons from 56 nucleons leaves 30 neutrons.

Question 2Atomic Structure and Properties

Which quantity is identical for the two isotopes ¹²C and ¹⁴C?

  1. Number of protons Correct answer.
  2. Number of neutrons
  3. Number of nucleons
  4. Mass number

Explanation

Isotopes of an element differ only in their neutron count; the atomic number, which fixes the proton count, is the same for both. The two atoms therefore have six protons each.

Question 3Atomic Structure and Properties

What is the ground-state electron configuration of a neutral chlorine atom (Z = 17)?

  1. 1s²2s²2p⁶3s²3p⁵ Correct answer.
  2. 1s²2s²2p⁵3s²3p⁶
  3. 1s²2s²2p⁶3s²3p⁴
  4. 1s²2s²2p⁶3s²3p⁶

Explanation

A neutral chlorine atom has 17 electrons, which fill orbitals in order of increasing energy: 1s, 2s, 2p, 3s, then 3p. Two electrons enter 3s and the remaining five enter 3p.

Question 4Atomic Structure and Properties

Which of the following atoms has the largest atomic radius?

  1. S
  2. Se
  3. Te Correct answer.
  4. O

Explanation

Atomic radius increases down a group because each successive element places valence electrons in a higher principal shell, farther from the nucleus. Tellurium lies below the other three in Group 16.

Question 5Atomic Structure and Properties

What is the maximum number of electrons that can occupy the entire n = 3 principal energy level?

  1. 32
  2. 6
  3. 18 Correct answer.
  4. 8

Explanation

The n = 3 level contains one 3s orbital, three 3p orbitals, and five 3d orbitals, for nine orbitals total. Because each orbital holds two electrons, the level holds at most 18 electrons.

Question 6Atomic Structure and Properties

Which color of visible light has the highest frequency?

  1. Red
  2. Yellow
  3. Green
  4. Blue Correct answer.

Explanation

Frequency and wavelength are inversely related through c = λν, so the shortest wavelength carries the highest frequency. Within the visible spectrum, blue-violet light has the shortest wavelength.

Question 7Atomic Structure and Properties

How many valence electrons does a neutral phosphorus atom (Z = 15) have?

  1. 3
  2. 5 Correct answer.
  3. 15
  4. 2

Explanation

The ground-state configuration of phosphorus is 1s²2s²2p⁶3s²3p³, so five electrons occupy the n = 3 shell. Phosphorus is in Group 15, consistent with five valence electrons.

Question 8Atomic Structure and Properties

Copper consists of 69.2% ⁶³Cu (62.93 amu) and 30.8% ⁶⁵Cu (64.93 amu). What is the average atomic mass of copper?

  1. 64.93 amu
  2. 63.93 amu
  3. 62.93 amu
  4. 63.55 amu Correct answer.

Explanation

The average atomic mass is the abundance-weighted mean of the isotope masses: (0.692)(62.93) + (0.308)(64.93). This equals about 43.55 + 20.00, or 63.55 amu.

Question 9Atomic Structure and Properties

A photoelectron spectrum of an element shows three peaks with relative areas 2 : 2 : 4, listed from lowest to highest binding energy. Which element produced the spectrum?

  1. Carbon (Z = 6)
  2. Neon (Z = 10)
  3. Nitrogen (Z = 7)
  4. Oxygen (Z = 8) Correct answer.

Explanation

Peak areas in a photoelectron spectrum are proportional to the number of electrons in each subshell. An area pattern of 2 : 2 : 4 corresponds to 1s²2s²2p⁴, a total of eight electrons, which identifies oxygen.

Question 10Atomic Structure and Properties

Moving left to right across Period 3, atomic radius decreases mainly because the number of occupied principal energy levels stays the same while which quantity changes?

  1. The number of occupied principal energy levels increases steadily.
  2. Effective nuclear charge increases while shielding stays nearly constant. Correct answer.
  3. The number of neutrons in the nucleus increases.
  4. Electron–electron repulsion grows faster than the nuclear charge does.

Explanation

Across a period, protons are added to the nucleus while new electrons enter the same principal shell, so the shielding from inner electrons changes very little. The resulting rise in effective nuclear charge pulls the valence shell closer to the nucleus.

Question 11Atomic Structure and Properties

Which statement correctly describes the ground-state arrangement of the three 2p electrons in a nitrogen atom?

  1. All three electrons are paired within a single 2p orbital.
  2. Each 2p orbital holds one electron, and all three spins are parallel. Correct answer.
  3. Two electrons are paired in one 2p orbital and the third occupies a second 2p orbital.
  4. Each 2p orbital holds one electron, with two spins parallel and one opposed.

Explanation

Hund's rule states that electrons occupy degenerate orbitals singly, with parallel spins, before any pairing occurs. Nitrogen's three 2p electrons therefore occupy the 2p orbital set one at a time, all with the same spin.

Question 12Atomic Structure and Properties

Which set of quantum numbers (n, ℓ, mℓ, ms) is allowed for an electron in an atom?

  1. (n = 3, ℓ = 1, mℓ = +2, ms = −½)
  2. (n = 1, ℓ = 0, mℓ = 0, ms = 0)
  3. (n = 2, ℓ = 2, mℓ = 0, ms = +½)
  4. (n = 3, ℓ = 2, mℓ = −1, ms = +½) Correct answer.

Explanation

The allowed values are ℓ = 0 through n − 1, mℓ = −ℓ through +ℓ, and ms = +½ or −½. Only (3, 2, −1, +½) satisfies all three restrictions.

Question 13Atomic Structure and Properties

An element's successive ionization energies are 578, 1817, 2745, and 11575 kJ/mol. In which group does the element most likely belong?

  1. Group 2
  2. Group 14
  3. Group 13 Correct answer.
  4. Group 1

Explanation

A very large jump between the third and fourth ionization energies means the fourth electron must be removed from a filled inner shell. The element therefore has three valence electrons, which places it in Group 13.

Question 14Atomic Structure and Properties

Which of the following ions has the smallest radius?

  1. O²⁻
  2. F⁻
  3. Na⁺
  4. Mg²⁺ Correct answer.

Explanation

All four ions are isoelectronic, each containing ten electrons. With an identical electron configuration, the ion with the largest nuclear charge attracts the electrons most strongly and is smallest, and magnesium has Z = 12.

Question 15Atomic Structure and Properties

The ground-state electron configuration of an element ends in 4p³. In which period and group is this element found?

  1. Period 3, Group 15
  2. Period 4, Group 13
  3. Period 3, Group 13
  4. Period 4, Group 15 Correct answer.

Explanation

The highest occupied shell is n = 4, so the element lies in Period 4. The configuration 4s²4p³ gives five valence electrons, which corresponds to Group 15.

Question 16Atomic Structure and Properties

What is the ground-state electron configuration of the Fe³⁺ ion (Z = 26)?

  1. [Ar]3d³4s²
  2. [Ar]3d⁶4s²
  3. [Ar]3d⁶
  4. [Ar]3d⁵ Correct answer.

Explanation

Neutral iron is [Ar]3d⁶4s², and the 4s electrons are removed first when a transition metal is ionized. Removing three electrons therefore leaves [Ar]3d⁵.

Question 17Atomic Structure and Properties

An excited hydrogen atom can undergo several different electronic transitions. Which transition emits a photon with the shortest wavelength?

  1. n = 5 → n = 3
  2. n = 4 → n = 2
  3. n = 2 → n = 1 Correct answer.
  4. n = 3 → n = 2

Explanation

The photon energy equals the energy difference between the two levels, and wavelength is inversely proportional to energy. The n = 2 → n = 1 gap is the largest of the listed transitions, so its photon has the greatest energy and the shortest wavelength.

Question 18Atomic Structure and Properties

The mass spectrum of chlorine gas shows peaks at m/z 70, 72, and 74. Which statement best explains these peaks?

  1. The peaks come from Cl₂ molecules that lost one, two, or three electrons.
  2. Three different elements are present, each contributing one peak.
  3. The peaks come from Cl⁺, Cl²⁺, and Cl³⁺ ions of a single isotope.
  4. Cl₂ molecules with different combinations of ³⁵Cl and ³⁷Cl are detected. Correct answer.

Explanation

Molecular chlorine produces ions containing two chlorine atoms, and each atom may be either ³⁵Cl or ³⁷Cl. The three possible combinations give molecular ions at m/z 70, 72, and 74 in a 9 : 6 : 1 ratio.

Question 19Atomic Structure and Properties

The minimum energy required to eject an electron from a metal surface is 3.0 eV. Light whose photons each carry 3.1 eV strikes the surface. What is the maximum kinetic energy of the ejected electrons?

  1. 0.10 eV Correct answer.
  2. 6.10 eV
  3. 3.10 eV
  4. 0.00 eV

Explanation

The energy of the photon is used first to overcome the binding energy, and any excess becomes kinetic energy of the electron. Here 3.1 eV − 3.0 eV = 0.10 eV.

Question 20Atomic Structure and Properties

The first ionization energy of Mg is 738 kJ/mol, while that of Al is only 578 kJ/mol even though Al has a larger nuclear charge. Which statement best explains this observation?

  1. Magnesium has a completely filled 3s subshell, so removing one electron gives it a noble-gas electron configuration.
  2. Aluminum's larger nuclear charge makes its outer electrons harder to remove than magnesium's.
  3. Aluminum's effective nuclear charge is smaller than magnesium's.
  4. Aluminum's 3p electron is higher in energy and better shielded than magnesium's 3s electron. Correct answer.

Explanation

Across Period 3 the 3p orbital lies higher in energy and penetrates the core less effectively than the 3s orbital. That reduced attraction outweighs aluminum's larger nuclear charge, so its first electron is easier to remove.

Question 21Atomic Structure and Properties

How many electrons in a ground-state arsenic atom (Z = 33) have the angular momentum quantum number ℓ = 1?

  1. 18
  2. 9
  3. 15 Correct answer.
  4. 12

Explanation

Electrons with ℓ = 1 are p electrons, so only the p subshells need to be counted. Arsenic's configuration [Ar]3d¹⁰4s²4p³ contains 2p⁶, 3p⁶, and 4p³, giving 6 + 6 + 3 = 15 p electrons.

Question 22Atomic Structure and Properties

What is the total energy, in kilojoules, of one mole of photons with a wavelength of 500 nm?

  1. 120 kJ
  2. 239 kJ Correct answer.
  3. 398 kJ
  4. 479 kJ

Explanation

Each photon carries E = hc/λ = (6.626 × 10⁻³⁴)(2.998 × 10⁸)/(5.00 × 10⁻⁷), which is about 3.97 × 10⁻¹⁹ J. Multiplying by Avogadro's number gives roughly 2.39 × 10⁵ J, or 239 kJ per mole.

Question 23Atomic Structure and Properties

Which of the following elements has the most exothermic electron affinity?

  1. F
  2. I
  3. Cl Correct answer.
  4. Br

Explanation

Electron affinity generally becomes more exothermic across a period as effective nuclear charge increases, which places chlorine above bromine and iodine. Fluorine is an exception: its very small 2p orbitals cause strong electron–electron repulsion when an extra electron is added, so its electron affinity is less exothermic than chlorine's.

Question 24Compound Structure and Properties

Which pair of elements is most likely to form an ionic compound when combined?

  1. H and H
  2. Na and Cl Correct answer.
  3. N and N
  4. C and O

Explanation

Ionic bonding is favored when a metal with low ionization energy transfers electrons to a nonmetal with high electron affinity, producing a large electronegativity difference. Na is a metal and Cl is a nonmetal, so the pair forms an ionic compound. The other pairs are nonmetal-nonmetal combinations that form covalent bonds.

Question 25Compound Structure and Properties

According to VSEPR theory, what is the molecular geometry of a molecule with the formula AX3 and no lone pairs on the central atom?

  1. trigonal planar Correct answer.
  2. tetrahedral
  3. T-shaped
  4. trigonal pyramidal

Explanation

VSEPR theory counts electron domains around the central atom. Three bonding domains and zero lone pairs adopt a trigonal planar arrangement to minimize repulsion.

Question 26Compound Structure and Properties

How many lone pairs are on the central oxygen atom in a correct Lewis structure of water, H2O?

  1. three
  2. two Correct answer.
  3. one
  4. zero

Explanation

In H2O, oxygen forms two O–H single bonds and completes its octet with two lone pairs. That gives four electron domains, two bonding and two nonbonding.

Question 27Compound Structure and Properties

In the carbonate ion, CO3^2−, all three C–O bonds are experimentally identical. Which concept explains this observation?

  1. resonance Correct answer.
  2. hybridization
  3. formal charge
  4. ionization energy

Explanation

The carbonate ion has three equivalent C–O bonds even though a single Lewis structure would show one double and two single bonds. Resonance describes the delocalization of pi electrons over the three oxygen atoms.

Question 28Compound Structure and Properties

Which property is characteristic of metals because their valence electrons are delocalized?

  1. Poor thermal conductivity
  2. Low melting point
  3. Electrical conductivity Correct answer.
  4. Brittleness

Explanation

Metallic bonding involves a lattice of cations surrounded by a sea of delocalized valence electrons. These mobile electrons can carry charge and heat through the solid.

Question 29Compound Structure and Properties

Which substance forms a network covalent solid with an extremely high melting point?

  1. Ice
  2. Diamond Correct answer.
  3. Sodium chloride
  4. Copper

Explanation

Diamond consists of carbon atoms covalently bonded in a three-dimensional network. Breaking the solid requires breaking many strong covalent bonds, so the melting point is extremely high.

Question 30Compound Structure and Properties

What is the hybridization of each carbon atom in ethyne, C2H2?

  1. sp3
  2. sp Correct answer.
  3. sp2
  4. sp3d

Explanation

Each carbon in ethyne is bonded to one hydrogen and triple-bonded to the other carbon. That gives two electron domains per carbon, corresponding to sp hybridization.

Question 31Compound Structure and Properties

In a Lewis structure of ozone, O3, the central oxygen is double bonded to one terminal oxygen, single bonded to the other, and has one lone pair. What is the formal charge on the central oxygen?

  1. +1 Correct answer.
  2. +2
  3. −1
  4. 0

Explanation

Formal charge equals valence electrons minus the sum of lone-pair electrons and half the bonding electrons. For the central oxygen, 6 − (2 + 3) = +1.

Question 32Compound Structure and Properties

What is the molecular geometry of the chlorate ion, ClO3−?

  1. T-shaped
  2. tetrahedral
  3. trigonal bipyramidal
  4. trigonal pyramidal Correct answer.

Explanation

The chlorate ion has three bonding domains and one lone pair on chlorine, giving four electron domains. The electron-domain geometry is tetrahedral, so the molecular geometry is trigonal pyramidal.

Question 33Compound Structure and Properties

Which molecule is polar?

  1. BF3
  2. CO2
  3. H2S Correct answer.
  4. CCl4

Explanation

A molecule is polar when polar bonds do not cancel in three dimensions. H2S is bent, so its two S–H bond dipoles add to give a net dipole moment.

Question 34Compound Structure and Properties

Which species has the shortest carbon–oxygen bond?

  1. CO Correct answer.
  2. H2CO
  3. CO2
  4. CO3^2−

Explanation

Bond length decreases as bond order increases for bonds between the same two elements. CO contains a C≡O triple bond, which is shorter than the double bonds in CO2 and H2CO and the resonance-averaged bonds in CO3^2−.

Question 35Compound Structure and Properties

Which ionic compound is expected to have the greatest lattice energy?

  1. CaO
  2. MgO Correct answer.
  3. NaCl
  4. KCl

Explanation

Lattice energy increases with the magnitudes of ionic charges and decreases as ionic radii increase. MgO has +2 and −2 ions that are small, giving stronger Coulombic attraction than the +1/−1 salts.

Question 36Compound Structure and Properties

What is the hybridization of the nitrogen atom in ammonia, NH3?

  1. sp2
  2. sp3d
  3. sp3 Correct answer.
  4. sp

Explanation

Nitrogen in ammonia forms three N–H bonds and keeps one lone pair. Four electron domains correspond to sp3 hybridization.

Question 37Compound Structure and Properties

How many equivalent resonance structures can be drawn for the nitrate ion, NO3−, while keeping each atom's octet satisfied?

  1. 3 Correct answer.
  2. 4
  3. 1
  4. 2

Explanation

Nitrate has three equivalent N–O bonds, and the octet rule is satisfied by placing the double bond on each oxygen in turn. That gives three resonance contributors.

Question 38Compound Structure and Properties

Which molecule has a net dipole moment of zero?

  1. CH4 Correct answer.
  2. H2O
  3. NH3
  4. HCl

Explanation

A molecule has zero net dipole when its polar bonds are arranged symmetrically so their dipoles cancel. CH4 is tetrahedral with four identical C–H bonds, so cancellation is complete.

Question 39Compound Structure and Properties

Why does MgO have a higher melting point than NaF?

  1. MgO has larger ions than NaF
  2. MgO has delocalized valence electrons
  3. MgO contains more highly charged ions Correct answer.
  4. NaF is held together by covalent bonds

Explanation

The melting point of an ionic solid rises with lattice energy, which depends on ionic charge and size. MgO contains Mg2+ and O2−, so the electrostatic attraction is much stronger than in NaF with Na+ and F−.

Question 40Compound Structure and Properties

Which bond is expected to be the shortest?

  1. C≡C Correct answer.
  2. C–N
  3. C–C
  4. C=C

Explanation

For bonds between similar atoms, higher bond order means a shorter bond. The C≡C triple bond has bond order 3 and is shorter than the C=C double bond, the C–C single bond, and the C–N single bond.

Question 41Compound Structure and Properties

Which molecule has a bond angle closest to 120°?

  1. NH3
  2. BF3 Correct answer.
  3. CH4
  4. H2O

Explanation

A trigonal planar electron geometry gives ideal bond angles of 120°. BF3 has three bonding pairs and no lone pair on boron, so it is trigonal planar.

Question 42Compound Structure and Properties

Which molecule is nonpolar even though it contains polar bonds?

  1. SF4
  2. XeF4 Correct answer.
  3. NH3
  4. H2O

Explanation

XeF4 has a square planar molecular geometry because xenon has four bonding pairs and two lone pairs. The four Xe–F bond dipoles cancel, so the molecule has no net dipole moment.

Question 43Compound Structure and Properties

In the dominant Lewis structure of the cyanide ion, CN−, what are the formal charges on carbon and nitrogen?

  1. C = −1, N = 0 Correct answer.
  2. C = +1, N = −2
  3. C = 0, N = −1
  4. C = −1, N = +1

Explanation

In the dominant C≡N− Lewis structure, carbon has one lone pair and three bonds, giving 4 − (2 + 3) = −1. Nitrogen has one lone pair and three bonds, giving 5 − (2 + 3) = 0.

Question 44Compound Structure and Properties

What is the average nitrogen–oxygen bond order in the nitrate ion, NO3−?

  1. 1.33 Correct answer.
  2. 2.00
  3. 1.50
  4. 1.00

Explanation

The nitrate ion has three equivalent resonance structures with one double and two single N–O bonds. Total bond order over three positions is 1 + 1 + 2 = 4, so the average is 4/3 ≈ 1.33.

Question 45Compound Structure and Properties

Which molecule has an sp2-hybridized central atom and is polar?

  1. CO2
  2. BF3
  3. CH4
  4. SO2 Correct answer.

Explanation

SO2 has a bent molecular geometry with a lone pair on sulfur, so its two S=O bond dipoles do not cancel. The sulfur atom is sp2 hybridized because it has three electron domains.

Question 46Compound Structure and Properties

Why does graphite conduct electricity while diamond does not?

  1. Diamond has an expanded octet around carbon
  2. Diamond contains mobile ions that carry charge
  3. Graphite has delocalized pi electrons Correct answer.
  4. Graphite's sigma bonds are weaker than diamond's

Explanation

In graphite, each carbon is sp2 hybridized and forms three sigma bonds in a layer, leaving one p electron delocalized over the sheet. These mobile electrons allow graphite to conduct electricity along the layers, while diamond's sp3 carbons have only localized sigma bonds.

Question 47Properties of Substances and Mixtures

Which intermolecular force operates between all molecules, regardless of whether they are polar or nonpolar?

  1. Dipole–dipole attractions
  2. London dispersion forces Correct answer.
  3. Ion–dipole attractions
  4. Hydrogen bonds

Explanation

London dispersion forces arise from instantaneous, temporary dipoles produced by fluctuating electron clouds, so they act between every atom and every molecule. The other forces listed require permanent dipoles, H atoms bonded to N/O/F, or ions, respectively.

Question 48Properties of Substances and Mixtures

Methanol (CH3OH) boils at 65 °C, whereas methanethiol (CH3SH) boils near 6 °C. Which intermolecular force is chiefly responsible for the higher boiling point of methanol?

  1. Ion–dipole attractions
  2. Covalent bonds linking one molecule to the next
  3. Hydrogen bonding Correct answer.
  4. London dispersion forces

Explanation

The O–H bond makes methanol molecules capable of hydrogen bonding to one another, an interaction much stronger than the dipole–dipole and dispersion forces available to CH3SH. Sulfur is far less electronegative than oxygen, so the S–H bond supports at most weak hydrogen bonding.

Question 49Properties of Substances and Mixtures

A fixed sample of gas at constant temperature is compressed to one-third of its original volume. The pressure of the gas becomes

  1. three times its original value Correct answer.
  2. nine times its original value
  3. the same as its original value
  4. one-third of its original value

Explanation

Boyle's law states that pressure and volume are inversely proportional for a fixed amount of gas at constant temperature, so PV is constant. Dividing the volume by three therefore multiplies the pressure by three.

Question 50Properties of Substances and Mixtures

A solution is made by dissolving 0.25 mol of KNO3 in enough water to give 500. mL of solution. What is the molarity of the solution?

  1. 0.25 M
  2. 0.50 M Correct answer.
  3. 0.125 M
  4. 1.0 M

Explanation

Molarity equals moles of solute divided by liters of solution. Converting 500. mL to 0.500 L gives 0.25 mol ÷ 0.500 L = 0.50 M.