1. Compounds (2 -1);

2. Compound
A. Pure substance made up of 2 or more elements in definite proportions.
2. Molecules - makes up compounds and contain 2 or more atoms, bonded together.
B. Chemical formula-
1. Shows the kind and proportion of atoms
2. Water has 2 hydrogens and 1 oxygen
3. Formula is written H2O
C. Compounds differ from each individual element.

3. D. Chemical Reactions
1. Occurs when atoms have unfilled energy levels and react to form bonds.
A. Bonds break
B. Atoms rearrange.
C. New substance is formed.

4. 2. Properties
A. Characteristic of an atom. Physical and chemical properties DO change when atoms combine with other different atoms.
3. Chemical bonds
A. When atoms bond they are held together by chemical bonds called ionic or covalent bonds.

5. Ionic Bonds
A. Atoms achieve stability when all outer shells are filled.
B. Ions – positively or negatively charged atoms.
C. Ionic bonds transfer electrons to another atom.

6. 2. When sodium releases an electron it becomes a positively charged ion and chlorine becomes negatively charged when it picks up an electron.

7. Ionic bond = transfers electrons Sodium has 11 electrons and Chlorine has 17 electrons.+

8. 5. Covalent Bonds
A. Forms when 2 atoms share one or more pairs of electrons. There are 3 different covalent bonds.

9. 3 Types of Covalent Bonds
1. Single – means that 2 electrons, or 1 pair are being shared between 2 atoms.
2. Double – means that 4 electrons, or 2 pairs are being shared between 2 atoms.
3. Triple - means that 6 electrons, or 3 pairs are being shared between 2 atoms.

11. 7. Isotopes
A. Atoms that have different numbers of neutrons then originally.
Isotopes are identified by their neutron number. (Mass number)
Because they have the same number of electrons each isotope has the same chemical property.
Some are radioactive, nucleus is unstable and break down over time.

12. 8. Importance of radioactive elements
A. Can determine the age of rocks and fossils.
Can be used to treat cancer
Can be used to kill bacteria
Can be used to follow the movement of substances within organisms “TRACERS”