AQA GCSE Chemistry Topic 1: Atomic Structure and the Periodic Table Notes

Atoms, Elements, and Compounds

• Atoms are the fundamental building blocks of all substances and the smallest unit of an element that retains its chemical properties.
• Chemical symbols represent atoms of elements (e.g., Na for sodium).
• Compounds are formed through chemical reactions, resulting in new substances and often involving energy changes.
• Compounds consist of two or more elements chemically combined in fixed proportions, represented by formulae (e.g., $HCl$).
• Separating compounds into elements requires chemical reactions.

Mixtures

• Mixtures contain two or more elements or compounds not chemically combined; each substance retains its chemical properties.
• Separation methods include filtration, crystallization, simple distillation, fractional distillation, and chromatography, which are physical processes that do not create new substances.

Development of the Atomic Model

• Initially, atoms were conceived as indivisible, tiny spheres.
• The discovery of the electron led to the plum pudding model: an atom with a ball of positive charge and embedded negative electrons.
• The alpha particle scattering experiment revealed that the mass of an atom is concentrated in a charged nucleus.

Alpha Particle Scattering Experiment

• Alpha particles were directed at thin gold foil, and their passage was observed.
• Observations:
  • Some alpha particles passed through.
  • Some deflected at various angles.
  • Some rebounded.
• Conclusions:
  • A small, concentrated positive charge (nucleus) repelled and deflected positively charged alpha particles.
• Neil Bohr proposed that electrons orbit the nucleus at specific distances, supported by experimental data.

Subatomic Particles and Atomic Structure

• Further experiments showed the positive charge in a nucleus could be divided into smaller particles (protons).
• James Chadwick discovered neutrons within the nucleus.

Relative Electrical Charges of Subatomic Particles

• Atomic number: the number of protons in an atom.
• All atoms of an element have the same number of protons.
• Atoms of different elements have different numbers of protons.

• Atoms are neutral: the number of protons equals the number of electrons.

Size and Mass of Atoms

• Atoms are extremely small (radius ≈ $0.1 nm$).
• The nucleus is much smaller (radius < $1/10,000$ of the atom) but holds almost all the mass.

• Mass number: the sum of protons and neutrons in an atom.
• Isotopes: atoms of the same element with different numbers of neutrons.

Relative Atomic Mass

• Relative atomic mass is an average value considering the abundance of isotopes.
• Calculation: $\frac{\sum(isotope \, mass \, \times \, abundance)}{100}$
• Example: Carbon isotopes – Carbon-14 (20% abundance) and Carbon-12 (80% abundance).
  • Calculation: $((14 \times 20) + (12 \times 80)) \div 100 = 1240 \div 100 = 12.4$

Electronic Structure

• Electrons occupy the lowest available energy levels (shells closest to the nucleus).
• The electronic structure indicates the number of electrons in each shell.
• Example: Sodium (2,8,1) – 2 electrons in the first shell, 8 in the second, and 1 in the third.