CCEA A Level Chemistry 1.2 Atomic Structure Exam Questions

About CCEA A Level Chemistry 1.2 Atomic Structure

Section 1.2 of Unit AS 1 covers what atoms are made of and how their electrons are arranged. You need the properties of protons, neutrons and electrons, isotopes, relative atomic, isotopic, molecular and formula masses on the carbon-12 scale, and how to interpret simple mass spectra such as that of chlorine. You must write electronic configurations up to krypton using s, p and d notation and electrons in boxes, know the shapes of s and p orbitals and the s, p, d and f blocks. Ionisation energy trends are tested every year: equations, evidence for shells and subshells, and explanations using nuclear charge, distance and shielding.

What you need to know

• Particles and isotopes Protons (relative mass 1, charge +1), neutrons (1, 0) and electrons (1/1840, -1). Isotopes have the same number of protons but different numbers of neutrons, so the same chemical properties.
• Relative masses Relative atomic mass is the average mass of an atom of an element compared with 1/12 of the mass of a carbon-12 atom. Calculate it from a mass spectrum using abundances.
• Mass spectra Chlorine gives peaks at m/z 35 and 37 (ratio 3:1) for atoms, and 70, 72 and 74 (ratio 9:6:1) for Cl2+ molecular ions.
• Electronic configuration Fill 1s 2s 2p 3s 3p 4s 3d 4p. Chromium is [Ar] 3d5 4s1 and copper is [Ar] 3d10 4s1. s orbitals are spherical and p orbitals are dumbbell shaped. Elements are classified into s, p, d and f blocks by their outer subshell.
• Ionisation energy First ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms, for example Na(g) gives Na+(g) + e-. Large jumps in successive ionisation energies show a new shell.
• Trends Ionisation energy decreases down a group (more shells, more shielding, greater distance). It generally increases across a period (higher nuclear charge, similar shielding), with dips at Group III (new p subshell) and Group VI (paired electrons repel). Filled and half-filled subshells are relatively stable.

Common mistakes in 1.2 Atomic Structure

• Omitting gas state symbols in ionisation energy equations. Every species is (g).
• Writing chromium as [Ar] 3d4 4s2. It is [Ar] 3d5 4s1 because a half-filled d subshell is more stable.
• Explaining the drop from Mg to Al by shielding alone. The 3p electron is in a higher energy subshell, so less energy is needed.
• Saying isotopes have different chemical properties. They have the same electron configuration, so react in the same way.
• Defining relative atomic mass without reference to carbon-12. Always include 1/12 of the mass of a carbon-12 atom.

Try three 1.2 Atomic Structure exam-style questions