Chemistry terms in Models of the particulate nature of matter

The lowest possible temperature, $0\text{ K}$ ($-273.15{}^{\circ }\text{C}$), at which particles have their minimum possible kinetic energy.

A continuous spectrum crossed by dark lines at the wavelengths absorbed by atoms as their electrons are promoted to higher energy levels.

A helium nucleus, $\mathrm{H}\mathrm{e}{\phantom{X}}^{2+}$, of two protons and two neutrons, emitted by some radioactive nuclei; used in the gold-foil experiment.

A measure of the number of specified particles in a sample, symbol $n$, measured in moles.

Containing no water; an anhydrous salt is the form left when the water of crystallization has been removed, such as white $\mathrm{C}\mathrm{u}\mathrm{S}\mathrm{O}{\phantom{X}}_4$.

A negatively charged ion, with more electrons than protons; non-metals form anions by gaining electrons.

The number of protons in the nucleus of an atom, symbol $Z$; it identifies the element and gives its position in the periodic table.

A region of space around the nucleus with a high probability of finding an electron; each orbital holds a maximum of two electrons of opposite spin.

Electrons occupy the orbitals of lowest available energy first; up to Z = 36 the order of filling is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.

The number of elementary entities in one mole, $N_{\text{A}}=6.02\times {10}^{23}{\text{mol}}^{-1}$ (fixed exactly at $6.02214076\times {10}^{23}{\text{mol}}^{-1}$).

Equal volumes of all gases, measured at the same temperature and pressure, contain equal numbers of molecules.

The tallest peak in a mass spectrum, from the most abundant ion, assigned a relative abundance of 100.

One of the four regions of the periodic table (s, p, d and f), named after the sublevel that receives the last electron in the Aufbau order.

Vaporization throughout a liquid, with bubbles of vapour forming, at the temperature where the vapour pressure of the liquid equals the external pressure.

The temperature at which a liquid boils at a given external pressure; data tables quote it at 101.325 kPa.

A graph of a measured property (such as absorbance) against the known concentrations of standard solutions, used to read off the concentration of an unknown sample.

A positively charged ion, with fewer electrons than protons; metals form cations by losing electrons.

A technique that separates the components of a mixture by their different distribution between a stationary phase and a mobile phase that moves through or over it.

For a fixed amount of gas, $P_1{V}_1\mathrm{/}{T}_1=P_2{V}_2\mathrm{/}{T}_2$, with temperatures in kelvin.

Finding the empirical formula of an organic compound by burning a weighed sample in excess oxygen and weighing the carbon dioxide and water formed; oxygen is found by difference.

A substance made of atoms of two or more different elements chemically bonded together in a fixed ratio; its properties differ from those of its elements.

The change of state from gas to liquid, which releases energy (exothermic).

An electron configuration in which the inner electrons are written as the preceding noble gas in square brackets, for example $[\text{Ar}]{\text{4s}}^1$ for potassium.

A spectrum containing all wavelengths over a range with no gaps, such as white light from a filament lamp passed through a prism.

The narrowing of the spacing between successive lines of a spectral series at higher frequency, until they merge, because the energy levels get closer together at higher energy.

The frequency at which the lines of a spectral series merge; for the series ending on the ground state it corresponds to ionization of the atom.

A graph of temperature against time for a substance as it cools; a pure substance gives a flat plateau at its freezing point, an impure one a gradual slope.

The formation of crystals of a solute from a saturated solution, usually by evaporating some solvent and then cooling slowly.

The change of state directly from gas to solid, which releases energy (exothermic), for example $\mathrm{I}{\phantom{X}}_2(\mathrm{g})\stackrel{}{\to }\mathrm{I}{\phantom{X}}_2(\mathrm{s})$.

The isotope of hydrogen with one proton and one neutron in its nucleus, ${\phantom{X}}_{\phantom{1}}^{\phantom{2}}{\phantom{X}}_{\phantom{2}1}^{\phantom{2}2}\mathrm{H}$ or D; heavy water is $\mathrm{D}{\phantom{X}}_2\mathrm{O}$.

The net movement of particles from a region of higher concentration to one of lower concentration, caused by their random motion.

Adding solvent to a solution. The amount of solute stays the same, so $C_1{V}_1=C_2{V}_2$.

Two quantities are directly proportional when doubling one doubles the other; a graph of one against the other is a straight line through the origin.

Separation of a liquid from a mixture by boiling it and condensing the vapour in a separate container; used for a solvent from dissolved solids or liquids with very different boiling points.

A collision in which no kinetic energy is lost; the total kinetic energy of the particles is the same before and after.

The full range of electromagnetic radiation, from gamma rays (shortest wavelength, highest frequency and photon energy) through X-rays, ultraviolet, visible light, infrared and microwaves to radio waves (longest wavelength).

A subatomic particle with a relative charge of −1 and a negligible mass (about 1/1836 of a proton), occupying the space around the nucleus.

The arrangement of the electrons of an atom or ion in sublevels, written with superscripts for the number of electrons in each, for example ${\text{1s}}^2{\text{2s}}^2{\text{2p}}^6{\text{3s}}^1$ for sodium.

A property of an electron with two possible values, drawn as up and down arrows; two electrons in the same orbital must have opposite spins.

A substance made of only one kind of atom (all with the same atomic number), which cannot be broken down into simpler substances by chemical reactions.

The particle being counted in a mole: an atom, molecule, ion, electron, formula unit or other specified particle or group of particles.

The light emitted by excited atoms, seen as bright lines of specific wavelengths on a dark background; each line comes from electrons falling between two energy levels.

A formula that gives the simplest whole-number ratio of the atoms of each element in a compound.

Describes a change that absorbs energy from the surroundings, such as melting, vaporization or sublimation; the surroundings get colder and $\mathrm{\Delta }H$ is positive.

One of the fixed (quantized) energies that an electron in an atom can have; electrons can move between levels but cannot have energies in between.

Vaporization from the surface of a liquid at any temperature below its boiling point, as the most energetic particles escape; it cools the remaining liquid. Also used to remove a solvent from a solution.

An arrangement of electrons with more energy than the ground state, in which one or more electrons have been promoted to a higher energy level.

Describes a change that releases energy to the surroundings, such as freezing, condensation or deposition; the surroundings get warmer and $\mathrm{\Delta }H$ is negative.

Extending a line or trend on a graph beyond the measured data, for example extending a cooling line back to the time of mixing to correct for heat loss.

The liquid or solution that passes through the filter paper during filtration.

Separation of an insoluble solid from a liquid or solution by passing the mixture through filter paper, which holds back the solid particles.

A test in which a compound is heated in a non-luminous flame; the colour of the flame, produced by the emission spectrum of the metal, identifies metal ions such as lithium, sodium, potassium, calcium and copper.

The group of ions or atoms given by the formula of a substance that is not made of molecules, such as one $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$ and one $\mathrm{C}\mathrm{l}{\phantom{X}}^{-}$ in $\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}$.

Distillation through a packed fractionating column, in which vapour repeatedly condenses and re-evaporates, to separate miscible liquids with similar boiling points; the most volatile component distils first.

The change of state from liquid to solid, which releases energy (exothermic).

The number of waves passing a fixed point per second, symbol $f$, in ${\text{s}}^{-1}$ or Hz; it is inversely proportional to wavelength, $c=f\lambda$.

The constant $R$ in the ideal gas equation, $8.31{\text{J K}}^{-1}{\text{mol}}^{-1}$.

The lowest-energy arrangement of the electrons in an atom or ion.

A graph of temperature against time for a substance heated at a constant rate; the flat parts (plateaus) show changes of state, during which the temperature is constant.

Repeated heating, cooling and weighing of a sample until two consecutive masses agree, showing that a change such as the loss of water is complete.

A mixture whose composition is not uniform, with two or more regions (phases) of different composition, for example sand in water or oil and water.

A mixture with a uniform composition and properties throughout (one phase), so that any sample has the same ratio of components, for example a salt solution, air or brass.

Electrons occupy the orbitals of a sublevel singly, with parallel spins, before any orbital gains a second electron.

A salt whose crystals contain water of crystallization, such as blue copper(II) sulfate pentahydrate, $\mathrm{C}\mathrm{u}\mathrm{S}\mathrm{O}{\phantom{X}}_4\cdot 5\mathrm{H}{\phantom{X}}_2\mathrm{O}$.

A model gas whose particles are in continuous random motion, have negligible volume, exert no intermolecular forces and collide perfectly elastically; it obeys $PV=nRT$ exactly.

$PV=nRT$: the relationship between the pressure (Pa), volume (${\text{m}}^3$), amount (mol) and absolute temperature (K) of an ideal gas.

Describes liquids that do not mix and form separate layers, such as oil and water.

Forces of attraction between separate molecules (or atoms of a noble gas), such as London (dispersion) forces, dipole–dipole forces and hydrogen bonds; much weaker than covalent bonds.

Reading a value from a graph between the measured data points, within the range over which the relationship has been tested.

Two quantities are inversely proportional when doubling one halves the other, so their product is constant; a graph of one against the reciprocal of the other is a straight line through the origin.

A charged particle formed when an atom or a group of atoms loses or gains electrons; the number of protons does not change.

The complete removal of one or more electrons from an atom or ion, forming a positive ion; drawn on an energy-level diagram as an upward arrow to $n=\mathrm{\infty }$.

Describes atoms or ions with the same number of electrons, and so the same electron configuration, such as $\mathrm{O}{\phantom{X}}^{2-}$, $\mathrm{F}{\phantom{X}}^{-}$, $\mathrm{N}\mathrm{e}$ and $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$.

Atoms of the same element (same number of protons) with different numbers of neutrons, and so different mass numbers; they have the same chemical properties but different physical properties.

An isotope, often a heavier or radioactive one, used to follow atoms through a reaction or process, because it reacts identically but can be detected by its mass or radiation.

The SI unit of temperature. The Kelvin scale starts at absolute zero and has degrees of the same size as Celsius degrees: $T\mathrm{/}\text{K}=\theta {\mathrm{/}}^{\circ }\text{C}+273.15$.

The energy a particle has because of its motion, $E_{\text{k}}=\frac{1}{2}m{v}^2$. The average kinetic energy of the particles in a substance is proportional to its absolute temperature.

A model in which all matter is made of particles in constant random motion, with attractions between them; it explains the properties of solids, liquids and gases and changes of state.

A spectrum containing only certain separate wavelengths, seen as sharp lines; the pattern of lines is characteristic of each element.

A scale on which the logarithm of a quantity is plotted, so that equal steps represent equal factors (such as ×10); used for data spanning several orders of magnitude, such as ionization energies.

A group of electron energy states with the same principal quantum number $n$; level $n$ holds a maximum of $2n^2$ electrons.

The mass of solute per unit volume of solution, in ${\text{g dm}}^{-3}$; equal to the molar concentration multiplied by the molar mass.

The total number of protons and neutrons in the nucleus of an atom, symbol $A$; always a whole number.

A technique that ionizes the particles of a sample and separates the ions by their mass-to-charge ratio; it is used to detect the isotopes of an element and measure their relative abundances and isotope ratios.

The output of a mass spectrometer: relative abundance against mass-to-charge ratio, $m\mathrm{/}z$. For an element each peak is one isotope and its height is the relative abundance.

The quantity $m\mathrm{/}z$ plotted on the horizontal axis of a mass spectrum; for ions with a charge of 1+ it equals the mass of the ion.

The change of state from solid to liquid, which absorbs energy (endothermic).

The temperature at which a solid turns into a liquid; a pure substance melts sharply at a fixed melting point, an impure one over a range of lower temperatures.

The curved surface of a liquid in a narrow tube. For water and aqueous solutions the volume is read at the bottom of the curve, with the eye level with the mark.

Describes liquids that mix completely in any proportion to form a single phase, such as ethanol and water.

Two or more elements or compounds that are not chemically bonded together and are present in no fixed ratio; each component keeps its own properties and they can be separated by physical methods.

The phase in chromatography that moves through or over the stationary phase, carrying the components of the mixture with it, usually a solvent or a gas.

The amount of solute per unit volume of solution, $C=n\mathrm{/}V$, in ${\text{mol dm}}^{-3}$; written with square brackets, e.g. $[\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}]$.

The mass of one mole of a substance, symbol $M$, in ${\text{g mol}}^{-1}$; numerically equal to its relative formula mass. $n=m\mathrm{/}M$.

The volume occupied by one mole of a gas, $V_{\text{m}}$; for an ideal gas at STP it is $22.7{\text{dm}}^3{\text{mol}}^{-1}$.

The SI unit of amount of substance. One mole contains exactly $6.02214076\times {10}^{23}$ elementary entities (the Avogadro constant).

A formula that gives the actual number of atoms of each element in one molecule of a compound; a whole-number multiple of the empirical formula.

The positive ion formed when a whole molecule loses one electron in a mass spectrometer, such as $\mathrm{C}\mathrm{l}{\phantom{X}}_2{\phantom{X}}^{+}$ or $\mathrm{M}{\phantom{X}}^{+}$; its peak is at an $m\mathrm{/}z$ equal to the relative molecular mass of that molecule.

An uncharged subatomic particle in the nucleus with a relative mass of 1.

The total positive charge of the nucleus, equal to the number of protons; a greater nuclear charge attracts the electrons more strongly.

The model of the atom as a tiny, dense, positively charged nucleus containing almost all the mass, with electrons occupying the much larger space around it; proposed by Rutherford in 1911 from alpha-particle scattering.

The notation ${}_Z^A\text{X}$ for an atom or ion, with the mass number at the top left, the atomic number at the bottom left and any charge at the top right.

A particle in the nucleus: a proton or a neutron.

The very small, dense, positively charged centre of an atom, containing its protons and neutrons and almost all of its mass.

A diagram showing each orbital as a box and each electron as an arrow whose direction shows its spin, arranged in order of sublevel energy.

A dumbbell-shaped atomic orbital with two lobes on opposite sides of the nucleus; a p sublevel has three, lying along the x, y and z axes.

An error in reading a scale caused by viewing it from an angle rather than straight on, so the reading appears higher or lower than it is.

The SI unit of pressure, equal to a force of one newton acting on one square metre: $1\text{ Pa}=1{\text{N m}}^{-2}$.

An orbital can hold a maximum of two electrons, and they must have opposite spins.

The mass of each element in 100 g of a compound: the number of atoms of the element in the formula times its $A_{\text{r}}$, divided by the $M_{\text{r}}$ of the compound, times 100.

A region of a sample with uniform composition and properties, separated from other regions by a boundary.

A single packet of electromagnetic energy; its energy is proportional to the frequency of the radiation ($E=hf$).

The constant $h=6.63\times {10}^{-34}\text{ J s}$ linking the energy of a photon to the frequency of the radiation, $E=hf$.

Force per unit area, $P=F\mathrm{/}A$, measured in pascals ($1\text{ Pa}=1{\text{N m}}^{-2}$). In a gas it results from collisions of particles with the walls of the container.

A subatomic particle in the nucleus with a relative mass of 1 and a relative charge of +1; the number of protons is the atomic number.

A single element or compound, containing only one kind of particle, with a fixed composition and sharp, fixed melting and boiling points.

An isotope with an unstable nucleus that decays, emitting radiation, such as carbon-14.

An actual gas, whose particles have a finite volume and attract one another; it deviates from ideal behaviour most at high pressure and low temperature.

Purification of a solid by dissolving it in the minimum volume of hot solvent, cooling so that the pure compound crystallizes while impurities stay dissolved, then filtering, washing with cold solvent and drying.

The weighted mean mass of the atoms of an element, taking into account the abundance of each isotope, compared with one-twelfth of the mass of an atom of carbon-12. Symbol $A_{\text{r}}$; it has no units.

The sum of the relative atomic masses of all the atoms in the formula of a substance. Symbol $M_{\text{r}}$; it has no units.

The mass of an atom of one isotope relative to one-twelfth of the mass of an atom of carbon-12; very close to its mass number.

The relative formula mass of a substance made of molecules: the sum of the relative atomic masses of the atoms in one molecule. It has no units.

The insoluble solid left on the filter paper after filtration.

In chromatography, the distance moved by a component divided by the distance moved by the solvent front, both measured from the baseline; always less than 1.

A spherical atomic orbital centred on the nucleus; each main energy level has one s orbital, and its size increases with $n$.

A series of dilutions, each by the same factor, used to make very dilute solutions or a set of standards accurately.

The substance that dissolves in a solvent to form a solution.

A homogeneous mixture formed when a solute dissolves in a solvent; a solution in water is an aqueous solution, shown by (aq).

The process in which solute particles become surrounded by solvent particles and spread through the solvent; used to separate a soluble component of a mixture from an insoluble one.

The substance, usually a liquid, in which a solute dissolves to form a solution.

An instrument that uses a slit and a prism or diffraction grating to separate light into its wavelengths, so that the lines of a spectrum can be seen and their wavelengths measured.

The reference conditions 298.15 K (25 °C) and 100 kPa; the molar volume of an ideal gas at SATP is about $24.8{\text{dm}}^3{\text{mol}}^{-1}$.

A solution whose concentration is accurately known, made by dissolving an accurately weighed pure solute and making it up to an exact volume in a volumetric flask.

The reference conditions 273.15 K (0 °C) and 100 kPa, at which the molar volume of an ideal gas is $22.7{\text{dm}}^3{\text{mol}}^{-1}$.

A symbol in brackets after a formula showing the state of the substance: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.

The phase in chromatography that does not move, such as the water held in the fibres of chromatography paper or the silica layer on a TLC plate.

The very strong, very short-range attraction between nucleons that holds the nucleus together despite the repulsion between its protons.

A particle smaller than an atom; in chemistry, the proton, neutron and electron.

A group of orbitals of equal energy within a main energy level, labelled s, p, d or f, holding a maximum of 2, 6, 10 or 14 electrons.

The change of state directly from solid to gas without melting, which absorbs energy (endothermic), for example $\mathrm{C}\mathrm{O}{\phantom{X}}_2(\mathrm{s})\stackrel{}{\to }\mathrm{C}\mathrm{O}{\phantom{X}}_2(\mathrm{g})$.

The energy needed to remove each further mole of electrons, one at a time, from one mole of gaseous ions, for example $\mathrm{X}{\phantom{X}}^{2+}(\mathrm{g})\stackrel{}{\to }\mathrm{X}{\phantom{X}}^{3+}(\mathrm{g})+\mathrm{e}{\phantom{X}}^{-}$ for the third.

An error that shifts every measurement in the same direction, such as heat loss in calorimetry; repeating the measurement does not reduce it.

A measure of the average kinetic energy of the particles in a substance, measured in kelvin or degrees Celsius; it does not depend on the amount of substance. The absolute temperature in kelvin is proportional to the average kinetic energy.

An electron that occupies an orbital on its own, without a second electron of opposite spin.

An electron in the outermost main energy level of an atom (for transition elements, the 3d electrons too); valence electrons take part in bonding and reactions.

The change of state from liquid to gas, by evaporation at the surface or by boiling throughout the liquid; it absorbs energy (endothermic).

The pressure exerted by the vapour of a liquid in equilibrium with the liquid in a closed container at a given temperature; it increases with temperature.

A flask with a long narrow neck and a single graduation mark, calibrated to contain one precise volume of solution at a stated temperature.

A pipette with a single graduation mark, calibrated to deliver one precise volume of liquid, such as $25.00{\text{cm}}^3$.

Water molecules held in fixed proportion within the crystal lattice of a salt, shown after a dot in the formula, as in $\mathrm{C}\mathrm{u}\mathrm{S}\mathrm{O}{\phantom{X}}_4\cdot 5\mathrm{H}{\phantom{X}}_2\mathrm{O}$.

The distance between two successive crests (or other identical points) of a wave, symbol $\lambda$; for electromagnetic radiation, a shorter wavelength means a higher frequency and photon energy.

Finding the mass of solid transferred as the difference between the masses of its container before and after transfer, so solid left in the container causes no error.