1. Chemical bonds and Equations 2.1 and 2.4
A molecule is a group of atoms held together by a covalent bond.
A compound is a substance composed of atoms of two or more different elements that are chemically combined.

2. Covalent Bonds
These occur when atoms share electrons.
Examples include sugars, fats, proteins and water.

3. Ionic Bonds
If an atom gains or loses an electron it becomes electrically charged, and is known as an ion.
If an negatively charged ion encounters a positively charged ion, they are attracted to one another because of the opposite electric charges they contain.

6. Chemical reactions (rxn) occur when bonds are formed or broken causing new substances to form.
In organisms, chemical rxns occur inside the cells.
All of the chemical rxns that occur in an organism are referred to as the organisms metabolism.
Scientists represent chemical rxns by writing chemical equations.

9. In a chemical equation, the substances that undergo chemical reactions are called reactants.
The substances formed by a chemical reaction are called products.
Reactants
Products

10. Mg + 2HCl → MgCl + H 2C2H2 + 5O2 → 2H O + 4 CO 2H2S + 3O2 → 2H O + 2SO
Writing subscripts
Mg + 2HCl → MgCl + H 2 2
2C2H2 + 5O2 → 2H O + 4 CO 2 2
2H2S + 3O2 → 2H O + 2SO 2 2

11. Writing Coefficients
N2O4 → NO2 2
C3H8 + O2 → CO2 + H2O 5 3 4

12. Acids and Bases pH is a measure of how acidic or basic a solution is.
A pH of 7 is neutral
Substances with a pH below 7 are acids.
Acids form hydrogen ions (H+) in water.
E.g. When hydrogen chloride (HCl) is added to water, hydrogen ions (H+) and chlorine ions (Cl¯) are formed. So, HCl in solution becomes hydrochloric acid.

13. Substances with a pH above 7 are bases.
A base is any substance that forms hydroxide ions (OH¯) in water.
E.g. Sodium (Na+) and hydroxide (OH¯) in water combines to form sodium hydroxide (NaOH).
More acidic
More basic
Neutral