AP | A-Level | IB | NCERT 11 + 12 – FREE NOTES, RESOURCES AND VIDEOS!
Edexcel® A-Level
YouTube Channel
YouTube Channel
@chemistry.student
*Revision Materials and Past Papers* 1 Atomic Structure and the Periodic Table 2 Bonding and Structure 3 Redox I 4 Inorganic Chemistry and the Periodic Table 5 Formulae, Equations and Amounts of Substance 6 Organic Chemistry I 7 Modern Analytical Techniques I 8 Energetics I 9 Kinetics I 10 Equilibrium I 11 Equilibrium II 12 Acid-base Equilibria 13 Energetics II 14 Redox II 15 Transition Metals 16 Kinetics II 17 Organic Chemistry II 18 Organic Chemistry III 19 Modern Analytical Techniques II RP Required Practicals

1 Atomic Structure and the Periodic Table

1A – Atomic Structure and Isotopes 1B – Relative Masses and Mass Spectrometry 1C – Ionisation Energy and Electronic Structure 1D – Orbitals and Electron Configuration 1E – Periodicity and Trends

Atomic Structure and Isotopes

Specification Reference Topic 1, points 1–6 (Edexcel A-Level Chemistry)

Quick Notes

  • Atoms consist of three fundamental particles:
    • Protons: +1 charge, mass ~1 amu, found in the nucleus.
    • Neutrons: 0 charge, mass ~1 amu, found in the nucleus.
    • Electrons: −1 charge, negligible mass (~1/1836 amu), found in orbitals around the nucleus.
  • Atomic Number (Z) = Number of protons (defines the element).
  • Mass Number (A) = Number of protons + neutrons.
  • Isotopes:
    • Atoms with the same number of protons but different numbers of neutrons
    • Same atomic number, different mass number
  • Relative Isotopic Mass:
    • The mass of a single isotope
    • Compared to 1/12th the mass of a 12C atom
  • Relative Atomic Mass (Ar):
    • The weighted average mass of an element's isotopes
    • Compared to 1/12th the mass of a 12C atom

Full Notes

The structure of the atom has been outlined in more detail here.
This page is just what you need to know for Edexcel A-level :)

Fundamental Particles

Atoms are the smallest, indivisible units of elements and are composed of three fundamental subatomic particles:

Edexcel A-Level Chemistry diagram of an atom showing protons and neutrons in the nucleus and electrons in surrounding shells.

Comparison of Subatomic Particles

Particle Charge Relative Mass Location
Proton +1 ≈ 1 Nucleus
Neutron 0 ≈ 1 Nucleus
Electron −1 ≈ 1/1836 Shells / orbitals

Atomic Number and Mass Number

The atomic number is the number of protons in the nucleus.

The mass number is the total number of protons and neutrons.

Edexcel A-Level Chemistry diagram illustrating atomic number as protons and mass number as protons plus neutrons using nuclide notation.

Determining Subatomic Particles

To determine subatomic particle numbers in a neutral atom:

You may be asked to apply this to atoms or simple molecules.

Isotopes

Isotopes are atoms of the same element that have the same number of protons and different numbers of neutrons.

This gives them the same atomic number but different mass numbers.

All isotopes of an element have the same chemical properties, because they have the same electron configuration.

For Example: Carbon-12 and Carbon-13

Both have 6 protons but different neutrons (Carbon-12 has 6 neutrons, Carbon-13 has 7 neutrons).

Edexcel A-Level Chemistry illustration comparing carbon isotopes 12C and 13C with equal protons and different neutron numbers.

Relative Isotopic Mass

Relative isotopic mass is the mass of a particular isotope compared to 1/12th the mass of a carbon-12 atom.

It refers to one specific isotope, not an average.

For Example: Carbon isotopes

Relative Atomic Mass (Ar)

The relative atomic mass (Ar) of an element is the average mass of an atom of that element compared to 1/12 of the mass of a carbon-12 atom.

It takes into account the masses of all naturally occurring isotopes and their relative abundances.

Ar has no units.

For Example

Edexcel A-Level Chemistry graphic showing chlorine isotopes 35Cl and 37Cl contributing to the element’s relative atomic mass Ar.

Chlorine has two isotopes, 35Cl and 37Cl. The average relative mass of a chlorine atom is based on the amounts of each isotope in a natural sample of chlorine.

Summary