1. Elements Combine to Form Compounds

2. Elements Combine to Form Compounds
Although there are just over 100 different elements, there are millions of different substances.
That is because most substances are compounds.
A compound is a substance made from atoms of two or more different elements.

3. Properties of Compounds depend on:
Which atoms the compounds contains
Water (H2O) and Carbon Dioxide (CO2) are very different even though they both contain oxygen.

4. Properties of Compounds depend on:
How many of each atom there is
Water (H2O) and Hydrogen Peroxide (H2O2) are very different even though they both contain hydrogen and oxygen.

5. Properties of Compounds depend on:
How the atoms are arranged
Carbon and hydrogen can combine to form thousands of different compounds.
These compounds include natural gas, components of automobile gasoline, and many plastics.
methane
propane
octane
polyethylene

6. Compounds are often very different from the elements that make them.
Sodium = very reactive metal – reacts violently with water, soft metal, can be cut with a knife
Chlorine = poisonous, yellow-green gas, sharp odor

7. Compounds are often very different from the elements that make them.
When these two react, they form a substance that is familiar to you – sodium chloride – table salt
2Na(s)  +  Cl2(g)  ——>  2NaCl(s)

8. Atoms combine in predictable numbers.
A given compound always contains atoms of elements in a specific ratio.
For example: the compound ammonia always has three hydrogen atoms for every one nitrogen atom. 3:1 ratio
A substance with a different ratio of hydrogen to nitrogen atoms is not ammonia.

9. Chemical Formulas
A chemical formula uses chemical symbols to represent the atoms of the elements and their ratios in a chemical compound.
Some examples:
H2O - water
CO2 – carbon dioxide
C6H12O6 – glucose (table sugar)

10. How to write chemical formulas
Find the symbols for the elements in the compound on the Periodic Table
Carbon (C) Oxygen (O)
To indicate that there is more than one atom of an element, place subscripts slightly below and to the right of the element’s symbol O2
If there is only one atom of an element, you do not need to use a subscript.
CO2

11. Chemical Formulas show the ratios of atoms in a chemical compound
The chart shows names, atoms, ratios, and chemical formulas for several chemical compounds.
Compound Name
Atoms
Atomic Ratio
Chemical Formula
Hydrogen Chloride
1:1
HCl
Water
2:1
H2O
Ammonia
1:3
NH3
Methane
1:4
CH4
Propane
3:8
C3H8

12. Chemical Formulas Name Formula # of Different Elements
Names of Elements
# of Atoms of Each Element
Total # of Atoms in One Molecule
Magnesium oxide
MgO 2
Magnesium 1
Oxygen

13. Name That Molecule!

14. Bonds Can Make the Same Element Look Different
The element Carbon can exist in very different forms:
Diamond
Graphite
Both substances are made only of carbon.
The properties of each form depend on how the carbon atoms are bonded to each other.

15. Diamond Diamond is the hardest natural substance.
This makes them useful for cutting other substances
Each carbon atom forms strong bonds with four other carbon atoms
The pattern of linked atoms extends throughout the entire diamond crystal.
This three-dimensional structure gives diamonds their strength.

16. Graphite Graphite is the dark, slippery component of pencil “lead”.
Each carbon atom in graphite forms strong bonds with three other atoms to form flat layers.
These layers stack up on top of one another like sheets of paper, with weak bonds between the layers.
The layers slide past each other easily.

17. Water is Special
Water is very unique because its solid phase (ice) is less dense than its liquid phase (water).
This structure has spaces in it that make ice less dense than water.

18. Water is Special
Because of the shape of the water molecule, it forms a six sided structure when frozen.
This is why snowflakes are six sided.

21. Most Molecules are Very Complex
Although we have talked about some simple molecules, most are very complex.
Vanilla, C8H8O3
Acetaminophen, C8H9NO2 
Aspartame, C14H18N2O5
Caffeine, C8H10N4O2
Sucrose, C6H12O6
DNA (Deoxyribonucleic Acid), (too complex to list)

22. Acetaminophen (Tylenol)
vanillin
Acetaminophen (Tylenol)
Aspartame (Nutra Sweet)

23. caffeine
sugar

24. (Deoxyribonucleic acid)
DNA
(Deoxyribonucleic acid)