the ratio of the weight of any volume of a substance to the weight of an equal volume of pure water at 4 degrees Celsius

specific gravity

the time in which half the atoms of a radioactive material disintegrate

half-life

a shiny, dense substance that easily conducts heat and electricity

metal

the group 17 (Group VIIA) elements; consists of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At)

halogens

a group of related objects that work together to perform a function

system

the spontaneous disintegration of the nucleus of an atom that results in the emission of radiation

radioactivity

one of six elemental gasses that does not naturally react with other elements

noble gas

the smallest unit of an element that has the properties of that element

atom

a Russian scientist who organized and classified the elements based on similar physical and chemical properties; his work was the basis for the modern periodic table of the elements

Mendeleev

the chart scientists use to organize and classify all the known elements

periodic table of elements

the way parts are put together

structure

a material with a particular chemical makeup

pure substance

the path or orbit electrons follow around the nucleus of an atom

electron shell

the number of protons in the nucleus of an atom

atomic number

an element that is a greenish gas at room temperature

chlorine

elements that lie between the metals and the nonmetals on the periodic table; these element exhibit both metallic and nonmetallic properties

metalloids

one of the horizontal rows in the periodic table

period

a generalization or rule that describes recurring facts or events in nature

law

a pure substance that cannot be broken down into simpler substances by chemical reactions

element

approximately equal to the number of protons and neutrons

atomic mass

the chemical element consisting of one proton and one electron

hydrogen

an element that makes up most of the air near Earth's surface

nitrogen