Chemistry (UK GCSE)
GCSE Chemistry deck covering atomic structure, bonding, quantitative chemistry, chemical changes, energy, rates, organic chemistry, analysis, atmosphere and resources. Aligned with AQA/Edexcel/OCR/WJEC common core (Years 10-11, ages 14-16).
Ämne: Kemi · Nivå: Högstadium (13–15) · 434 kort
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- An atom is the smallest particle of an element that retains the properties of that element. Atoms have a radius of about 0.1 nm (1 × 10⁻¹⁰ m).
- An element is a substance made of only one type of atom. Elements are arranged in the periodic table.
- A compound contains two or more elements chemically combined in fixed proportions, and can only be separated by chemical reactions.
- A mixture contains two or more substances not chemically combined. The components keep their properties and can be separated by physical methods.
- Filtration separates an insoluble solid from a liquid using filter paper. The solid stays as residue; the liquid passes through as filtrate.
- Crystallisation produces solid crystals from a solution by evaporating the solvent (often water). It is used to obtain a soluble salt as crystals.
- Simple distillation separates a solvent from a solution by boiling and condensing the solvent. Fractional distillation separates a mixture of liquids with different boiling points.
- Paper chromatography separates substances based on different solubilities in a solvent. The Rf value = distance moved by substance ÷ distance moved by solvent.
- John Dalton (early 1800s) proposed that atoms were tiny solid spheres that could not be divided, with different elements made of different atoms.
- J. J. Thomson discovered the electron in 1897 and proposed the plum pudding model: a ball of positive charge with negative electrons embedded in it.
- Rutherford's alpha-scattering experiment (1909) showed atoms are mostly empty space with a small, dense, positively charged nucleus at the centre.
- Niels Bohr suggested that electrons orbit the nucleus at specific distances in fixed shells (energy levels), explaining stability of atoms.
- James Chadwick discovered the neutron in 1932, completing the modern picture of the atom with protons, neutrons and electrons.
- Protons have a relative charge of +1 and relative mass of 1. Neutrons have charge 0 and mass 1. Electrons have charge −1 and mass approximately 1/1836 (negligible).
- The atomic number (Z) equals the number of protons in an atom. The mass number (A) equals the number of protons plus neutrons.
- Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties.
- Relative atomic mass (Ar) is the mean mass of an atom of an element, weighted by the natural abundance of each isotope, compared with 1/12 of the mass of a carbon-12 atom.
- Electrons fill shells from the nearest to the nucleus first. The first shell holds up to 2 electrons, the second 8, and the third 8 (at GCSE).
- John Newlands proposed the law of octaves in 1864, arranging elements by atomic mass in groups of eight, but his scheme broke down for heavier elements.
- Dmitri Mendeleev (1869) arranged elements by atomic mass but left gaps for undiscovered elements and predicted their properties, which proved correct.
- The modern periodic table arranges elements in order of increasing atomic number. Elements in the same group have the same number of outer-shell electrons.
- Group 1 elements (alkali metals: lithium, sodium, potassium, rubidium, caesium, francium) are soft, silvery metals that react vigorously with water to give an alkaline hydroxide and hydrogen.
- Reactivity increases down Group 1 because the outer electron is further from the nucleus, more shielded by inner shells, and lost more easily.
- Group 7 elements (halogens: fluorine, chlorine, bromine, iodine, astatine) are diatomic non-metals (F₂, Cl₂, Br₂, I₂) that form salts called halides with metals.
- A more reactive halogen will displace a less reactive halogen from a solution of its salt. For example, chlorine displaces bromine from potassium bromide.
- Reactivity decreases down Group 7 because the outer shell is further from the nucleus, more shielded, and an electron is gained less easily.
- Group 0 (noble gases: helium, neon, argon, krypton, xenon, radon) are unreactive monatomic gases. Their outer shells are full, so they don't easily form bonds.
- Transition metals (centre block of the periodic table, e.g. iron, copper, nickel) are typically hard, dense, good conductors that form coloured compounds and act as catalysts.
- Metals are on the left and centre of the periodic table; non-metals are on the right. The dividing line steps down from boron to polonium.
- Ionic bonding occurs between metals and non-metals. Metal atoms lose electrons to form positive ions; non-metal atoms gain electrons to form negative ions.
- An ionic compound is held together by strong electrostatic forces between oppositely charged ions in a regular giant lattice structure.
- Dot-and-cross diagrams show electron transfer in ionic bonding (with electrons drawn as dots and crosses to distinguish atoms of different elements).
- Ionic compounds have high melting and boiling points because strong electrostatic forces between oppositely charged ions require much energy to overcome.
- Ionic compounds conduct electricity when molten or dissolved in water (ions are free to move) but not when solid (ions held in lattice).
- Covalent bonding occurs between non-metal atoms, which share pairs of electrons to gain a full outer shell. Each shared pair counts as one covalent bond.
- Simple molecular substances (e.g. H₂O, CO₂, Cl₂) have strong covalent bonds within molecules but weak intermolecular forces between molecules.
- Simple molecular substances have low melting and boiling points because only weak intermolecular forces (not the covalent bonds) need to be overcome to melt or boil them.
- Diamond is a giant covalent structure where each carbon atom is bonded to four others tetrahedrally. It is very hard, has a very high melting point and does not conduct electricity.
- Graphite has carbon atoms in layers of hexagonal rings, each atom bonded to three others. Delocalised electrons between layers let it conduct electricity; layers can slide so it is slippery.
- Graphene is a single layer of graphite, just one atom thick. It is very strong, light and an excellent conductor of electricity and heat.
- Fullerenes are molecules of carbon atoms with hollow shapes. Buckminsterfullerene (C₆₀) is a hollow ball. They are used in drug delivery, lubricants and catalysts.
- Carbon nanotubes are cylindrical fullerenes with high length-to-diameter ratios. They have high tensile strength and conduct electricity, useful in electronics and composites.
- Metallic bonding is the strong electrostatic attraction between positive metal ions and a sea of delocalised outer-shell electrons that move freely through the lattice.
- Metals conduct electricity and heat because the delocalised electrons in the sea can move and transfer energy. Metals are malleable because layers of ions can slide over each other.
- An alloy is a mixture of a metal with at least one other element. Different sized atoms distort the layers, making it harder for them to slide. Alloys are usually harder than the pure metal.
- Substances exist as solid, liquid or gas. The state depends on the strength of forces between particles and the energy they have at a given temperature.
- State symbols in chemical equations: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
- Polymers are very large molecules made of many small repeating units (monomers) joined by covalent bonds. Examples include poly(ethene) and poly(propene).
- Nanoparticles are 1–100 nm in diameter (a few hundred atoms across). They have a very high surface-area-to-volume ratio, giving unusual properties used in catalysts, sunscreens and electronics.
- The law of conservation of mass states that no atoms are lost or made during a chemical reaction. The total mass of reactants equals the total mass of products.