JCE Chemistry key terms

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

A pure substance made of only one kind of atom, in which every atom has the same number of protons. Elements cannot be split into simpler substances by chemical means.

A pure substance formed when two or more different elements are chemically bonded together in fixed proportions. Its properties differ from those of the elements that make it up.

Two or more substances placed together without chemical bonding, in no fixed proportion. Each substance keeps its own properties, and the parts can usually be separated by physical methods such as filtration or distillation.

The number of protons in the nucleus of an atom, which identifies the element. In a neutral atom it equals the number of electrons, and it fixes the atom's position in the periodic table.

The total number of protons plus neutrons in the nucleus of an atom. Electrons are ignored because their mass is negligible, so it is always a whole number.

Atoms of the same element with the same number of protons but different numbers of neutrons, and therefore different mass numbers. Isotopes have identical chemical properties but may differ in physical properties and radioactivity.

The average mass of the atoms of an element compared with one-twelfth of the mass of a carbon-12 atom. It takes account of the abundance of each isotope and is not a whole number.

A region at a fixed distance from the nucleus where electrons are most likely to be found. Electrons fill the lowest available shell first, and the outer shell holds the electrons involved in bonding.

An electron in the outermost shell of an atom. The number of valence electrons, equal to the group number for main-group elements, controls how the atom reacts and bonds.

A vertical column of the periodic table. Elements in the same group have the same number of outer-shell electrons and so show similar chemical properties, with reactivity often changing steadily down the column.

A horizontal row of the periodic table. Across a period the number of protons increases by one from element to element while the outer shell stays the same, so metallic character falls and non-metallic character rises.

The strong electrostatic attraction between oppositely charged ions formed when a metal transfers electrons to a non-metal. The resulting giant lattice gives high melting points and conducts electricity only when molten or dissolved.

A bond formed when two atoms share one or more pairs of electrons. It is typical of non-metal elements and compounds and may produce simple molecules or giant covalent structures.

A network of many atoms joined by covalent bonds in a continuous three-dimensional lattice, as in diamond, graphite or silicon dioxide. Such substances have very high melting points and are usually insoluble and non-conducting.

One of two or more structural forms of the same element in the same physical state, such as diamond and graphite for carbon. Allotropes differ in arrangement and bonding, so their properties differ.

A measure of how strongly an atom attracts the shared pair of electrons in a covalent bond. It increases across a period and decreases down a group, and large differences lead to ionic character.

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

The mass of one mole of a substance, in grams per mole. Its numerical value equals the relative formula mass, so it links the mass weighed in the laboratory to the number of moles.

The sum of the relative atomic masses of all the atoms shown in the formula of a compound. It is used for ionic compounds and for molecules, where it is also called relative molecular mass.

The simplest whole-number ratio of the atoms of each element present in a compound. It is found from percentage composition or from masses in a reaction, and may differ from the molecular formula.

The reactant that is used up first in a reaction and therefore decides the maximum amount of product that can form. Any other reactant is in excess and remains partly unreacted.

The actual mass of product obtained expressed as a percentage of the maximum mass predicted by calculation. It is below 100 percent because of incomplete reactions, side reactions and loss during separation.

The relative formula mass of the desired product divided by the total relative formula mass of all reactants, multiplied by 100. It measures how much of the reactant atoms end up in the wanted product.

A substance that donates hydrogen ions, or protons, in solution. Acids turn litmus red, react with metals to release hydrogen and are neutralised by bases to give a salt and water.

A substance that accepts hydrogen ions, or protons. A soluble base is called an alkali; bases neutralise acids and turn litmus blue.

A scale from 0 to 14 expressing the concentration of hydrogen ions in a solution. Values below 7 are acidic, 7 is neutral, and above 7 is alkaline; each unit represents a tenfold change in hydrogen ion concentration.

The reaction in which an acid and a base react to form a salt and water, with the hydrogen ions of the acid combining with the hydroxide ions of the base. It releases heat and brings the pH towards 7.

An ionic compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion. The name of the salt depends on the acid used and on the positive ion involved.

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

A volumetric technique in which a solution of known concentration is added from a burette to a measured volume of another solution until the indicator shows the end point. It allows an unknown concentration to be calculated.

An insoluble solid that forms when two solutions are mixed and a reaction produces a product that cannot stay dissolved. Precipitates are used in tests to identify ions and can be removed by filtration.

A separation method that passes a mixture through filter paper, which traps insoluble solid particles while allowing the liquid or solution to run through. It separates an insoluble solid from a liquid.

A reaction that transfers energy to the surroundings, usually as heat, so the temperature of the surroundings rises. The products have less chemical energy than the reactants, giving a negative enthalpy change.

A reaction that takes in energy from the surroundings, so the temperature of the surroundings falls. The products have more chemical energy than the reactants and energy must be supplied for the reaction to occur.

The minimum energy that colliding particles must have for a reaction to take place. It corresponds to the energy barrier between reactants and products and is lowered by a catalyst.

The explanation that particles react only when they collide with at least the activation energy and in the correct orientation. Raising temperature, concentration or surface area increases the frequency of such successful collisions.

A substance that speeds up a reaction by providing an alternative pathway of lower activation energy, without being used up or changing the products. It is often specific and may be heterogeneous or homogeneous.

The breakdown of an ionic compound, molten or in aqueous solution, by passing a direct electric current through it. Positive ions move to the cathode and negative ions to the anode, where they are discharged.

Loss of electrons by a substance, or gain of oxygen, or loss of hydrogen. In terms of oxidation number it is an increase; it always occurs together with reduction in a redox reaction.

Gain of electrons by a substance, or loss of oxygen, or gain of hydrogen. In terms of oxidation number it is a decrease, and it is the opposite process to oxidation.

A reaction in which a more reactive element takes the place of a less reactive one in a compound. In metals it is used to confirm the order of the reactivity series.

A list of metals in order of how readily they lose electrons to form positive ions. It predicts reactions with water, acids and oxygen, and which metals can be extracted by heating with carbon.

A rock or mineral containing a metal compound in a concentration high enough to make extraction economically worthwhile. Ores are mined and then processed to obtain the metal.

A family of organic compounds with the same general formula and similar chemical properties, in which each member differs from the next by a CH2 unit. Members show a gradual change in physical properties.

An atom or group of atoms that gives an organic compound its characteristic reactions and properties, such as the hydroxyl group in alcohols or the carboxyl group in carboxylic acids.

One of two or more compounds with the same molecular formula but a different arrangement of atoms, and therefore different properties. Isomers may differ in the position of a functional group or in chain branching.

The joining of many unsaturated monomer molecules, each containing a carbon-carbon double bond, into a long saturated chain. No other product is formed, so all the atoms of the monomers appear in the polymer.

A separation technique that uses differences in boiling point. A mixture is heated and its components vaporise and condense at different heights in a column, giving separate fractions.

A separation method in which a mobile phase carries substances along a stationary phase. Components travel at different rates according to their solubility and attraction to the stationary phase, allowing identification by their positions.

A property of a substance or procedure that could cause harm, such as being corrosive, flammable or toxic. Hazards are communicated by hazard symbols and assessed before practical work begins.

The chance that a hazard will actually cause harm, taking into account how the substance is used and how much is involved. Risk is reduced by control measures such as goggles, fume cupboards or smaller quantities.