All terms in this list:

atom: The smallest mediaeval unit of time, equal to fifteen ninety-fourths of a second.

proton: A positively charged subatomic particle forming part of the nucleus of an atom and determining the atomic number of an element; the nucleus of the most common isotope of hydrogen; composed of two up quarks and a down antiquark

neutron: A subatomic particle forming part of the nucleus of an atom and having no charge; it is a combination of an up quark and two down quarks

nucleus: The core, central part (of something), round which others are assembled.

electron: The subatomic particle having a negative charge and orbiting the nucleus; the flow of electrons in a conductor constitutes electricity.

electron cloud: A mathematically defined region around an atomic nucleus defined by a high probability that the atom's electrons will be present in that region.

atomic number: The number, equal to the number of protons in an atom that determines its chemical properties. Symbol: Z

mass number: The total number of protons and neutrons in an atomic nucleus.

periodic table: A tabular chart of the chemical elements according to their atomic numbers so that elements with similar properties are in the same column.

chemical symbol': a one-, two or three-letter abbreviation of the name of an element

average atomic mass: the weighted average of the masses of all naturally occurring isotope of an element

metal: Any of a number of chemical elements in the periodic table that form a metallic bond with other metal atoms; generally shiny, somewhat malleable and hard, often a conductor of heat and electricity.

nonmetal: an element, such as phosphorus or chlorine, that does not have the chemical or physical properties of a metal

metalloid: an element, such as silicon or germanium, intermediate in properties between that of a metal and a nonmetal; especially one that exhibits the external characteristics of a metal, but behaves chemically more as a nonmetal

group: A number of things or persons being in some relation to one another.

period: Appropriate for a given historical era.

pure substance: a sample of matter either a single element or a single compound that has definite chemical and physical properties.

heterogeneous: Diverse in kind or nature; composed of diverse parts.

element: Any one of the simplest chemical substances that cannot be decomposed in a chemical reaction or by any chemical means and made up of atoms all having the same number of protons.

compound: a group of buildings situated close together, eg. for a school or block of offices

mixture: Something produced by mixing.

homogeneous: having the same composition throughout; uniform

atmosphere: The gases surrounding the Earth or any astronomical body.

air pressure: atmospheric pressure

thermosphere: layer of the Earth's atmosphere directly above the mesosphere and directly below the exosphere

mesosphere: layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere

stratosphere: The region of the uppermost atmosphere where temperature increases along with the altitude due to the absorption of solar UV radiation by ozone. The stratosphere extends from the tropopause (10-15 kilometers) to approximately 50 kilometers.

troposphere: The lower levels of the atmosphere extending from the earth's surface up to the tropopause. It is characterized by convective air movements and a large vertical temperature change.

ozone layer: A region of the stratosphere, between 15 and 30 kilometres in altitude, containing a relatively high concentration of ozone; it absorbs most solar ultraviolet radiation

greenhouse effect: (with the) The process by which a planet is warmed by its atmosphere. (Do not confuse with global warming.)