Constituents of the atom and isotopesAQA A-Level Physics: Revision notes
Section 1
The simple model of the atom
An atom has a tiny, dense nucleus containing protons and neutrons (together called nucleons), surrounded by electrons.
- Proton: charge +1.60×10⁻¹⁹ C (relative +1), mass 1.67×10⁻²⁷ kg (relative 1)
- Neutron: charge 0 (relative 0), mass 1.67×10⁻²⁷ kg (relative 1)
- Electron: charge −1.60×10⁻¹⁹ C (relative −1), mass 9.11×10⁻³¹ kg (relative 1/1840, about 0.0005)
The proton and neutron have almost the same mass, and the electron's mass is about 1/1840 of a proton's. The atomic mass unit is covered in the nuclear physics topic.
Section 2
Nuclide notation
A nuclide is written , where X is the chemical symbol, Z is the proton number (number of protons) and A is the nucleon number (protons + neutrons).
Number of neutrons N = A − Z.
Worked example. has 17 protons and 35 − 17 = 18 neutrons. A neutral atom has 17 electrons. For an ion, the number of electrons = Z − charge number, so has 18 electrons.
The proton number gives the number of electrons only in a neutral atom. An ion has gained or lost electrons, so its electron number differs from Z.
Section 3
Specific charge
The specific charge of a particle is its charge divided by its mass, in C kg⁻¹.
- Proton: 1.60×10⁻¹⁹ ÷ 1.67×10⁻²⁷ = 9.58×10⁷ C kg⁻¹
- Electron: 1.60×10⁻¹⁹ ÷ 9.11×10⁻³¹ = 1.76×10¹¹ C kg⁻¹
For a nucleus or ion, use its total charge (number of protons minus number of electrons, multiplied by e) and its total mass. For a nucleus the mass can be taken as the number of nucleons × 1.67×10⁻²⁷ kg.
Worked example. A ⁴He²⁺ nucleus has charge 3.20×10⁻¹⁹ C and mass 6.64×10⁻²⁷ kg, so its specific charge is 4.82×10⁷ C kg⁻¹, about half that of a proton.
Charge goes on top and mass on the bottom. Check the answer is sensible: the electron's specific charge is about 1800 times that of the proton.
Section 4
Isotopes and isotopic data
Isotopes are nuclei of the same element, so with the same number of protons, but with different numbers of neutrons (and so different nucleon numbers). They have identical chemical properties because they have the same number of electrons, but different masses, so they have different specific charges.
Use of isotopic data: the relative atomic mass of an element is the weighted mean of its isotopes. For chlorine with 75.8% ³⁵Cl and 24.2% ³⁷Cl:
(75.8 × 35 + 24.2 × 37) ÷ 100 = 35.5
Isotopes are not different elements and do not differ in protons or electrons. Only the number of neutrons changes.
Must Know
- Proton +1.60×10⁻¹⁹ C, 1.67×10⁻²⁷ kg; neutron 0, 1.67×10⁻²⁷ kg; electron −1.60×10⁻¹⁹ C, 9.11×10⁻³¹ kg
- Relative masses 1, 1 and about 1/1840; relative charges +1, 0 and −1
- Notation: Z protons, A nucleons, N = A − Z
- Specific charge = Q ÷ m (C kg⁻¹); proton 9.58×10⁷, electron 1.76×10¹¹
- Isotopes: same Z, different N
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Constituents of the atom and isotopes
- A student studies the structure of lithium. A neutral atom of lithium-7 is written . Use e = 1.60×10⁻¹⁹ C.Another isotope of lithium is . State the number of protons, neutrons and electrons in a neutral atom of this isotope.2 marks
- Chlorine, proton number 17, occurs naturally as two stable isotopes, chlorine-35 and chlorine-37. In sodium chloride chlorine forms chloride ions, Cl⁻.State one way in which atoms of chlorine-35 and chlorine-37 are the same and one way in which they differ, in terms of particles.2 marks
- Naturally occurring chlorine consists of 75.8% chlorine-35 and 24.2% chlorine-37 by number of atoms. Use e = 1.60×10⁻¹⁹ C and take the mass of a nucleus to be the number of nucleons multiplied by 1.67×10⁻²⁷ kg.Calculate the relative atomic mass of naturally occurring chlorine.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).