Properties of Subatomic Particles
Protons, neutrons, and electrons are the three fundamental particles within an atom, and their relative masses, charges, and locations underpin all further chemistry topics.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The atom consists of a central nucleus surrounded by electrons.
- The nucleus contains protons and neutrons, and therefore contains nearly all the atom's mass.
- Protons have a relative charge of +1 and a relative mass of 1.
- Neutrons have a relative charge of 0 (they are neutral) and a relative mass of 1.
- Electrons have a relative charge of -1 and a negligible relative mass (approximately 1/2000).
- An atom is electrically neutral because it has an equal number of protons and electrons.
Definitions
- Proton
- A subatomic particle with a relative mass of 1 and a relative charge of +1, found in the nucleus.
- Neutron
- A subatomic particle with a relative mass of 1 and a charge of 0, found in the nucleus.
- Electron
- A subatomic particle with a negligible relative mass and a relative charge of -1, found orbiting the nucleus.
- Nucleus
- The very dense, positively charged central core of an atom, containing protons and neutrons.
Worked example
A particle contains 12 neutrons, 11 protons and 10 electrons. Which of the following statements is correct? A) The particle is an atom with a mass number of 21. B) The particle is an ion with a charge of +1. C) The particle has most of its mass outside the nucleus. D) Electrons contribute more to the particle's mass than protons.
- 1
Analyse the particle's composition:
11 protons (+1 charge each), 12 neutrons (0 charge each), 10 electrons (-1 charge each).
- 2
Calculate the overall charge:
(11 × +1) + (10 × -1) = +11 - 10 = +1.
Since the charge is not zero, it is an ion.
- 3
Calculate the mass number:
number of protons + number of neutrons = 11 + 12 = 23.
- 4
Evaluate the options:
A is incorrect because the mass number is 23, not 21.
B is correct because the particle has an unequal number of protons and electrons, resulting in a net charge of +1.
C is incorrect because protons and neutrons are in the nucleus and have a relative mass of 1 each, while electrons outside the nucleus have a negligible mass.
D is incorrect because protons (mass ~1) are far more massive than electrons (mass ~1/2000).
Answer: B) The particle is an ion with a charge of +1.
Common mistakes
- ×Mistaking the electron's mass as zero. It is extremely small ('negligible') compared to protons and neutrons, but not zero.
- ×Confusing the properties of protons and electrons, especially their charges.
- ×Forgetting that the nucleus contains almost all the mass, despite being tiny compared to the whole atom.
No-calculator tips
- ✓To find the charge of any atom or ion, just find the difference between the number of protons and the number of electrons. No complex calculation is needed.
- ✓When comparing masses, you can effectively ignore the contribution of electrons. The mass is determined by the sum of protons and neutrons.
- ✓Remember the mnemonic 'PEN' for Protons, Electrons, Neutrons. For a neutral atom, P = E. The charge is determined by the difference between P and E.