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2.1.1/2.1.2 Atomic structure, compounds, formulae and equations GapFill
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Atoms consist of a nucleus, made of protons and neutrons, surrounded by electrons. Protons have a relative mass of and a relative charge of +1; the particles surrounding the nucleus have a negligibly small relative mass (approximately 1/1840) and a relative charge of ; neutrons have a relative mass of and a relative charge of 0. The atomic number of an element is equal to the number of protons each of its atoms contains. As atoms are electrically neutral, the atomic number is also equal to the number of in each atom. When this is not the case, the particles are called instead. The charge on these particles can be predicted by considering the number of electrons that must be lost (for metals) or gained (for non-metals) to attain a full outer shell. For instance, oxygen (group 6) gains electrons to form O2−, while aluminium (group 3) loses electrons to form .
Atoms of the same element with a different mass are called . The relative abundances of these forms of an element are measured using . This information can be used to determine the relative atomic mass of an element, which is the weighted mean mass of its atoms relative to . For a molecule, the relative atomic masses of the constituent elements can be added together to give the relative molecular mass; for extended solids, the equivalent is the relative mass.