Atoms, Electrons & Periodicity — Chapter quiz · Physical Science: Chemistry, Grade 11

Connect atomic evidence, isotopes, electron arrangements, spectra, and periodic patterns.

Problem 1

A potassium atom is written with mass number 39 and atomic number 19. How many neutrons does it have?
Answer and explanation

20

Neutrons = mass number − atomic number = 39 − 19 = 20.

Problem 2

Which experiment provided the first evidence for a small, dense, positively charged nucleus?
  1. Dalton's ratios of combining masses
  2. Thomson's cathode-ray tube
  3. Rutherford's gold-foil experiment
  4. Bohr's study of hydrogen light
Answer and explanation

Rutherford's gold-foil experiment

The sharp backward deflection of a few alpha particles in Rutherford's gold-foil experiment revealed the nucleus.

Problem 3

Why is the atomic mass on the periodic table almost never a whole number?
  1. Neutrons weigh slightly less than 1 amu
  2. Instruments cannot measure whole numbers
  3. Electrons add fractional mass
  4. It is a weighted average of the element's isotopes
Answer and explanation

It is a weighted average of the element's isotopes

The listed mass is the abundance-weighted average of all the element's naturally occurring isotopes.

Problem 4

An element has two isotopes: mass 69.0 amu at 60.0% abundance and mass 71.0 amu at 40.0% abundance. Find its average atomic mass in amu.
Answer and explanation

69.8 amu

69.0 × 0.600 + 71.0 × 0.400 = 41.4 + 28.4 = 69.8 amu.

Problem 5

Which is the correct ground-state electron configuration for oxygen (Z = 8)?
  1. 1 s² 2 s² 2p⁴
  2. 1 s² 2 s² 2p⁶
  3. 1 s² 2 s² 2p²
  4. 1 s² 2p⁶
Answer and explanation

1 s² 2 s² 2p⁴

Filling in order until 8 electrons are placed gives 1 s² 2 s² 2p⁴ (2 + 2 + 4 = 8).

Problem 6

Using the rule 2n², what is the maximum number of electrons the n = 2 energy level can hold?
Answer and explanation

8

2n² = 2 × 2² = 2 × 4 = 8 electrons.

Problem 7

According to the Aufbau order, which sublevel fills first?
  1. 3d before 4 s
  2. 4 s before 3d
  3. 4p before 4 s
  4. 3d before 3p
Answer and explanation

4 s before 3d

The 4 s sublevel is slightly lower in energy than 3d, so 4 s fills first.

Problem 8

A vertical column of the periodic table, whose elements share the same number of valence electrons, is called a…
  1. shell
  2. period
  3. group
  4. block
Answer and explanation

group

Vertical columns are groups (families); their shared valence-electron count drives similar chemistry.

Problem 9

A main-group element is in Group 15. How many valence electrons does it have?
Answer and explanation

5

For main-group elements, valence electrons = group number − 10 = 15 − 10 = 5.

Problem 10

Using electron configurations, explain why all the elements in a group have similar chemical properties.
Answer and explanation

Elements in the same group have the same number of valence electrons in their outermost energy level. Because valence electrons determine how an atom bonds and reacts, group-mates form similar bonds and behave alike chemically.

Problem 11

Moving left to right across a period, first ionization energy generally…
  1. decreases
  2. stays constant
  3. drops to zero
  4. increases
Answer and explanation

increases

Rising effective nuclear charge across a period holds the outer electron more tightly, so ionization energy increases.

Problem 12

Which has the larger atomic radius, sodium (Na) or potassium (K)?
  1. Potassium
  2. They are equal
  3. It cannot be determined
  4. Sodium
Answer and explanation

Potassium

Potassium sits below sodium in Group 1; atomic radius increases down a group because a new energy level is added, so potassium is larger.

Problem 13

Explain the following claim and supply a valid example or counterexample: A shell diagram is an organizing model, not a literal path for an electron.
Answer and explanation

Charge=17−18=−1; atomic number 17 identifies chlorine, so Cl⁻. A shell diagram is an organizing model, not a literal path for an electron.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 14

Independent chapter check: A species has 17 protons, 18 neutrons and 18 electrons. Give Z, mass number, element and charge. Explain which change would make an isotope rather than an ion.
Answer and explanation

Z=17, A=35, chlorine with charge −1: ³⁵Cl⁻. Changing neutron number while keeping 17 protons makes another chlorine isotope. Changing only electron number changes its ionization state.

Z=17, A=35, chlorine with charge −1: ³⁵Cl⁻. Changing neutron number while keeping 17 protons makes another chlorine isotope. Changing only electron number changes its ionization state.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 15

Explain the following claim and supply a valid example or counterexample: Mass number counts nucleons; average atomic mass is a weighted value, not a nucleon count.
Answer and explanation

10(.20)+11(.80)=10.8u. The result lies between the two masses and closer to the more abundant one. Mass number counts nucleons; average atomic mass is a weighted value, not a nucleon count.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 16

Independent chapter check: Two modeled isotopes have masses 24.0 and 26.0 u. A sample mean is 24.6 u. Find the fraction of each and check the answer.
Answer and explanation

24.6=24.0x+26.0(1−x)=26.0−2.0x, so x=0.70. Fractions are 70% lighter and 30% heavier; 0.70(24.0)+0.30(26.0)=24.6 u.

24.6=24.0x+26.0(1−x)=26.0−2.0x, so x=0.70. Fractions are 70% lighter and 30% heavier; 0.70(24.0)+0.30(26.0)=24.6 u.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 17

Explain the following claim and supply a valid example or counterexample: The 2n² shell capacity is not the actual filling order of sublevels.
Answer and explanation

1 s²2 s²2p⁶3s¹; eleven electrons total and one outer-shell electron. The 2n² shell capacity is not the actual filling order of sublevels.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 18

Independent chapter check: A neutral atom has Z=16. Write its ground-state configuration and electron total for its common 2− ion. Explain why gaining electrons does not change the element.
Answer and explanation

Neutral sulfur: 1s²2s²2p⁶3s²3p⁴, totaling 16 electrons. S²⁻ has 18 electrons, with 3p⁶. Its 16 protons retain sulfur identity.

Neutral sulfur: 1s²2s²2p⁶3s²3p⁴, totaling 16 electrons. S²⁻ has 18 electrons, with 3p⁶. Its 16 protons retain sulfur identity.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 19

Explain the following claim and supply a valid example or counterexample: The simple group-number valence rule applies to main-group patterns, not all transition elements.
Answer and explanation

Outer n=3 gives period 3; five valence electrons give main-group 15; this is phosphorus. The simple group-number valence rule applies to main-group patterns, not all transition elements.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 20

Independent chapter check: Element A has outer configuration 3s²3p⁵ and B has 4s²4p⁵. Identify their main-group relationship, explain one likely similarity and one reason properties need not match.
Answer and explanation

Both fit Group 17. Similar valence patterns support related bonding tendencies, while B has an additional occupied shell and different nuclear charge/shielding. Exact radius, energy and reaction behavior need the specified species and conditions.

Both fit Group 17. Similar valence patterns support related bonding tendencies, while B has an additional occupied shell and different nuclear charge/shielding. Exact radius, energy and reaction behavior need the specified species and conditions.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 21

Explain the following claim and supply a valid example or counterexample: General trends have exceptions; do not use a trend arrow as an exact numerical table.
Answer and explanation

K has an occupied outer shell farther from the nucleus with more inner shielding, outweighing its greater proton count in this comparison. General trends have exceptions; do not use a trend arrow as an exact numerical table.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 22

Independent chapter check: Rank Ne, F⁻ and Mg²⁺ by the usual radius trend for an isoelectronic series. Give electron/proton counts and the model limitation.
Answer and explanation

All have ten electrons; proton counts are F=9, Ne=10, Mg=12. The usual order is F⁻ > Ne > Mg²⁺ under a compatible radius comparison. Defined atomic/ionic radii and environments must be matched before treating reference numbers as directly interchangeable.

All have ten electrons; proton counts are F=9, Ne=10, Mg=12. The usual order is F⁻ > Ne > Mg²⁺ under a compatible radius comparison. Defined atomic/ionic radii and environments must be matched before treating reference numbers as directly interchangeable.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 23

Levels at 9 and 4 units give a downward transition. Find released energy.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

5 energy units

9−4 = 5.

Problem 24

Does a line spectrum prove classical circular electron orbits?
Answer and explanation

No. It supports discrete energy states; modern orbitals are probability distributions.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 25

Independent chapter check: A selected atom has levels at 0, 1.5 and 4.5 eV. Find the energy and approximate vacuum wavelength for a 4.5-to-1.5 eV emission. Could a further drop emit another photon?
Answer and explanation

The first photon has 3.0 eV, or about 4.81×10⁻¹⁹ J, giving λ≈4.14×10⁻⁷ m =414 nm. A later allowed 1.5-to-0 transition could emit a 1.5 eV photon; the selected model does not supply transition probabilities.

The first photon has 3.0 eV, or about 4.81×10⁻¹⁹ J, giving λ≈4.14×10⁻⁷ m =414 nm. A later allowed 1.5-to-0 transition could emit a 1.5 eV photon; the selected model does not supply transition probabilities.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Browse all courses