When elements are chemically combined, they form compounds having properties that

are different from those of the uncombined elements.

Compounds• A compound is a pure

substance made of two or more elements chemically combined in a set ratio.

• A compound may be represented by a chemical formula which shows the elements in the compound and their ratios.

Chemical Formulas

• A subscript, the number below the symbol, tells how many of that specific atom is present in the compound.

• In the chemical formula for carbon dioxide, CO2, the subscript for oxygen tells you that the ratio of carbon to oxygen is 1 to 2.

• If there is no number after the element’s symbol, the number 1 is understood.

Chemical Formulas & Valence Electrons

4 valenceelectrons

C: :6 valence electrons

O: :..

6 valence electrons

O: :..

Carbon & oxygen form a covalent bond – they share electrons.

The ratio of carbon to oxygen is 1 to 2.

Chemical Equations• A chemical equation is a way to

show a chemical reaction using symbols instead of words.

• All chemical equations use formulas to represent the substances involved in the reaction.

Structure of an Equation

The principle of conservation of mass states that in a chemical

reaction, the total mass of the reactants must equal

the total mass of the products.

All the atoms present at the start of the reaction are present at the end.

Conservation of Mass

Balancing Chemical Equations

• To describe a chemical reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation.

• An equation is balanced when it accurately shows conservation of mass.

H2 O2+Reactant

s

H2

OProducts

Write the equation –

H2 O2+ H2

O

Count the atoms –

2 22 1

Use coefficients to balance atoms –

H2 O2+ H2

O

2

4 222

H2 O2+ H2

O

2

4 224

2

Look back and check!

H2 O2+Reactant

s

H2

OProducts