The temperature, defined as 0K on the Kelvin scale, at which the motion of molecules in a solid virtually ceases. Absolute zero is equal to −459.67°F (−273.15°C).
The smallest particle of an element that retains the chemical and physical properties of the element. An atom can exist either alone or in combination with other atoms in a molecule. Atoms are madeup of protons, neutrons, and electrons. Anatom that loses or gains one or more electrons, and thus has a net charge, is an ion. Atoms that have the same number of protons—that is, are of the same element—but differ in number of neutrons, are known as isotopes.
An SI unit (abbreviated amu), equal to 1.66 · 10 24 g, for measuring the mass of atoms.
The number of protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table in order of atomic number.
A form of crystalline solid in which atoms of one element bond to one another. Examples include diamonds (made of pure carbon), silicon, and all metals.
A figure used by chemists to specify the mass—in atomic mass units—of the average atom in a large sample. If a substance is a compound, the average atomic mass of all atoms in a molecule of that substance must be added together to yield the average molecular mass of that substance.
A figure, named after Italian physicist Amedeo Avogadro (1776-1856), equal to 6.022137× 10 23 . Avogadro's number indicates the number of atoms, molecules, or other elementary particles in a mole.
The transition from one phase of matter to another.
A coordinate, plotted on a phase diagram, above which a substance cannot exist in anything other than the gaseous state. Located at a position of very high temperature and pressure, the critical point marks the termination of the vaporization curve.
A substance made up of atoms of more than one element. These atoms are usually, but not always, joined inmolecules.
A type of solid in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions. Types of crystalline solids include atomicsolids, ionic solids, and molecular solids.
A substance made up of only one kind of atom.
A negatively charged particle in an atom. Electrons, which spin around the protons and neutrons that make up the atom's nucleus, constitute a very small portion of the atom's mass. In most atoms, the number of electrons and protons is the same, thus canceling out one another. When an atom loses one or more electrons, however—thus becoming anion—it acquires a net electric charge.
The ability to accomplishwork—that is, the exertion of force over a given distance to displace or move an object.
The boundary between solid and liquid for any given substance as plotted on a phase diagram.
A phase of matter in which molecules move at high speeds, and therefore exert little or no attraction toward one another.
An atom or atoms that has lost or gained one or more electrons, and thus has a net electric charge.
A form of crystalline solid that contains ions. When mixed with a solvent such as water, ions from table salt—an example of an ionic solid—move freely throughout the solution, making it possible to conduct an electric current.
Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons.
The energy that an object possesses by virtue of its movement.
A phase of matter in which molecules move at moderate speeds, and therefore exert moderate attractions toward one another.
Physical substance that occupies space, has mass, is composed of atoms (or in the case of subatomic particles, is part of an atom), and is convertible to energy.
The amount of energy required to melt 1 mole of a solid substance.
The amount of energy required to turn 1 mole of a liquid into a gas. Because gases possess much more energy than liquids or solids, molar heat of vaporization is usually much higher than molar heat of fusion.
The mass, in grams, of 1 mole of a given substance. The value in grams of molar mass is always equal to the value, in atomic mass units, of the average atomic mass of that substance: thus carbon has a molar mass of 12.01 g, and anaverage atomic mass of 12.01 amu.
The SI fundamental unit for "amount of substance." A mole is, generally speaking, Avogadro's number of atoms, molecules, or other elementary particles; however, in the more precise SI definition, a mole is equal to the number of carbon atoms in 12.01 g of carbon.
A form of crystalline solid in which the molecules have a neutral electric charge, meaning that there are no ions present as there are in an ionicsolid. Table sugar (sucrose) is an example of a molecular solid. When mixed with a solvent such as water, the resulting solution does not conduct electricity.
A group of atoms, usually of more than one element, joined in astructure.
A subatomic particle that has no electric charge. Neutrons are found at the nucleus of an atom, alongside protons.
The center of an atom, a region where protons and neutrons are located, and around which electrons spin.
A chart, plotted for any particular substance, identifying the particular phase of matter for that substance at a given temperature and pressure level. A phase diagram usually shows temperature along the x-axis, and pressure along the y-axis.
The various forms of material substance (matter), which are defined primarily in terms of the behavior exhibited by their atomic or molecular structures. On Earth, three principal phases of matter exist, namely solid, liquid, and gas. Other forms of matter include plasma.
One of the phases of matter, closely related to gas. Plasma is found primarily in stars. Containing neither atoms nor molecules, plasma is made up of electrons and positive ions.
A positively charged particle in an atom. Protons and neutrons, which together form the nucleus around which electrons spin, have approximately the same mass—a mass that is many times greater than that of an electron. The number of protons in the nucleus of an atom is the atomic number of an element.
A phase of matter in which molecules move slowly relative to one another, and therefore exert strong attractions toward one another.
The boundary between solid and gas for any given substance, as plotted on a phase diagram.
In chemistry and other sciences, the term "system" usually refers to any set of interactions isolated from the rest of the universe. Anything outside of the system, including all factors and forces irrelevant to a discussion of that system, is known as the environment.
"Heat" energy, a form of kinetic energy produced by the relative motion of atoms or molecules. The greater the movement of these particles, the greater the thermal energy.
The boundary between liquid and gas for any given substance as plotted on a phase diagram.