1. Putting Atoms Together
Chemistry

2. Learning Goals
To be able to count the number of atoms in a molecule or compound

3. All matter (substances) is made of the elements found on the periodic table
The way the atoms in a substance are arranged determines the substance’s properties
Carbon atoms in graphite are arranged in layers that are loosely held together
That is why graphite is soft and brittle
This allows some graphite to slide off your pencil when you write

4. Molecules
A molecule is a group of atoms that are chemically joined together
Oxygen gas that we breathe is actually made up of two oxygen atoms  O2
If three oxygen atoms are joined together, a different molecule is created  O3, known as ozone
In addition to oxygen, some other elements also occur as molecules, so they are referred to as molecular elements
Examples: H2, N2, F2, and Cl2

5. Compounds Most substances we encounter are actually compounds
A compound is a pure substance made of two or more different elements chemically joined together
Examples: water (H2O), carbon dioxide (CO2), sodium chloride (NaCl), ammonia (NH3), and methane (CH4)
There are millions of compounds

6. Interpreting Formulas
A chemical formula tells you important information about a pure substance
The symbols in the formula tell you the elements in the substance
If there is only on symbol, there is only one element
If there is more than one symbol, the substance is a compound
The subscript (small number) tells you how many atoms of each element are in each molecule of a substance

7. sodium chloride (table salt) sodium bicarbonate (baking soda)
Chemical Name
Chemical Formula
Elements Present
Atoms of each Element
ozone O3
water
H2O
carbon dioxide
CO2
ammonia
NH3
Chemical Name
Chemical Formula
Elements Present
Atoms of each Element
sodium chloride (table salt)
NaCl
sodium bicarbonate (baking soda)
NaHCO3
Oxygen
Hydrogen
Carbon
Nitrogen 3 2 1
Sodium
Chlorine
Hydrogen
Carbon
Oxygen 1 3

8. Success Criteria
I CAN count the number of atoms in a molecule or compound

9. To Do…
Worksheet
Prepare for Practice Quiz (Monday)