OCR GCSE Chemistry A C1.2 Atomic Structure Exam Questions

About OCR GCSE Chemistry A C1.2 Atomic Structure

C1.2 covers the structure of the atom and how scientists' models of it developed over time. On the OCR Gateway Chemistry A course it is part of Topic C1 Particles, assessed in Paper 1 (Foundation) or Paper 3 (Higher). It leads directly into electronic structure and the periodic table in C2.2 and underpins every topic involving formulae and moles. Examiners regularly test the history of the atomic model from Dalton to Chadwick, working out numbers of subatomic particles in atoms and ions, the meaning of isotopes, the relative size of the atom and nucleus, and calculating relative atomic mass from isotopic abundances.

What you need to know

• Development of the model Dalton described atoms as solid spheres. Thomson discovered the electron and proposed the plum pudding model; Rutherford's alpha particle scattering experiment showed a small, dense, positive nucleus; Bohr placed electrons in shells; Chadwick discovered the neutron.
• Subatomic particles Protons have relative charge +1 and relative mass 1; neutrons have charge 0 and mass 1; electrons have charge -1 and a very small mass (about 1/1840). Protons and neutrons are in the nucleus, electrons in shells around it.
• Size of atoms Atoms have a radius of about 1 x 10^-10 m. The nucleus is roughly 1/10 000 of the radius of the atom but contains almost all the mass.
• Atomic and mass numbers Atomic number is the number of protons. Mass number is protons plus neutrons, so neutrons = mass number - atomic number. In a neutral atom, electrons equal protons.
• Isotopes and ions Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Ions form when atoms gain or lose electrons, so a 2+ ion has two fewer electrons than protons.
• Relative atomic mass Relative atomic mass is the weighted mean mass of an element's isotopes compared with carbon-12. Ar = sum of (isotope mass x percentage abundance) / 100.

Common mistakes in C1.2 Atomic Structure

• Saying ions form by gaining or losing protons. Ions form when electrons are gained or lost; the number of protons never changes.
• Writing that most alpha particles bounced back in Rutherford's experiment. Most went straight through, showing the atom is mostly empty space; only a few were deflected.
• Giving relative atomic mass as a simple mean of the isotope masses. It must be weighted by abundance.
• Stating that isotopes have different chemical properties. Isotopes have the same electron arrangement, so they react in the same way.
• Confusing atomic number and mass number. The smaller number on the periodic table is the atomic (proton) number.

Try three C1.2 Atomic Structure exam-style questions