Cambridge IGCSE Chemistry 2 Atoms, Elements and Compounds Exam Questions

About Cambridge IGCSE Chemistry 2 Atoms, Elements and Compounds

Atoms, elements and compounds is the big structure and bonding topic in Cambridge IGCSE Chemistry 0620. It starts with the difference between elements, compounds and mixtures, then builds up atomic structure, electronic configurations and isotopes. You then explain how atoms bond: ionic bonding, simple covalent molecules, giant covalent structures such as diamond and graphite, and (Supplement) metallic bonding. Almost every later topic depends on it, especially the Periodic Table, electrolysis and stoichiometry. Examiners regularly ask for dot-and-cross diagrams, explanations of melting point and conductivity in terms of structure, and calculations of relative atomic mass from isotope abundances.

What you need to know

• 2.1 Elements, compounds and mixtures An element contains only one type of atom; a compound contains two or more elements chemically combined; a mixture contains two or more substances not chemically combined, so it can be separated by physical methods.
• 2.2 Atomic structure Protons (relative charge +1, relative mass 1), neutrons (0, 1) and electrons (-1, 1/1840). Proton number is the number of protons; mass number is protons plus neutrons. Electronic configurations are written for the first 20 elements, e.g. calcium 2,8,8,2.
• 2.2 Links to the Periodic Table The number of outer shell electrons equals the group number (except helium and the noble gases); the number of occupied shells equals the period number. Noble gases have full outer shells.
• 2.3 Isotopes Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties because they have the same electronic configuration. Extended candidates calculate relative atomic mass from percentage abundances.
• 2.4 Ionic bonding Electrons are transferred from a metal to a non-metal, forming ions with full outer shells held by strong electrostatic attraction. Ionic compounds have high melting and boiling points and conduct electricity when molten or in aqueous solution, not as solids. Extended candidates describe the giant lattice of alternating ions.
• 2.5 Simple molecules Covalent bonds form when pairs of electrons are shared. Learn dot-and-cross diagrams for H2, Cl2, H2O, CH4, NH3 and HCl, and for Extended also CH3OH, C2H4, O2, CO2 and N2. Simple molecular substances have low melting and boiling points because of weak intermolecular forces, and do not conduct.
• 2.6 Giant covalent structures Diamond: each carbon bonded to four others, very hard, used in cutting tools, does not conduct. Graphite: layers of carbon atoms each bonded to three others, with delocalised electrons, so it conducts and the layers slide, used as electrodes and a lubricant. Extended candidates also learn silicon(IV) oxide.
• 2.7 Metallic bonding (Supplement) A lattice of positive metal ions in a sea of delocalised electrons. This explains good electrical conductivity, and malleability and ductility because layers of ions can slide.

Common mistakes in 2 Atoms, Elements and Compounds

• Saying ionic compounds conduct when solid. The ions can only move and carry charge when molten or dissolved.
• Explaining the low melting point of a simple molecular substance by breaking covalent bonds. Only the weak intermolecular forces are overcome; the covalent bonds stay intact.
• Saying isotopes have different chemical properties. They react the same way because they have the same number of outer electrons.
• Saying graphite conducts because of free ions or moving atoms. It is the delocalised electrons between the layers that move.
• Drawing dot-and-cross diagrams with only the bonding electrons. Non-bonding pairs on atoms such as oxygen, nitrogen and chlorine must be shown too.

Try three 2 Atoms, Elements and Compounds exam-style questions