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