AQA GCSE Physics 4.4.1 Atoms and Isotopes Exam Questions

About AQA GCSE Physics 4.4.1 Atoms and Isotopes

This first section of the AQA GCSE Physics Atomic Structure topic covers the nuclear model of the atom, the meaning of atomic number and mass number, isotopes, and how the model of the atom developed through new evidence. It is examined on Paper 1 and underpins radioactive decay, nuclear equations, fission and fusion. Examiners regularly ask about the size of the atom and nucleus, how electrons move between energy levels, and why the plum pudding model was replaced after the alpha particle scattering experiment.

What you need to know

• Structure of an atom Atoms are very small, with a radius of about 1 x 10^-10 m. The nucleus is positively charged, made of protons and neutrons, and its radius is less than 1/10 000 of the radius of the atom. Most of the mass is in the nucleus.
• Electrons and energy levels Electrons are arranged at different distances from the nucleus (different energy levels). Absorbing electromagnetic radiation can move an electron further from the nucleus; emitting radiation moves it closer.
• Atomic and mass number Atoms have no overall charge because the number of electrons equals the number of protons. Atomic number is the number of protons; mass number is the total number of protons and neutrons.
• Isotopes and ions Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Atoms become positive ions if they lose one or more outer electrons.
• Plum pudding model Electron discovery led to the plum pudding model: a ball of positive charge with negative electrons embedded in it.
• Alpha scattering and the nuclear model In the alpha particle scattering experiment most alpha particles passed straight through gold foil but a few were deflected by large angles. This showed the mass is concentrated in a small, positive nucleus, giving the nuclear model.
• Later developments Niels Bohr suggested electrons orbit at specific distances, which matched experimental observations. Later experiments showed the positive charge is divided into smaller particles (protons). James Chadwick provided evidence for neutrons about 20 years after the nucleus became accepted.

Common mistakes in 4.4.1 Atoms and Isotopes

• Saying isotopes have different numbers of protons. Isotopes have the same number of protons but different numbers of neutrons.
• Mixing up atomic number and mass number. Atomic number is protons; mass number is protons plus neutrons.
• Saying alpha particles bounced back because they hit electrons. A few were deflected by the small, dense, positive nucleus.
• Saying most alpha particles went straight through because the atom is 'empty'. Say most of the atom is empty space.
• Getting the scale wrong: the nucleus is about 10 000 times smaller in radius than the atom, not just a bit smaller.

Try three 4.4.1 Atoms and Isotopes exam-style questions