WJEC GCSE Chemistry 1.2 Atomic Structure and the Periodic Table Exam Questions

About WJEC GCSE Chemistry 1.2 Atomic Structure and the Periodic Table

This Unit 1 topic explains how the structure of atoms accounts for the arrangement and properties of elements in the periodic table. You need the relative masses and charges of subatomic particles, isotopes and relative atomic mass calculations, and electronic structures for the first 20 elements. You must explain trends in reactivity in Group 1 and Group 7, describe their reactions, and know the properties and uses of the noble gases. The topic also includes tests for ions such as flame tests and precipitate tests, with a specified practical identifying unknown ionic compounds. Examiners often ask you to explain reactivity trends in terms of electrons and shells.

What you need to know

• Atoms Protons (mass 1, charge +1) and neutrons (mass 1, charge 0) are in the nucleus; electrons (very small mass, charge -1) are in shells. Atomic number = number of protons; mass number = protons + neutrons.
• Isotopes Isotopes are atoms of the same element with different numbers of neutrons. Relative atomic mass is the mean mass weighted by the abundance of each isotope.
• Electronic structure Shells fill 2, 8, 8, e.g. sodium is 2,8,1. The group number equals the number of outer electrons and the period number equals the number of occupied shells.
• Group 1 Alkali metals react with water to form a metal hydroxide and hydrogen, e.g. 2Na + 2H2O → 2NaOH + H2. Reactivity increases down the group because the outer electron is further from the nucleus and more shielded, so it is lost more easily.
• Group 7 and Group 0 Halogens become less reactive down the group because it is harder to gain an electron. A more reactive halogen displaces a less reactive one from solutions of its salts. Noble gases are unreactive because they have full outer shells; uses include helium in balloons and argon in light bulbs.
• Ion tests Flame tests: lithium red, sodium yellow-orange, potassium lilac, calcium brick red. Sodium hydroxide gives coloured precipitates with Cu2+ (blue), Fe2+ (green) and Fe3+ (brown). Acidified silver nitrate gives white (chloride), cream (bromide) and yellow (iodide) precipitates. Identifying unknown ionic compounds is a specified practical.

Common mistakes in 1.2 Atomic Structure and the Periodic Table

• Explaining Group 1 reactivity only as 'more shells'. Full answers say the outer electron is further from the nucleus and more shielded, so attraction is weaker and it is lost more easily.
• Saying halogens get more reactive down the group like Group 1. Halogens gain electrons, which becomes harder further from the nucleus, so reactivity decreases down Group 7.
• Giving the mass of an electron as zero. It is very small (about 1/2000 of a proton) but not zero.
• Mixing up the precipitate colours for Fe2+ and Fe3+. Fe2+ gives a green precipitate and Fe3+ a brown precipitate with sodium hydroxide.

Try three 1.2 Atomic Structure and the Periodic Table exam-style questions