OCR GCSE Chemistry B C2 Chemical Patterns Exam Questions

About OCR GCSE Chemistry B C2 Chemical Patterns

Chapter C2 is about the patterns that let chemists predict how elements behave. On OCR Chemistry B you follow how models of the atom developed from Dalton to Bohr, use atomic structure to explain the arrangement of the Periodic Table, and compare the trends in groups 1, 7 and 0. You then see how metals and non-metals combine by ionic bonding and learn to write formulae and balanced equations with state symbols. Separate chemistry students also study transition metals. This chapter underpins almost every later topic, so examiners test it often through particle counts, reactivity explanations, dot and cross diagrams and balanced equations.

What you need to know

• C2.1 Atomic models Dalton pictured solid spheres, Thomson the plum pudding model, Rutherford's gold foil experiment showed a small dense positive nucleus, and Bohr placed electrons in shells. Atoms are about 10^-10 m across.
• C2.1 Subatomic particles Protons have relative charge +1 and mass 1, neutrons charge 0 and mass 1, electrons charge -1 and very small mass. Atomic number gives protons; mass number minus atomic number gives neutrons.
• C2.2 The Periodic Table Elements are arranged by atomic number; the group number equals the number of outer shell electrons and the period equals the number of occupied shells. Mendeleev left gaps for undiscovered elements and predicted their properties.
• C2.2 Group 1 and group 7 Alkali metals react with water to form hydroxides and hydrogen, getting more reactive down the group. Halogens get less reactive down the group, shown by displacement reactions, for example chlorine displaces bromide ions.
• C2.3 Ionic bonding Metals lose electrons to form positive ions and non-metals gain electrons to form negative ions. Ionic compounds form giant lattices with high melting points and conduct only when molten or dissolved. Group 0 elements are unreactive because they have full outer shells.
• C2.4 Formulae and equations Use ion charges to work out formulae, such as MgCl2 or Al2O3, and write balanced symbol equations with state symbols (s), (l), (g) and (aq).
• C2.5 Transition metals Separate chemistry only: transition metals have high melting points and densities, form coloured compounds and ions with different charges, and are used as catalysts, for example iron in the Haber process.

Common mistakes in C2 Chemical Patterns

• Saying halogens get more reactive down the group. Group 7 reactivity decreases down the group because the outer shell is further from the nucleus, so gaining an electron is harder.
• Explaining group 1 trends in terms of gaining electrons. Alkali metals lose their single outer electron, which becomes easier further down the group.
• Forgetting to change electron numbers for ions. A Na+ ion has 11 protons but only 10 electrons.
• Saying ionic compounds conduct when solid. The ions are fixed in the lattice, so they only conduct when molten or in solution.
• Changing small numbers inside formulae to balance equations. Only change the large numbers in front of formulae.

Try three C2 Chemical Patterns exam-style questions