CSEC Chemistry key terms

A pure substance made of only one kind of atom, which cannot be broken down into simpler substances by chemical means. Each element has a unique atomic number.

A pure substance formed when two or more different elements are chemically bonded together in fixed proportions by mass. Its properties differ from those of its constituent elements.

Two or more substances combined physically but not chemically bonded, so each keeps its own properties. Components can usually be separated by physical methods and occur in variable proportions.

The smallest particle of an element that can take part in a chemical reaction. It consists of a nucleus of protons and neutrons surrounded by electrons.

An atom or group of atoms that has gained or lost electrons, giving it an overall electric charge. Positive ions are called cations; negative ions are called anions.

One of two or more atoms of the same element with the same number of protons but different numbers of neutrons, so they have different mass numbers but identical chemical properties.

The number of protons in the nucleus of an atom, which identifies the element and equals the number of electrons in a neutral atom.

The total number of protons and neutrons in the nucleus of an atom. It is always a whole number and is used to compare isotopes.

The average mass of the atoms of an element compared with one-twelfth the mass of a carbon-12 atom, taking natural isotopic abundance into account. It has no units.

The amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12, namely about 6.02 x 10^23 particles.

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

The simplest whole-number ratio of the atoms of each element present in a compound. It may be the same as or a simple multiple of the molecular formula.

The actual number of atoms of each element in one molecule of a compound. It is a whole-number multiple of the empirical formula.

A symbolic representation of a chemical reaction using formulas, with reactants on the left and products on the right. It must be balanced so atoms are conserved.

A shorthand notation using element symbols and subscript numbers to show the type and number of atoms in a substance, for example H2O or CaCO3.

An arrangement of elements in order of increasing atomic number, organised in periods and groups so that elements with similar properties fall in the same vertical column.

A vertical column of the periodic table. Elements in the same group have the same number of outer-shell electrons and therefore similar chemical properties.

A horizontal row of the periodic table. Across a period the number of outer-shell electrons increases by one from element to element.

The combining power of an element or radical, equal to the number of electrons it gains, loses or shares to achieve a stable outer shell. It is used to write formulas.

The strong electrostatic attraction between oppositely charged ions formed when electrons are transferred from a metal atom to a non-metal atom.

A bond formed when two atoms share one or more pairs of electrons, usually between non-metal atoms. The shared pair holds the atoms together.

The smallest particle of an element or compound that can exist independently and consists of two or more atoms held together by covalent bonds.

One of two or more different structural forms of the same element in the same physical state, such as diamond and graphite for carbon.

The decomposition of an ionic compound, either molten or in solution, by passing a direct electric current through it using inert electrodes.

A substance that conducts electricity when molten or dissolved in water because it contains free-moving ions. It is decomposed by the current during electrolysis.

A reaction in which a substance loses electrons, gains oxygen or loses hydrogen. In terms of oxidation number, the value increases.

A reaction in which a substance gains electrons, loses oxygen or gains hydrogen. In terms of oxidation number, the value decreases.

A reaction in which oxidation and reduction occur together, with electrons transferred from one species to another. The two changes are always simultaneous.

A substance that donates protons (hydrogen ions) to another substance and produces hydrogen ions in aqueous solution. Acids turn blue litmus red and react with bases.

A substance that accepts protons (hydrogen ions). A soluble base is called an alkali and produces hydroxide ions in aqueous solution.

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

The reaction between an acid and a base to form a salt and water, in which hydrogen ions and hydroxide ions combine. The pH moves towards 7.

A compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion. Salts are ionic and often crystalline.

A weak acid or base that changes colour over a particular pH range, used to show whether a solution is acidic or alkaline or to detect the end point of a titration.

A technique in which a solution of known concentration is added from a burette to a measured volume of another solution until the reaction is complete, usually shown by an indicator.

The change in concentration of a reactant or product per unit time. It is increased by higher temperature, higher concentration, greater surface area or a catalyst.

A substance that increases the rate of a chemical reaction by providing an alternative pathway of lower activation energy, without being used up itself.

The minimum energy that colliding particles must possess for a reaction to occur. A catalyst lowers this value.

The idea that particles must collide with sufficient energy and correct orientation for a reaction to take place. Only effective collisions lead to products.

Describing a reaction that releases heat energy to the surroundings, so the temperature rises. The products have less energy than the reactants.

Describing a reaction that absorbs heat energy from the surroundings, so the temperature falls. The products have more energy than the reactants.

The heat energy transferred at constant pressure during a reaction, given the symbol delta H. It is negative for exothermic and positive for endothermic reactions.

A family of organic compounds with the same general formula and similar chemical properties, in which successive members differ by a CH2 unit.

An atom or group of atoms within a molecule that determines its characteristic chemical reactions, such as the hydroxyl group in alcohols or the double bond in alkenes.

One of two or more compounds that have the same molecular formula but different structural formulas, and therefore different physical or chemical properties.

A very large molecule formed when many small monomer units join together by addition or condensation polymerisation, such as polythene or nylon.

A small reactive molecule that can join with many identical or similar molecules to form a long-chain polymer.