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Glossary: Atoms, elements, and ions
- A form of an element that has isotopic abundances that are different from the naturally occuring form. For example, "depleted" uranium has had most of the uranium-235 removed, and is an allobar of natural uranium.
- allotrope. allotropy; allotropic; allotropism. Compare with isotope and polymorph.
- Some elements occur in several distinct forms called allotropes. Allotropes have different chemical and physical properties. For example, graphite and diamond are allotropes of carbon.
- alpha particle. (42He)
- A particle that is commonly ejected from radioactive nuclei, consisting of two protons and two neutrons. Alpha particles are helium nuclei. Alpha particles have a mass of 6.644 655 98 × 10-27 kg or 4.001 506 1747 atomic mass units. [1998 CODATA values]
- alpha ray. (-ray) alpha radiation.
- A stream of alpha particles. Alpha rays rapidly dissipate their energy as they pass through materials, and are far less penetrating than beta particles and gamma rays.
- anion. Compare with cation.
- An anion is a negatively charged ion. Nonmetals typically form anions.
- anode. Compare with cathode.
- The electrode at which oxidation occurs in a cell. Anions migrate to the anode.
- atomic mass unit. (amu,u) amu; dalton.
- A unit of mass equal to 1/12 the mass of a carbon-12 nucleus, which is 1.660 538 73 × 10-27 kg ± 0.000 000 13 × 10-27 kg [1998 CODATA values]. Abbreviated as amu or u. Sometimes called the dalton, after John Dalton, architect of the first modern atomic theory.
- atomic nucleus. nucleus; nuclei; atomic nuclei.
- A tiny, incredibly dense positively charged mass at the heart of the atom. The nucleus is composed of protons and neutrons (and other particles). It contains almost all of the mass of the atom but occupies only a tiny fraction of the atom's volume.
- atomic number. (Z)
- The number of protons in an atomic nucleus. The atomic number and the element symbol are two alternate ways to label an element. In nuclide symbols, the atomic number is a leading subscript; for example, in 126C, the "6" is the atomic number.
- atomic theory.
- An explanation of chemical properties and processes that assumes that tiny particles called atoms are the ultimate building blocks of matter.
- atomic weight. atomic mass.
- The average mass of an atom of an element, usually expressed in atomic mass units. The terms mass and weight are used interchangeably in this case. The atomic weight given on the periodic table is a weighted average of isotopic masses found in a typical terrestrial sample of the element.
- atom. Compare with molecule and ion.
- An atom is the smallest particle of an element that retains the chemical properties of the element. Atoms are electrically neutral, with a positively charged nucleus that binds one or more electrons in motion around it.
- beta particle. (ß-)
- An electron emitted by an unstable nucleus, when a neutron decays into a proton and an electron. In some cases, beta radiation consists of positrons ("antielectrons" which are identical to electrons but carry a +1 charge.") Note that beta particles are created in nuclear decay; they do not exist as independent particles within the nucleus.
- Brownian motion. Brownian movement.
- Small particles suspended in liquid move spontaneously in a random fashion. The motion is caused by unbalanced impacts of molecules on the particle. Brownian motion provided strong circumstantial evidence for the existence of molecules.
- A device that separates isotopes (e. g. 235U from 238U) by ionizing the sample, accellerating the ions in a strong electric field, and then passing them through a strong magnetic field. The magnetic field bends the trajectories of the ions with high charge-to-mass ratio more, allowing ions to be separated by mass and collected.
- cathode ray.
- A negatively charged beam that emanates from the cathode of a discharge tube. Cathode rays are streams of electrons.
- cathode. Compare with anode.
- The electrode at which reduction occurs.
- cation. Compare with anion.
- A cation is a positively charged ion. Metals typically form cations.
- chemical change. reaction; chemical reaction. Compare with physical change.
- A chemical change is a dissociation, recombination, or rearrangement of atoms.
- compound Compare with element and mixture.
- A compound is a material formed from elements chemically combined in definite proportions by mass. For example, water is formed from chemically bound hydrogen and oxygen. Any pure water sample contains 2 g of hydrogen for every 16 g of oxygen.
- deuterium. (D, 2H)
- An isotope of hydrogen that contains one neutron and one proton in its nucleus.
- electric charge. charge.
- A property used to explain attractions and repulsions between certain objects. Two types of charge are possible: negative and positive. Objects with different charge attract; objects with the same charge repel each other.
- electron. (e-) Compare with proton and neutron.
- A fundamental consituent of matter, having a negative charge of 1.602 176 462 × 10-19 coulombs ± 0.000 000 063 × 10-19 coulombs and a mass of 9.109 381 88 × 10-31 kg ± 0.000 000 72 × 10-31 kg [1998 CODATA values].
- element Compare with compound and mixture.
- An element is a substance composed of atoms with identical atomic number. The older definition of element (an element is a pure substance that can't be decomposed chemically) was made obsolete by the discovery of isotopes.
- element symbol.
- An international abbreviation for element names, usually consisting of the first one or two distinctive letters in element name. Some symbols are abbreviations for ancient names.
- 1. A substructure that imparts characteristic chemical behaviors to a molecule, for example, a carboxylic acid group. (also: functional group). 2. A vertical column on the periodic table, for example, the halogens. Elements that belong to the same group usually show chemical similarities, although the element at the top of the group is usually atypical.
- heavy water. (D2O)
- Water that contains 2H, rather than 1H. Heavy water is about 11% denser than ordinary water.
- An atom or molecule that has acquired a charge by either gaining or losing electrons. An atom or molecule with missing electrons has a net positive charge and is called a cation; one with extra electrons has a net negative charge and is called an anion.
- isotope. isotopic; isotopy. Compare with isomer, allotrope, isobar, and isotone.
- Atoms or ions of an element with different numbers of neutronsin their atomic nucleus. Isotopes have the same atomic number but different mass number. Isotopes have very similar chemical properties but sometimes differ greatly in nuclear stability.
- isotopic abundance. Compare with natural abundance.
- The fraction of atoms of a given isotope in a sample of an element.
- isotopic mass. isotopic masses.
- The mass of a single atom of a given isotope, usually given in daltons.
- International Union of Pure and Applied Chemistry, an organization which sets international standards for chemical nomenclature, atomic weights, and the names of newly discovered elements.
- law of conservation of mass.
- There is no change in total mass during a chemical change. The demonstration of conservation of mass by Antoine Lavoisier in the late 18th century was a milestone in the development of modern chemistry.
- law of definite proportions.
- When two pure substances react to form a compound, they do so in a definite proportion by mass. For example, when water is formed from the reaction between hydrogen and oxygen, the 'definite proportion' is 1 g of H for every 8 g of O.
- law of multiple proportions.
- When one element can combine with another to form more than one compound, the mass ratios of the elements in the compounds are simple whole-number ratios of each other. For example, in CO and in CO2, the oxygen-to-carbon ratios are 16:12 and 32:12, respectively. Note that the second ratio is exactly twice the first, because there are exactly twice as many oxygens in CO2 per carbon as there are in CO.
- mass number. (M,A) Compare with atomic number and atomic weight.
- The total number of protons and neutrons in an atom or ion. In nuclide symbols the mass number is given as a leading superscript. In isotope names (e. g. carbon-14, sodium-23) the mass number is the number following the element name.
- mass spectrometer.
- An instrument that measures the masses and relative abundances of a sample that has been vaporized and ionized.
- mass spectrometry. mass spectroscopy.
- (of elements) A method for experimentally determining isotopic masses and isotopic abundances. A sample of an element is converted into a stream of ions and passed through an electromagnetic field. Ions with different charge-to-mass ratios are deflected by different amounts, and strike different spots on a film plate or other detector. From the position of the spots, the mass of the ions can be determined; from the intensity of the spot, the relative number of ions (the isotopic abundance) can be determined.
- mass spectrum. mass spectra.
- A plot showing the results of a mass spectrometry experiment, which shows the presence of particles with different masses as a series of sharp, separate peaks. The position of the peaks on the x-axis indicates the mass of the particles; the peak heights indicate the relative abundance of the particles.
- metal. metallic. Compare with nonmetal and metalloid.
- A metal is a substance that conducts heat and electricity, is shiny and reflects many colors of light, and can be hammered into sheets or drawn into wire. Metals lose electrons easily to form cations. About 80% of the known chemical elements are metals.
- natural abundance. Compare with isotopic abundance.
- The average fraction of atoms of a given isotope of an element on Earth.
- 1. having no net electrical charge. Atoms are electrically neutral; ions are not. 2. A solution containing equal concentrations of H+ and OH-.
- neutron. (n, 10n) Compare with proton and electron.
- An elementary particle found the atomic nucleus of all stable atoms except the hydrogen-1 atom. Neutrons have no charge and have a mass of 1.008665 daltons.
- nonmetal. (metal,metalloid) non-metal.
- A nonmetal is a substance that conducts heat and electricity poorly, is brittle or waxy or gaseous, and cannot be hammered into sheets or drawn into wire. Nonmetals gain electrons easily to form anions. About 20% of the known chemical elements are nonmetals.
- nuclear binding energy.
- Energy needed to break an atomic nucleus into separate protons and neutrons.
- nucleon. Compare with proton, neutron and atomic nucleus.
- A proton or a neutron in the atomic nucleus.
- nuclide symbol. Compare with atomic nucleus, nuclide and element symbol.
- A symbol for an nuclide that contains the mass number as a leading superscript and the atomic number as a leading subscript. For ions, the ionic charge is given as a trailing superscript. For example, the nuclide symbol for the most common form of the chloride ion is 3517Cl-, where 35 is the mass number, 17 is the atomic number, and the charge on the ion is -1. The atomic number is sometimes omitted from nuclide symbols.
- nuclide. Compare with atomic nucleus and nuclide symbol.
- An atom or ion with a specified mass number and atomic number. For example, uranium-235 and carbon-14 are nuclides.
- periodic table.
- An arrangement of the elements according to increasing atomic number that shows relationships between element properties.
- Rows in the periodic table are called periods. For example, all of the elements in the second row are referred to as 'second period elements'. All elements currently known fall in the first seven periods.
- polymorph. polymorphism; polymorphic. Compare with isotope and allotrope.
- Solid substances that occur in several distinct forms. Polymorphs have different chemical and physical properties. allotropes are polymorphs of elements.
- proton. (p+) Compare with electron and neutron.
- An elementary particle found the atomic nucleus with a positive charge equal and opposite that of the electron. Protons have a mass of 1.007276 daltons.
- radioactivity. radiation; radioactive.
- Spontaneous emission of particles or high-energy electromagnetic radiation from the nuclei of unstable atoms. "Radiation" refers to the emissions, and "radioactive source" refers to the source of the radiation.
- 1. Ratios of atoms in a compound. 2. Ratios of moles of compounds in a reaction. 3. A branch of chemistry that quantitatively relates amounts of elements and compounds involved in chemical reactions, based on the law of conservation of mass and the law of definite proportions.
- x-ray spectrum. x-ray spectra.
- A set of characteristic x-ray frequencies or wavelengths produced by a substance used as a target in an x-ray tube. Each element has a characteristic x-ray spectrum, and there is a strong correlation between atomic number and the frequencies of certain lines in the x-ray spectrum.