Unit 1 · 7–9% of the exam Beta

Unit 1 practice test: Atomic 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: Moles and Molar Mass; Mass Spectra of Elements; Elemental Composition of Pure Substances; Composition of Mixtures; Atomic Structure and Electron Configuration; Photoelectron Spectroscopy; Periodic Trends; Valence Electrons and Ionic Compounds.

Particle view

Three boxes of gas particles

Box ABox BBox C

Key: small white circle, hydrogen atom; orange circle, oxygen atom; dark gray circle, carbon atom; light blue circle, neon atom. Molar masses: H₂O 18.02 g/mol, Ne 20.18 g/mol, CO₂ 44.01 g/mol.

1. Which box holds the greatest mass of gas?

  1. Box A, because its particles contain the most atoms
  2. Box B, because it holds as many particles as Box A and each is heavier
  3. The three boxes have equal masses
  4. Box C, with 3 carbon dioxide molecules
Show the answer

Total mass = number of particles × mass of each. A: 6 × 18.02 = 108.1 amu; B: 6 × 20.18 = 121.1 amu; C: 3 × 44.01 = 132.0 amu.

  • Box A, because its particles contain the most atoms: Box A has the most atoms (18), but they are light hydrogen and oxygen atoms: 108.1 amu in all.
  • Box B, because it holds as many particles as Box A and each is heavier: Box B outweighs Box A, but Box C is heavier still.
  • The three boxes have equal masses: The boxes hold different numbers of particles with different masses, so the totals differ.
  • Correct: Box C, with 3 carbon dioxide molecules: Right: 3 × 44.01 = 132.0 amu, more than 6 × 20.18 = 121.1 amu or 6 × 18.02 = 108.1 amu.

2. Boxes A and B hold the same number of particles. How do their masses compare, and why?

  1. Box B is heavier, because a neon atom has more mass than a water molecule
  2. They have equal masses, because they hold equal numbers of particles
  3. Box A is heavier, because each water molecule has three atoms
  4. Box A is heavier, because water is a compound and neon is an element
Show the answer

Same number of particles, so the box whose particles are heavier is heavier: 20.18 amu (Ne) > 18.02 amu (H₂O).

  • Correct: Box B is heavier, because a neon atom has more mass than a water molecule: Right: 20.18 amu per neon atom against 18.02 amu per water molecule.
  • They have equal masses, because they hold equal numbers of particles: Equal numbers of particles have equal masses only if the particles have the same mass.
  • Box A is heavier, because each water molecule has three atoms: Counting atoms is not enough: one neon atom outweighs one O atom plus two light H atoms.
  • Box A is heavier, because water is a compound and neon is an element: Being a compound says nothing about mass; the mass of each particle decides.

3. Which box contains the greatest number of atoms?

  1. Box B
  2. Box C
  3. Box A
  4. Boxes A and B, which tie
Show the answer

Count atoms, not particles: A has 18, B has 6, C has 9.

  • Box B: Box B has 6 particles, but each is a single atom: 6 atoms.
  • Box C: 3 molecules × 3 atoms = 9 atoms.
  • Correct: Box A: Right: 6 molecules × 3 atoms = 18 atoms.
  • Boxes A and B, which tie: They tie on particles (6 each), not on atoms (18 against 6).

Data table

Burning magnesium in three labs

Three lab groups heat different masses of pure magnesium in air until it has all turned into a white solid, magnesium oxide, and weigh the product.

Mass of magnesium used and of magnesium oxide made
GroupMass of Mg (g)Mass of magnesium oxide (g)Mass of O gained (g)
11.2162.0160.800
23.0405.0402.000
30.72931.20930.4800

4. Which conclusion do the three groups' results support?

  1. Magnesium oxide is about 60.3% magnesium by mass in each sample
  2. Larger samples of magnesium oxide contain a larger percent of magnesium
  3. Magnesium and oxygen combine in a 1 : 1 ratio by mass
  4. The groups made different compounds, since their product masses differ
Show the answer

All three products are 60.3% Mg by mass. A compound has the same composition by mass whatever the sample size.

  • Correct: Magnesium oxide is about 60.3% magnesium by mass in each sample: Right: 60.32%, 60.32% and 60.31%: a fixed composition, the law of definite proportions.
  • Larger samples of magnesium oxide contain a larger percent of magnesium: The masses differ, but the percent is the same in every group.
  • Magnesium and oxygen combine in a 1 : 1 ratio by mass: Group 1 used 1.216 g Mg with 0.800 g O, a mass ratio of about 1.5 : 1, not 1 : 1.
  • The groups made different compounds, since their product masses differ: Different amounts of one compound have different masses but the same composition.

5. Using Group 2's data, what is the empirical formula of magnesium oxide?

  1. Mg₃O₂
  2. MgO₂
  3. Mg₂O
  4. MgO
Show the answer

Formulas count atoms, so compare moles: 0.1251 mol Mg : 0.1250 mol O = 1 : 1, so MgO.

  • Mg₃O₂: This uses the mass ratio, about 3 : 2, as if it were a ratio of atoms. Convert masses to moles first.
  • MgO₂: This would need twice as many moles of O as Mg; the moles are equal.
  • Mg₂O: This would need twice as many moles of Mg as O; the moles are equal.
  • Correct: MgO: Right: 3.040/24.31 = 0.1251 mol Mg and 2.000/16.00 = 0.1250 mol O, a 1 : 1 ratio.

6. A fourth group's product had a magnesium mass percent of 66%. Which error best explains this result?

  1. The group used more magnesium than the other groups
  2. Some magnesium did not react, so the product was too light
  3. The product absorbed water from the air after heating
  4. The balance read each mass 0.05 g too high
Show the answer

Mass percent Mg = mass Mg ÷ mass product. If the product is too light because not all Mg reacted with oxygen, the percent rises above the true 60.3%.

  • The group used more magnesium than the other groups: Sample size does not change the percent of a pure compound.
  • Correct: Some magnesium did not react, so the product was too light: Right: too little oxygen gained makes the product too light, and the Mg share comes out too high.
  • The product absorbed water from the air after heating: Extra water would add mass and lower the Mg percent, not raise it.
  • The balance read each mass 0.05 g too high: A balance that reads every mass 0.05 g high moves the percent by only about 1 to 2 percentage points for these samples, not 6.

7. An atom of chlorine has 17 protons and 20 neutrons. What is its mass number?

  1. 37
  2. 17
  3. 20
  4. 3
Show the answer

Mass number = protons + neutrons = 37. This atom is chlorine-37, written ³⁷Cl.

  • Correct: 37: Right: mass number = protons + neutrons = 17 + 20.
  • 17: That is the atomic number, the protons alone.
  • 20: That is the number of neutrons alone.
  • 3: Subtracting gives no standard quantity; the mass number adds the two.

8. Two samples of a gray powder are both iron and sulfur. Sample 1 is 63.5% Fe by mass every time it is tested. Sample 2 is 30% Fe in one scoop and 55% Fe in another. Which conclusion is best supported?

  1. Both are mixtures, because both contain two elements
  2. Sample 1 is likely a compound; Sample 2, an uneven mixture
  3. Both are compounds, because both contain iron and sulfur
  4. Sample 2 is likely a compound because its iron content is lower
Show the answer

A compound has a fixed composition (FeS is 63.5% Fe). A mixture's composition can vary, and if it varies within one sample, the mixture is heterogeneous.

  • Both are mixtures, because both contain two elements: Compounds contain two or more elements too; fixed vs varying composition is what tells them apart.
  • Correct: Sample 1 is likely a compound; Sample 2, an uneven mixture: Right: fixed composition points to a compound; composition that varies between scoops points to an uneven mixture.
  • Both are compounds, because both contain iron and sulfur: A compound has one fixed composition; Sample 2 varies from scoop to scoop.
  • Sample 2 is likely a compound because its iron content is lower: The size of the percent does not decide it; whether it stays fixed does.

9. Which is the electron configuration of iron (26 electrons) in noble gas notation?

  1. [Ar] 3d⁸
  2. [Ar] 4s² 4d⁶
  3. [Kr] 4s² 3d⁶
  4. [Ar] 4s² 3d⁶
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Argon (18) + 4s² + 3d⁶ = 26. The 3d subshell fills after 4s in the neutral atom.

  • [Ar] 3d⁸: The 4s subshell is lower in energy than 3d in a neutral atom, so it fills first.
  • [Ar] 4s² 4d⁶: After 4s comes 3d, not 4d. The d subshell is one shell behind.
  • [Kr] 4s² 3d⁶: Krypton has 36 electrons, more than iron has in total.
  • Correct: [Ar] 4s² 3d⁶: Right: 18 electrons in the argon core, then 4s fills before 3d: 2 + 6 = 8 more.

10. In a photoelectron spectrometer, how is the binding energy of an electron found?

  1. By weighing the atom before and after the electron leaves
  2. By counting how many electrons leave each second
  3. From the size of the atom measured under a microscope
  4. Light energy minus the ejected electron's energy of motion
Show the answer

The light supplies a known energy. Part of it frees the electron (the binding energy); the rest becomes the electron's motion, which is measured. Subtracting gives the binding energy.

  • By weighing the atom before and after the electron leaves: An electron is too light for its loss to be weighed this way, and mass is not energy here.
  • By counting how many electrons leave each second: The count gives peak height, not binding energy.
  • From the size of the atom measured under a microscope: Atoms are far too small for that, and size does not give the energy directly.
  • Correct: Light energy minus the ejected electron's energy of motion: Right: whatever energy is not used to free the electron stays with it as motion.

11. Why is the first ionization energy of potassium lower than that of sodium?

  1. K's valence electron is farther out and more shielded
  2. K has more protons, which push the electron away
  3. K has more mass, so its electron moves faster
  4. K is a metal and Na is not
Show the answer

Down a group, each element adds a shell. The valence electron is farther from the nucleus and shielded by more core electrons, so it is held less tightly.

  • Correct: K's valence electron is farther out and more shielded: Right: distance and shielding increase down a group, outweighing the extra protons.
  • K has more protons, which push the electron away: Protons attract electrons; they never push them away.
  • K has more mass, so its electron moves faster: Mass is not the cause in Coulomb's law.
  • K is a metal and Na is not: Both are metals in group 1.

12. Sodium and potassium both react with water in similar ways. Which explanation is best?

  1. Both have one loosely held s valence electron
  2. They have similar masses
  3. They have the same number of protons
  4. They have the same number of shells
Show the answer

Elements in a group share the same valence configuration (here ns¹), so they gain or lose electrons in the same way and react alike.

  • Correct: Both have one loosely held s valence electron: Right: same group, same number of valence electrons, so similar chemistry.
  • They have similar masses: Potassium is much heavier than sodium; mass is not the reason.
  • They have the same number of protons: Na has 11 and K has 19.
  • They have the same number of shells: K has four occupied shells and Na three.

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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.