AQA GCSE Chemistry 4.1.1 Atomic Structure, Symbols, Relative Atomic Mass and Isotopes Exam Questions

About AQA GCSE Chemistry 4.1.1 Atomic Structure, Symbols, Relative Atomic Mass and Isotopes

This opening AQA GCSE Chemistry topic sets up everything else in the course. You learn how atoms, elements, compounds and mixtures differ, how mixtures are separated, how the model of the atom developed from the plum pudding model to the nuclear model, and how to work out numbers of protons, neutrons and electrons from an atomic symbol. It sits in section 4.1 and is examined on Paper 1. Examiners regularly test the alpha scattering experiment, isotope calculations of relative atomic mass, electronic structures for the first 20 elements and the sizes of atoms and nuclei in standard form.

What you need to know

• Atoms, elements and compounds An element contains only one type of atom; a compound contains two or more elements chemically combined in fixed proportions. Compounds can only be separated by chemical reactions, while mixtures can be separated by physical methods such as filtration, crystallisation, simple and fractional distillation and chromatography.
• Development of the atomic model Dalton pictured atoms as tiny solid spheres. Thomson's discovery of the electron led to the plum pudding model. Rutherford's alpha particle scattering experiment showed most of the atom is empty space with a small, dense, positive nucleus; Bohr placed electrons in shells at fixed distances, and Chadwick later showed the nucleus contains neutrons.
• Subatomic particles Proton: relative mass 1, charge +1. Neutron: relative mass 1, charge 0. Electron: very small mass, charge -1. Atoms are neutral because they have equal numbers of protons and electrons.
• Atomic symbols The atomic number is the number of protons; the mass number is protons plus neutrons. Neutrons = mass number minus atomic number.
• Size and mass of atoms Atoms have a radius of about 0.1 nm (1 x 10^-10 m). The nucleus has a radius less than 1/10 000 of the atom but contains almost all of its mass.
• Isotopes and relative atomic mass Isotopes are atoms of the same element with different numbers of neutrons. Relative atomic mass is the mean mass of an atom of the element, weighted by the abundance of each isotope, compared with carbon-12.
• Electronic structure Electrons fill the lowest available shells first: up to 2 in the first shell, then 8, then 8 for the first 20 elements. Sodium is 2,8,1 and calcium is 2,8,8,2.

Common mistakes in 4.1.1 Atomic Structure, Symbols, Relative Atomic Mass and Isotopes

• Saying the nucleus contains electrons. The nucleus contains protons and neutrons only; electrons are in shells around it.
• Describing isotopes as having different numbers of protons. Isotopes have the same number of protons but different numbers of neutrons.
• Concluding from alpha scattering only that 'the atom has a nucleus'. Full answers link observations to conclusions: most alpha particles passed straight through, so most of the atom is empty space; a few were deflected, so the nucleus is positive; very few bounced back, so the mass is concentrated in a tiny nucleus.
• Calculating relative atomic mass by simply averaging the isotope masses. You must weight each mass by its percentage abundance and divide by 100.
• Confusing mixtures with compounds. A mixture has no chemical bonds between its parts, so it can be separated physically.

Try three 4.1.1 Atomic Structure, Symbols, Relative Atomic Mass and Isotopes exam-style questions