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AS · Practice questions · Particles in the atom

Count the particles.

Six original Cambridge-style questions on the subatomic particles, the nuclide symbol, ions, and the behaviour of charged beams in an electric field.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Two numbers give everything.

01
Recall
[3 marks]

Complete the table for the three ions.

Ionprotonsneutronselectrons
3919K+
7935Br−
5626Fe2+
  • K+: 19 protons, 39 − 19 = 20 neutrons, 19 − 1 = 18 electrons. ✓
  • Br−: 35 protons, 79 − 35 = 44 neutrons, 35 + 1 = 36 electrons. ✓
  • Fe2+: 26 protons, 56 − 26 = 30 neutrons, 26 − 2 = 24 electrons. ✓
02
Explain
[3 marks]

Describe the distribution of mass and charge within an atom.

  • The nucleus contains the protons and neutrons, so nearly all the mass is concentrated in the nucleus. ✓
  • The nucleus carries all the positive charge (from the protons); neutrons are uncharged. ✓
  • The electrons, with negligible mass, occupy the space around the nucleus and carry all the negative charge. ✓
03
Explain
[4 marks]

Separate beams of protons, neutrons and electrons are passed between two oppositely charged plates. Describe and explain the path of each beam.

  • Protons are deflected towards the negatively charged plate because they are positively charged. ✓
  • Electrons are deflected towards the positively charged plate because they are negatively charged. ✓
  • The electron beam is deflected much more than the proton beam: the size of the charge is the same but the electron's mass is about 1/1836 of the proton's, so its charge-to-mass ratio is far larger. ✓
  • Neutrons are not deflected: they have no charge, so the field exerts no force on them. ✓
04
Analysis
[3 marks]

An alpha particle is a helium nucleus, 42He2+. State the numbers of protons, neutrons and electrons it contains, and compare its deflection in an electric field with that of a beam of protons moving at the same speed.

  • 2 protons, 4 − 2 = 2 neutrons, 2 − 2 = 0 electrons. ✓
  • It is deflected in the same direction as the protons (towards the negative plate) because it is positive. ✓
  • It is deflected less: its charge-to-mass ratio is 2/4 = 0.5, half that of a proton (1/1), so the same field bends it about half as much. ✓
05
Analysis
[3 marks]

An atom X has 17 protons and 18 neutrons. Identify X, write its nuclide symbol, and give the formula and electron count of the ion it forms most readily.

  • 17 protons means Z = 17, which is chlorine. ✓
  • A = 17 + 18 = 35, so the symbol is 3517Cl. ✓
  • Chlorine gains one electron to form Cl−, which has 17 + 1 = 18 electrons. ✓
06
Explain
[2 marks]

The nucleon number of a chlorine atom is always a whole number, yet the relative atomic mass of chlorine in the Periodic Table is 35.5. Explain the difference.

  • Nucleon number counts the protons and neutrons in one particular atom, so it must be a whole number (35 or 37 for chlorine). ✓
  • Relative atomic mass is a weighted average over the natural mixture of isotopes (about 75% 35Cl and 25% 37Cl), so it need not be a whole number. ✓

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