4. The structure of an atom  Atoms are the building blocks of matter.  Atoms are made up of smaller particles:  Protons are positively charged particles (p + )  Neutrons are particles that have no charge (n 0 )  Electrons are negatively charged particles (e - )

5.  Protons and neutrons are located in the center of the atom, called the nucleus.  Electrons are located outside the nucleus.

6.  An element is a pure substance that cannot be broken down into other substance by physical or chemical means.  There are over 100 know elements, 92 of which occur naturally.  Each element has a unique name and symbol.

7. The periodic table of elements  Horizontal rows are called periods, and the vertical columns are called groups.

10. Isotopes  Atoms of the same element that have the same number of protons and electrons but a different number of neutrons are called isotopes.

11. Radioactive isotopes  Some isotopes are unstable and break apart or decay – called radioactive isotopes.  In this process, a new element, new elements, are formed.  A decaying nucleus gives off radiation that can be detected and used for many applications.

13.  A compound is a pure substance formed when two or more different elements chemically bond.  Always formed from a specific combination of elements in a fixed ratio.  Cannot be broken down into simpler compounds or elements by simple physical processes.  Can be broken down by chemical processes.

15.  The force that holds atoms together is called a chemical bond.  Chemical bonds are formed when atoms gain, lose, or share valence electrons.  Valence electrons are found in the outermost energy level of the electron cloud.  Atoms become more stable when they bond.

16. Covalent Bonds  The chemical bond that forms when valence electrons are shared is called a covalent bond.  A molecule is the basic unit of a pure substance in which the atoms are held together by covalent bonds.

17. Ionic Bonds  An atom that has lost or gained one or more electrons is an ion.  Metals tend to lose, and become positive cations.  Nonmetals gain, and become negative anions.  An ionic bond is an electrical attraction between two oppositely charged ions.

18.  Most ionic compounds dissolve in water, are crystalline at room temperature, and have higher melting points than compounds formed by covalent bonds.

19.  Weak attractions between molecules are called van der Waals forces.  When molecules come close together, the attractive forces between slightly positive and negative regions pull on the molecules and hold them together.  The strength of the attraction depends on the size of the molecule, its shape, and its ability to attract electrons.

20.  A special kind of van der Waals force occurs between water molecules called hydrogen bonds.  Responsible for: ▪ Surface tension ▪ Water droplet formation