Atomic Models & Subatomic Particles — Practice · Grade 11 Science

Chapter: Atoms, Electrons & Periodicity

What this practice covers

By the end of Atomic Models & Subatomic Particles you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: atom, proton, neutron, electron, nucleus, atomic number (Z), mass number (A), ion, atomic mass unit (amu).

Problem 1

Rutherford's gold-foil experiment, in which a few alpha particles bounced sharply backward, was the direct evidence for which idea?
  1. Electrons exist as probability clouds
  2. Atoms are indivisible solid spheres
  3. An atom's mass and positive charge are concentrated in a tiny nucleus
  4. Electrons travel in fixed orbits
Answer and explanation

An atom's mass and positive charge are concentrated in a tiny nucleus

Most particles passed through (mostly empty space), but the rare sharp deflections showed a small, dense, positively charged nucleus.

Problem 2

Which particle has a mass roughly 1/1836 that of a proton?
  1. nucleus
  2. alpha particle
  3. neutron
  4. electron
Answer and explanation

electron

Electrons are almost weightless compared with protons and neutrons, so they contribute almost nothing to atomic mass.

Problem 3

An atom of selenium has atomic number 34 and mass number 80. How many neutrons does it have?
Answer and explanation

46

Neutrons = A − Z = 80 − 34 = 46.

Problem 4

A sulfide ion, S with mass number 32 and a 2− charge, has atomic number 16. How many electrons does it have?
Answer and explanation

18

A 2− ion has gained 2 electrons: electrons = protons + 2 = 16 + 2 = 18.

Problem 5

Thomson's plum-pudding model and Rutherford's nuclear model both had electrons and positive charge. What key difference did Rutherford's experiment force, and why?
Answer and explanation

Thomson spread the positive charge thinly throughout the whole atom, but Rutherford's sharp alpha-particle deflections showed the positive charge and nearly all the mass are packed into a tiny, dense central nucleus instead of being spread out.

Problem 6

A neutral atom of phosphorus is written with mass number 31 and atomic number 15. How many protons does it have?
Answer and explanation

15

The atomic number is the proton count, so protons = 15.

Problem 7

An ion has 12 protons and 10 electrons. Find its charge and element. Show the reasoning, not just the conclusion.
Hints

Electrons affect ion charge; protons identify the element.

Answer and explanation

Magnesium, charge 2+; positive charges exceed negative by two.

Problem 8

Why are simple circular electron paths an incomplete atomic model?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Quantum models describe orbital probability distributions rather than definite classical trajectories; simple drawings omit that distinction.

Problem 9

An orbital describes a quantum state rather than a tiny trackAn orbital describes a quantum state rather than a tiny trackOrbital: probability descriptionGroundExcitedAllowed energy changeNeither picture specifies an electron’s classical orbit.
Original qualitative cloud and energy-level schematic. Cloud colors are not electron material, and level separation is in arbitrary energy units.
Interpret the supplied visual. What information is missing if a drawing shows only a shaded orbital cloud?
Answer and explanation

It does not specify a particular electron position or trajectory. Its shading must be interpreted as a probability representation under the stated model.

An orbital describes an electron state with a spatial probability distribution. A cloud or boundary drawing does not show a solid shell, exact electron location or a classical path around the nucleus. Allowed energy levels account for discrete transitions; different orbitals can have different shapes and energies. Bohr-style shells remain useful bookkeeping for simple comparisons, but a multi-electron atom requires a quantum model and electron interactions rather than planets moving on fixed circular roads.

It does not specify a particular electron position or trajectory. Its shading must be interpreted as a probability representation under the stated model.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 10

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.

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