📄 Cheat Sheets — Chemistry

Key facts, formulas and common mistakes for 1 of 1 chapters

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7 key facts·1 formulas·5 common mistakes·12 definitions·5 exam tips

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Definitions

atom: The smallest part of an element that can take part in a chemical change.
nucleus: A tiny region in the centre of the atom where nearly all of its mass is concentrated.
nucleons: Particles that make up the nucleus, specifically protons and neutrons.
protons: Positively charged particles found in the nucleus of an atom.
neutrons: Neutral (uncharged) particles found in the nucleus of an atom.
electrons: Negatively charged particles that move around the nucleus in regions of space called orbitals or shells.
orbitals: Regions of space outside the nucleus where there is a probability of finding a particular electron.
electron shells: A simpler model of the atom where electrons move around the nucleus at certain distances, each at its own particular energy level.
atomic number (proton number): The number of protons in the nucleus of an atom, denoted by Z.
mass number (nucleon number): The number of protons plus neutrons in the nucleus of an atom, denoted by A.
isotopes: Atoms of the same element that have different numbers of neutrons.
ions: Electrically charged particles formed from atoms or molecules by the gain or loss of electrons.

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Ch 01

Number of neutrons

number of neutrons = mass number - atomic number

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An atom consists of a tiny, dense nucleus (protons and neutrons) surrounded by electrons in shells, with most of the atom being empty space.Ch 01
Protons have a relative charge of +1 and relative mass of 1; neutrons have a relative charge of 0 and relative mass of 1; electrons have a relative charge of -1 and negligible relative mass.Ch 01
Atomic (proton) number (Z) is the number of protons; Mass (nucleon) number (A) is the total number of protons and neutrons.Ch 01
Protons are deflected towards the negative plate, electrons towards the positive plate, and neutrons are not deflected in an electric field.Ch 01
Ions are formed by the gain or loss of electrons, resulting in a net positive (loss) or negative (gain) charge.Ch 01
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.Ch 01
Isotopes of an element have identical chemical properties due to the same electron configuration, but different physical properties (e.g., mass, density) due to different numbers of neutrons.Ch 01

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✗Don't confuse atomic number (protons) with mass number (protons + neutrons); this is crucial for calculating neutrons.Ch 01
✗Don't think atoms are solid spheres; remember they are mostly empty space.Ch 01
✗Don't assume isotopes have different chemical properties; their chemical properties are the same.Ch 01
✗Don't think ions are formed by gaining or losing protons; ion formation always involves the gain or loss of electrons.Ch 01
✗Don't forget to account for the charge when calculating the number of electrons in an ion.Ch 01

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→When asked to 'describe' atomic structure, include details about the nucleus (protons, neutrons, small, dense) and electrons (shells, empty space).Ch 01
→Practice deducing the number of protons, neutrons, and electrons for both atoms and ions, clearly showing your working for full marks.Ch 01
→For questions on isotopes, define them precisely in terms of protons and neutrons, then explain chemical properties (same electrons) and physical properties (different mass/density).Ch 01
→Be precise with relative charges and masses for subatomic particles; use the standard values (+1, 0, -1 for charge; 1, 1, negligible for mass).Ch 01
→When explaining the behaviour of subatomic particles in an electric field, link deflection direction directly to their charge (e.g., positive protons attracted to negative plate).Ch 01