Properties of Subatomic Particles
This topic covers the properties of the three fundamental particles within an atom: protons, neutrons, and electrons. Understanding their relative masses, charges, and locations is essential for 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.