making jewelry. There are also highly reactive nonmetals, such as chlorine and oxygen, and
some nonreactive gases, such as helium (He) and neon (N), which you might recognize from
helium balloons and neon signs.
Figure 2.8: The periodic table groups the elements based on their properties. ( 30 )Chemical Reactions
Amoleculeis any combination of two or more atoms. The oxygen in the air we breathe is
two oxygen atoms connected by a chemical bond to form O 2 , or molecular oxygen. A carbon
dioxide molecule is a combination of one carbon atom and two oxygen atoms. Because
carbon dioxide includes two different elements it is a ”compound” as well as a molecule.
Acompoundisanycombinationoftwoormoreelements. Acompoundusuallyhasverydif-
ferent properties from the elements that it contains. Elements and combinations of elements