Atomic Structure
Atomic structure is the study of the sub-atomic particles that make up atoms and ions - protons, neutrons and electrons - and how electrons are arranged into shells, sub-shells and orbitals. It also covers isotopes, relative atomic mass and time-of-flight mass spectrometry, and underpins A Level Chemistry topics on bonding and the periodic table.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Identify the mass number and atomic number of the isotope from its symbol (e.g. 37Cl has mass number 37, atomic number 17) to find the number of protons, neutrons and electrons.
- Adjust the electron count for charge: subtract electrons for a positive ion, add electrons for a negative ion, while the number of protons and neutrons stays the same.
- To find relative atomic mass, multiply each isotope's mass number by its percentage abundance (as a decimal), sum the products, and check the abundances add up to 100%.
- Write electron configurations using the sub-shell filling order (1s, 2s, 2p, 3s, 3p, 4s, 3d...), remembering that 4s fills before 3d but empties first when a d-block atom forms a positive ion.
- Use Hund's rule to arrange electrons singly, with parallel spins, across degenerate orbitals before any pairing occurs, and use the Pauli exclusion principle to explain why each orbital holds a maximum of two electrons.
- For mass spectrometry calculations, work through the four stages in order (ionisation, acceleration, ion drift/flight, detection) and use Ek = q x V and Ek = 1/2 x m x v^2 to link accelerating voltage to ion velocity.
Worked example
Gallium exists as two isotopes, 69Ga (60.0% abundance) and 71Ga (40.0% abundance). Calculate the relative atomic mass of gallium, giving your answer to 1 decimal place.
- Convert each percentage abundance to a decimal: 60.0% = 0.600 and 40.0% = 0.400.
- Multiply each isotope's mass number by its decimal abundance: 69 x 0.600 = 41.4, and 71 x 0.400 = 28.4.
- Add the two products together: 41.4 + 28.4 = 69.8.
- Check the abundances sum to 100% (60.0 + 40.0 = 100), confirming no isotopes are missing.
- Final answer: relative atomic mass of gallium = 69.8
Practice questions
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Q1State the relative charge of a neutron.Show answer
Answer: 0 (neutrons are electrically neutral)
Q2Define the term isotopes.Show answer
Answer: Atoms of the same element (same number of protons) with different numbers of neutrons
Q3An atom of potassium-39 has atomic number 19. State the number of protons, neutrons and electrons it contains.Show answer
Answer: 19 protons, 20 neutrons, 19 electrons (neutrons = mass number - atomic number = 39 - 19)
Q4State the number of orbitals in a d sub-shell and the maximum number of electrons it can hold.Show answer
Answer: 5 orbitals, maximum 10 electrons (2 electrons per orbital)
Q5Silicon has two main isotopes: 28Si (92.0% abundance) and 30Si (8.00% abundance). Calculate the relative atomic mass of silicon to 1 decimal place.Show answer
Answer: 28.2 ((28 x 0.920) + (30 x 0.0800) = 25.76 + 2.40 = 28.16, rounds to 28.2)
Q6Write the full electron configuration of the Ni2+ ion (nickel has atomic number 28).Show answer
Answer: 1s2 2s2 2p6 3s2 3p6 3d8 (the two 4s electrons are removed first from the atom's 1s2 2s2 2p6 3s2 3p6 3d8 4s2 configuration)
Exam-style questions
Written in the style of a A Level Science exam paper, with a full mark scheme.
An ion of bromine has the formula 81Br-. Bromine has atomic number 35. State the number of protons, neutrons and electrons present in this ion.
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In a time-of-flight mass spectrometer, singly charged Na+ ions (relative mass 23, charge = 1.60 x 10^-19 C) are accelerated through a potential difference of 12.0 kV. Calculate the velocity of the Na+ ions as they leave the acceleration stage. Use Ek = q x V, Ek = 1/2 x m x v^2, and 1 unified atomic mass unit = 1.66 x 10^-27 kg. Give your answer to 3 significant figures.
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Manganese has atomic number 25. Write the full electron configuration of a manganese atom, Mn, and of the Mn2+ ion. Hence state the number of unpaired electrons in Mn2+, using Hund's rule to justify your answer.
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