Scio NSZ Chemistry key terms

The smallest unit of an element that still keeps that element's chemical properties. It consists of a nucleus of protons and neutrons, surrounded by electrons.

A pure substance made of only one kind of atom, defined by its number of protons. Elements cannot be split into simpler substances by chemical means.

A positively charged particle in the atomic nucleus. Its count determines the element's identity and is called the atomic number.

An electrically neutral particle found in the atomic nucleus. Its number can vary between atoms of the same element, giving different isotopes.

A negatively charged particle that orbits the nucleus in shells or orbitals. Electrons take part in bonding and determine chemical behaviour.

The number of protons in the nucleus of an atom. It uniquely identifies an element and fixes its position in the periodic table.

The total number of protons and neutrons in one atom. It is written as a superscript before the element symbol and differs between isotopes.

One of two or more atoms of the same element that have the same number of protons but different numbers of neutrons, and therefore different mass numbers.

The average mass of an element's atoms compared with one-twelfth the mass of a carbon-12 atom, weighted by natural isotope abundance. It is dimensionless.

A table arranging elements by increasing atomic number so that elements with similar chemical properties line up in vertical groups and horizontal periods.

A vertical column in the periodic table. Elements in one group share the same number of valence electrons and therefore similar chemical properties.

A horizontal row in the periodic table. Across a period the atomic number rises by one per element and the number of electron shells stays the same.

An electron in the outermost shell of an atom. Valence electrons are the ones involved in forming chemical bonds and largely explain an element's reactivity.

An atom or group of atoms that has gained or lost electrons, giving it a net electrical charge. Cations are positive, anions are negative.

The electrostatic attraction between oppositely charged ions, usually formed when a metal transfers electrons to a non-metal. The resulting compound forms a lattice.

A bond formed when two atoms share one or more pairs of electrons. It typically joins non-metal atoms and can be single, double or triple.

Two or more atoms held together by covalent bonds. A molecule is the smallest particle of a covalent substance that can exist independently.

A measure of how strongly an atom attracts the shared electrons in a covalent bond. Large differences between bonded atoms make the bond more ionic.

A covalent bond in which the shared electrons are pulled unevenly towards the more electronegative atom, creating partial positive and negative charges.

A strong intermolecular attraction between a hydrogen atom bonded to N, O or F and a lone pair on a nearby electronegative atom. It explains water's high boiling point.

Any attractive force acting between separate molecules, such as London dispersion forces, dipole-dipole attraction or hydrogen bonding. These forces are weaker than covalent bonds.

The amount of substance containing 6.022 × 10²³ particles, known as Avogadro's number. It links countable particles to measurable masses in chemistry.

The mass of one mole of a substance, expressed in grams per mole. Numerically it equals the relative atomic or molecular mass of the substance.

The number of particles in one mole, approximately 6.022 × 10²³ per mole. It allows conversion between particle counts and amounts in moles.

The volume occupied by one mole of a gas. At standard conditions for gases it is about 22.4 litres per mole, whatever the gas.

The calculation of the amounts of reactants and products in a chemical reaction, using the balanced equation and molar ratios.

The reactant that is used up first in a reaction and therefore determines the maximum amount of product that can form.

The amount of product actually obtained from a reaction. Percentage yield compares this actual amount with the theoretical maximum predicted by stoichiometry.

A substance that donates protons (H⁺ ions) to another substance, or accepts an electron pair. Acids turn blue litmus red and react with bases.

A substance that accepts protons (H⁺ ions) or donates an electron pair. Soluble bases are called alkalis and turn red litmus blue.

A logarithmic scale from 0 to 14 measuring the concentration of hydrogen ions in a solution. Values below 7 are acidic, above 7 are alkaline.

The reaction between an acid and a base to form a salt and water. The hydrogen ions and hydroxide ions combine to produce water.

A reaction in which electrons are transferred between species. One substance is oxidised by losing electrons while another is reduced by gaining them.

The loss of electrons by a species, or an increase in its oxidation number. In terms of oxygen, it means gaining oxygen or losing hydrogen.

The gain of electrons by a species, or a decrease in its oxidation number. It always accompanies oxidation in a redox reaction.

A number assigned to an atom in a compound that tracks its electron balance, assuming bonds are fully ionic. It helps identify what is oxidised and reduced.

The use of an electric current to drive a non-spontaneous chemical reaction, such as splitting molten or dissolved compounds into their elements.

A substance that speeds up a reaction by providing an alternative pathway with lower activation energy, without being consumed in the overall process.

The minimum energy that colliding particles must possess for a reaction to occur. A lower activation energy means the reaction proceeds faster at a given temperature.

The change in concentration of a reactant or product per unit time. It depends on concentration, temperature, surface area and the presence of a catalyst.

The state of a reversible reaction in which the forward and reverse rates are equal, so concentrations of reactants and products stay constant over time.

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, it shifts in the direction that opposes the change.

A reaction that releases energy to its surroundings, usually as heat, so the surroundings warm up. The products have less enthalpy than the reactants.

A reaction that absorbs energy from its surroundings, usually as heat, so the surroundings cool down. The products have more enthalpy than the reactants.

A measure of the total heat content of a system at constant pressure. Changes in enthalpy show whether a reaction releases or absorbs heat.

A compound based on carbon chains or rings, usually containing hydrogen and often other elements such as oxygen, nitrogen or halogens.

An organic compound containing only carbon and hydrogen atoms. Examples include alkanes, alkenes and alkynes, which differ in bond saturation.

A specific group of atoms within an organic molecule that gives the compound characteristic chemical properties, such as hydroxyl, carboxyl or amino groups.

One of two or more compounds with the same molecular formula but a different arrangement of atoms, and therefore different physical or chemical properties.