Elements - Key terms



ANION:

The negative ion that results when an atom gains one or more electrons. An anion (pronounced "AN-ie-un") of an element is never called, for instance, the chlorine anion. Rather, for an anion involving a single element, it is named by adding the suffix -ide to the name of the original element—hence, "chloride." Other rules apply for more complex anions.

ATOM:

The smallest particle of an element. An atom can exist either alone or in combination with other atoms in a molecule. Atoms are made up of protons, neutrons, and electrons. An atom that loses orgains 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.

ATOMIC MASS UNIT:

An SI unit (abbreviated amu), equal to 1.66 · 10 −24 g, for measuring the mass of atoms.

ATOMIC NUMBER:

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.

AVERAGE ATOMIC MASS:

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.

CATION:

The positive ion that results when an atom loses one or more electrons. A cation (pronounced KAT-ie-un) is named after the element of which it is anion and thus is called, for instance, the aluminum ion or the aluminum cation.

CHEMICAL SYMBOL:

Another term for element symbol.

COMPOUND:

A substance made up of atoms of more than one element. These atoms are usually joined in molecules.

DIATOMIC:

A term describing an element that exists as molecules composed of two atoms. This is in contrast to monatomic elements.

ELECTRON:

Negatively charged particles 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. The number of electrons and protons is the same, thus canceling out one another. But when an atom loses or gains one or more electrons, however—thus becoming anion—it acquires a net electric charge.

ELEMENT:

A substance made up of only one kind of atom. Unlike compounds, elements cannot be chemically broken into other substances.

ELEMENT SYMBOL:

A one-or two-letter abbreviation for the name of an element. These may be a single capitalized letter (O for oxygen), or a capitalized letter followed by a lowercase one (Ne for neon). Sometimes the second letter is not the second letter of the element name (for example, Pt for platinum). In addition, the symbol may refer to an original Greek or Latinname, rather than the name used now, is the case with Au (aurum) for gold.

ION:

An atom that has lost or gained one or more electrons, and thus has a net electric charge.

ISOTOPES:

Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons.

MASS NUMBER:

The sum of protons and neutrons in an atom's nucleus. Where an isotope is represented, the mass number is placed above the atomic number to the left of the element symbol.

MOLECULE:

A group of atoms, usu-allybut not always representing more than one element, joined in a structure. Compounds are typically made of up molecules.

MONATOMIC:

A term describing an element that exists as single atoms. This in contrast to diatomic elements.

NEUTRON:

A subatomic particle that has no electric charge. Neutrons are found at the nucleus of an atom, alongside protons.

NUCLEUS:

The center of an atom, a region where protons and neutrons are located, and around which electrons spin.

PERIODIC TABLE OF ELEMENTS:

A chart that shows the elements arranged in order of atomic number, along with element symbol and the average atomic mass (in atomic mass units) for that particular element. Vertical columns within the periodic table indicate groups or "families" of elements with similar chemical characteristics.

PROTON:

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.

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