Bulgaria DZI Chemistry key terms

The number of protons in the nucleus of an atom, which uniquely identifies a chemical element and determines its position in the periodic table.

Atoms of the same element that have the same number of protons but different numbers of neutrons, giving them different mass numbers but nearly identical chemical properties.

The principle that the properties of elements vary periodically with their atomic number, so elements with similar outer-electron configurations appear at regular intervals in the periodic table.

A measure of the tendency of an atom in a chemical bond to attract the shared electrons toward itself, generally increasing across a period and decreasing down a group.

The minimum energy required to remove one electron from a gaseous atom or ion in its ground state, providing evidence for the shell structure of atoms.

The energy change when a gaseous atom gains an electron to form a singly charged negative ion; a more negative value indicates a greater tendency to accept electrons.

The weighted average mass of the naturally occurring isotopes of an element compared with one-twelfth the mass of a carbon-12 atom.

The electrostatic attraction between oppositely charged ions formed when one atom transfers one or more electrons to another, typically between a metal and a nonmetal.

A chemical bond formed by the sharing of one or more pairs of electrons between two atoms, usually between nonmetal atoms.

A covalent bond in which the shared electrons are unequally attracted by the two atoms because of a difference in electronegativity, creating partial positive and negative charges.

A relatively strong intermolecular attraction between a hydrogen atom bonded to a highly electronegative atom (such as O, N or F) and a lone pair on another electronegative atom.

The energy released when one mole of a solid ionic compound is formed from its constituent gaseous ions, or the energy needed to separate them; it reflects ionic bond strength.

The amount of substance that contains as many elementary entities as there are atoms in exactly 12 grams of carbon-12, namely Avogadro's number of particles.

The mass of one mole of a substance, expressed in grams per mole, numerically equal to its relative atomic or molecular mass.

The quantitative relationship between the amounts of reactants and products in a balanced chemical equation, used to calculate masses, volumes or concentrations.

The reactant that is completely consumed first in a chemical reaction and therefore determines the maximum amount of product that can be formed.

A substance that, when dissolved in water or melted, dissociates into ions and conducts an electric current, such as acids, bases and salts.

The splitting of a compound into ions when it dissolves in water or melts, allowing the resulting solution or melt to conduct electricity.

A logarithmic scale from 0 to 14 used to express the acidity or alkalinity of an aqueous solution, defined as the negative logarithm of the hydrogen ion concentration.

A solution that resists large changes in pH when small amounts of acid or base are added, usually containing a weak acid and its conjugate base or a weak base and its conjugate acid.

The existence of an element in two or more different structural forms in the same physical state, such as diamond and graphite for carbon or oxygen and ozone.

A substance that accepts electrons from another substance in a redox reaction, causing the other substance to be oxidised while itself being reduced.

A substance that donates electrons to another substance in a redox reaction, causing the other substance to be reduced while itself being oxidised.

A redox reaction in which the same element simultaneously undergoes oxidation and reduction, for example chlorine reacting with cold dilute alkali.

A hydroxide that can react both with acids to form salts and with strong bases to form complex ions, typical of certain metals such as aluminium and zinc.

An ion consisting of a central metal atom or ion bonded to one or more ligands by coordinate covalent bonds, such as the tetraamminecopper(II) ion.

A molecule or ion that donates one or more lone pairs of electrons to a central metal ion to form a coordinate bond in a complex.

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

The existence of two or more compounds with the same molecular formula but different arrangements of atoms, leading to different physical or chemical properties.

A specific group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule, such as the hydroxyl or carboxyl group.

The reaction between a carboxylic acid and an alcohol, usually catalysed by a strong acid, to produce an ester and water; it is reversible.

A reaction in which a compound is broken down by water, such as the hydrolysis of an ester to an alcohol and a carboxylic acid or of a salt to an acidic or alkaline solution.

The heat energy transferred at constant pressure during a chemical reaction, given the symbol ΔH; negative for exothermic and positive for endothermic reactions.

The minimum extra energy that colliding particles must possess for a reaction to occur, corresponding to the energy barrier between reactants and products.

A substance that increases the rate of a chemical reaction by providing an alternative pathway with lower activation energy, without being consumed in the overall reaction.

A dynamic state in a reversible reaction in which the rates of the forward and reverse reactions are equal, so the concentrations of reactants and products remain constant.

When a system at equilibrium is subjected to a change in concentration, pressure or temperature, the equilibrium shifts in the direction that opposes the change.

The total enthalpy change of a chemical reaction is independent of the route by which the reaction takes place, provided the initial and final conditions are the same.