Glossary · Bright Atoms

Activation energy (Eₐ): The minimum energy that colliding particles must have for a reaction to happen. Lesson: Why do reactions go faster when it’s hot?
Alkali: A base that dissolves in water, producing hydroxide ions (OH⁻). Its solutions have a pH above 7. Lesson: What does pH actually measure?
Anion: A negatively charged ion, such as Cl⁻. It has more electrons than protons.
Atomic number (Z): The number of protons in the nucleus of an atom. It decides which element the atom is.
Avogadro constant (Nₐ): The number of particles in one mole, exactly 6.022 140 76 × 10²³ per mole since 2019. Usually rounded to 6.02 × 10²³. Lesson: How do you count atoms you can’t see?
Bond angle: The angle between two bonds from the same atom, such as the 104.5° between the two O–H bonds in water. Lesson: Why is a water molecule bent?
Catalyst: A substance that speeds up a reaction without being used up, by providing a route with a lower activation energy. Lesson: Why do reactions go faster when it’s hot?
Cation: A positively charged ion, such as Na⁺. It has fewer electrons than protons.
Collision theory: The idea that particles must collide, with at least the activation energy, for a reaction to happen. Lesson: Why do reactions go faster when it’s hot?
Concentration: The amount of a substance dissolved in a given volume of solution, usually in mol/dm³ (or g/dm³).
Covalent bond: A shared pair of electrons between two atoms. Lesson: Why is a water molecule bent?
Dynamic equilibrium: The state of a reversible reaction in a closed system when the forward and reverse reactions go at the same rate, so the amounts of everything stay constant. Lesson: Why does a reaction push back when you change it?
Electron configuration: The arrangement of an atom’s electrons in shells, subshells and orbitals, such as 1s² 2s² 2p⁶ 3s¹ for sodium. Lesson: Where are the electrons in an atom?
Electronegativity: How strongly an atom attracts the pair of electrons in a covalent bond. Fluorine is the most electronegative element.
Emission spectrum: The set of exact wavelengths of light given out by an element when its electrons fall from higher to lower energy levels. Lesson: Why do metals burn with different colours?
Endothermic: A reaction or process that takes in energy from the surroundings, so ΔH is positive.
Enthalpy change (ΔH): The heat energy taken in or given out by a reaction at constant pressure, in kJ/mol.
Entropy (S): A measure of the number of ways particles and their energy can be arranged. Dissolving and turning into a gas usually increase it. Lesson: Why does salt dissolve in water?
Exothermic: A reaction or process that gives out energy to the surroundings, so ΔH is negative.
First ionisation energy: The energy needed to remove one electron from each atom in one mole of gaseous atoms, forming one mole of gaseous 1+ ions.
Flame test: A test for metal ions: a compound held in a hot flame gives a colour that depends on the metal. Lesson: Why do metals burn with different colours?
Group: A column of the periodic table, numbered 1 to 18. Elements in a group have the same outer electron arrangement.
Hund’s rule: Electrons fill the orbitals of a subshell singly, with the same spin, before any pair up. Lesson: Where are the electrons in an atom?
Hydration enthalpy: The enthalpy change when one mole of gaseous ions dissolves in water to give an infinitely dilute solution. Always negative. Lesson: Why does salt dissolve in water?
Ion: An atom or group of atoms with an electrical charge, because it has gained or lost electrons. Lesson: Why does salt dissolve in water?
Ionic bond: The strong electrostatic attraction between oppositely charged ions. Lesson: Why does salt dissolve in water?
Isotopes: Atoms of the same element with different numbers of neutrons, so different mass numbers.
Lattice enthalpy: The enthalpy change when one mole of an ionic solid is formed from its gaseous ions (formation, negative) or separated into them (dissociation, positive). Check which one a question uses. Lesson: Why does salt dissolve in water?
Le Chatelier’s principle: If a change is made to the conditions of a system at equilibrium, the position of equilibrium moves to counteract the change. Lesson: Why does a reaction push back when you change it?
Lone pair: A pair of outer electrons on an atom that is not used in bonding. Lesson: Why is a water molecule bent?
Molar mass (M): The mass of one mole of a substance, in g/mol. It has the same number as the relative formula mass. Lesson: How do you count atoms you can’t see?
Mole (mol): The SI unit of amount of substance. One mole contains exactly 6.022 140 76 × 10²³ particles. Lesson: How do you count atoms you can’t see?
Neutralisation: The reaction of an acid with a base. In solution it is H⁺ + OH⁻ → H₂O. Lesson: What does pH actually measure?
Orbital: A region around a nucleus where an electron is likely to be found. Each orbital holds up to two electrons with opposite spins. Lesson: Where are the electrons in an atom?
Period: A row of the periodic table. Elements in a period have the same number of electron shells.
pH: A measure of acidity, pH = −log₁₀[H⁺]. Below 7 is acidic, 7 is neutral at 25 °C, above 7 is alkaline. One unit is a factor of ten in [H⁺]. Lesson: What does pH actually measure?
Polar molecule: A molecule with a slightly positive end and a slightly negative end, because its bonds are polar and its shape doesn’t cancel them out. Water is polar. Lesson: Why is a water molecule bent?
Relative atomic mass (Aᵣ): The average mass of an atom of an element, taking account of its isotopes, compared with one twelfth of the mass of a carbon-12 atom. No units. Lesson: How do you count atoms you can’t see?
Relative formula mass (Mᵣ): The sum of the relative atomic masses of the atoms in a formula. Lesson: How do you count atoms you can’t see?
Reversible reaction: A reaction that can go both ways, shown with the ⇌ sign. Lesson: Why does a reaction push back when you change it?
Strong acid: An acid that is completely ionised in water, such as hydrochloric acid. Not the same as concentrated. Lesson: What does pH actually measure?
Subshell: A part of an electron shell, labelled s, p, d or f, holding up to 2, 6, 10 or 14 electrons. Lesson: Where are the electrons in an atom?
Successful collision: A collision between particles with at least the activation energy, which leads to a reaction. Lesson: Why do reactions go faster when it’s hot?
VSEPR: Valence shell electron pair repulsion. The rule that electron pairs around an atom repel and get as far apart as possible, which sets the shape of a molecule. Lesson: Why is a water molecule bent?
Weak acid: An acid that is only partly ionised in water, such as ethanoic acid. Lesson: What does pH actually measure?