Edexcel International A Level Chemistry Atomic Structure and the Periodic Table Exam Questions

About Edexcel International A Level Chemistry Atomic Structure and the Periodic Table

This is one of the opening topics of Unit 1 in Edexcel International A Level Chemistry and it supplies the ideas used throughout the course. You learn how isotopes are detected with a mass spectrometer and used to calculate relative atomic mass, then how ionisation energy data provides evidence for shells and subshells. Electronic configurations in s, p and d orbitals explain the blocks of the periodic table and trends such as melting temperature and first ionisation energy across Period 2 and Period 3. Examiners like calculations of relative atomic mass from spectra, explaining dips in first ionisation energy, and interpreting successive ionisation energy graphs.

What you need to know

• Atoms and isotopes Protons (relative mass 1, charge +1), neutrons (1, 0) and electrons (1/1836, -1). Isotopes have the same number of protons but different numbers of neutrons, so they have the same chemical properties but different masses.
• Mass spectrometry A mass spectrum gives m/z values and relative abundances. Relative atomic mass is the weighted mean mass of the atoms of an element relative to 1/12 of the mass of a carbon-12 atom. You can also use the M+ peak to find the relative molecular mass of a molecule.
• Ionisation energy First ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions. It depends on nuclear charge, distance from the nucleus and shielding.
• Successive ionisation energies Large jumps show where an electron is removed from a new shell closer to the nucleus, so they reveal the group of the element. They provide evidence for quantum shells.
• Orbitals and configurations s orbitals are spherical, p orbitals are dumbbell shaped. Subshells fill in the order 1s 2s 2p 3s 3p 4s 3d. Chromium is [Ar]3d5 4s1 and copper is [Ar]3d10 4s1.
• Trends across a period First ionisation energy generally increases across a period as nuclear charge increases with similar shielding. Dips occur at Group 3 (electron removed from a higher energy p subshell) and Group 6 (electron removed from a doubly occupied p orbital, where repulsion makes it easier).
• Periodicity of melting temperature Across Period 3 melting temperature rises from Na to Si (metallic bonding getting stronger, then giant covalent Si) and falls sharply for P4, S8, Cl2 and Ar, which have weak London forces between simple molecules or atoms.

Common mistakes in Atomic Structure and the Periodic Table

• Leaving out gaseous state symbols in an ionisation energy equation. It must be X(g) gives X+(g) + e-.
• Explaining the Group 3 dip by electron pair repulsion. That reason applies to Group 6; the Group 3 dip happens because the electron is in a p subshell of higher energy, further from the nucleus.
• Writing chromium as [Ar]3d4 4s2. It is [Ar]3d5 4s1, and copper is [Ar]3d10 4s1.
• Giving relative atomic mass to the wrong precision or forgetting to divide by total abundance. Always divide the sum of mass x abundance by the total abundance.
• Saying shielding increases across a period. Electrons are added to the same shell, so shielding stays roughly the same while nuclear charge increases.

Try three Atomic Structure and the Periodic Table exam-style questions