Fundamental particles, mass number and isotopesAQA A-Level Chemistry: Revision notes
Section 1
How ideas about atomic structure developed
Scientific models change when new evidence appears. Dalton pictured atoms as tiny solid spheres. Thomson's discovery of the electron led to the plum pudding model, a sphere of positive charge with electrons embedded in it.
In the alpha particle scattering experiment, positively charged alpha particles were fired at thin gold foil. Most passed straight through, a few were deflected through large angles and a very few bounced back. This showed that an atom is mostly empty space with a small, dense, positively charged nucleus, which led to the nuclear model. The nucleus was later found to contain uncharged neutrons (Chadwick), and Bohr proposed that electrons occupy shells around the nucleus.
Link every observation to its conclusion: most pass straight through → mostly empty space; a few deflected → positive nucleus; a few rebound → dense, massive nucleus.
Section 2
Protons, neutrons and electrons
An atom has a nucleus containing protons and neutrons, surrounded by electrons in shells. The nucleus is tiny but contains almost all of the mass.
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | 1/1840 |
An atom is electrically neutral because the number of protons equals the number of electrons.
The mass of an electron is not 'zero'. It is 1/1840 of a proton, which is negligible but should be quoted as 1/1840 (about 0.0005).
Section 3
Mass number and atomic number
The atomic (proton) number, Z, is the number of protons in the nucleus and defines the element. The mass number, A, is the total number of protons and neutrons.
In the symbol for an atom, the mass number is written top left and the atomic number bottom left, e.g. ²³₁₁Na.
- Number of neutrons = A − Z
- In a neutral atom, number of electrons = Z
- In an ion, number of electrons = Z − charge (a 2+ ion has lost two electrons; a 2− ion has gained two)
The number of protons never changes when an ion forms. Only the number of electrons changes.
Section 4
Worked example: particles in an ion
How many protons, neutrons and electrons are in ²⁷Al³⁺ (Z = 13)?
- Protons = Z = 13
- Neutrons = 27 − 13 = 14
- Electrons = 13 − 3 = 10 (the 3+ charge means three electrons have been lost)
For ³⁵Cl⁻ (Z = 17): protons 17, neutrons 18, electrons 17 + 1 = 18.
Section 5
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, so they have the same atomic number but different mass numbers. For example, ³⁵Cl and ³⁷Cl both have 17 protons, but 18 and 20 neutrons respectively.
Isotopes have the same chemical properties because chemical reactions depend on the electron arrangement, and isotopes have the same number of electrons. Neutrons have no charge, so they do not change how the atom reacts.
Isotopes have different physical properties that depend on mass, such as density and rate of diffusion, because the extra neutrons give a different mass.
Isotopes differ in neutrons, not electrons or protons. Do not define them as 'atoms with different mass numbers' without saying they are of the same element.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Fundamental particles, mass number and isotopes
- In 1909 Geiger and Marsden, working with Rutherford, fired a beam of positively charged alpha particles at a very thin sheet of gold foil and recorded where each particle landed using a detector screen. At the time, the accepted 'plum pudding' model pictured an atom as a sphere of positive charge with electrons embedded throughout it.Explain how the results of this experiment led scientists to conclude that the nucleus is positively charged and that it contains most of the mass of the atom.2 marks
- A forensic laboratory identifies an iron compound in a paint sample. The iron is present as the ion ⁵⁶Fe³⁺, and the analyst's data sheet gives the atomic number of iron as 26.A second sample contains the ion ⁵⁴Fe²⁺. State the number of protons, the number of neutrons and the number of electrons in one ⁵⁴Fe²⁺ ion.2 marks
- Magnesium has three naturally occurring isotopes, ²⁴Mg, ²⁵Mg and ²⁶Mg. A chemist is comparing atoms of these isotopes. The atomic number of magnesium is 12.Explain what is meant by the term isotopes, and state the number of protons, neutrons and electrons in an atom of ²⁵Mg.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).