1. Science
Chapter B1
Classifying
Matter

2. Lesson 2:
Text pages B16-B21
What are Compounds?

3. Vocabulary molecule: compound:
two or more atoms joined together; the smallest unit of many substances
only one kind of atom
compound:
a substance made up of atoms of more than one element joined together in a molecule
more than one kind of atom
A compound is simply a special kind of molecule!

4. Combining Atoms into Molecules
molecule = two or more atoms joined together
A molecule is the smallest piece of any substance that is still that substance.
A water molecule is made up of 2 hydrogen atoms and 1 oxygen atom. O H
If the atoms are separated, they will no longer be water.
A water molecule is the smallest piece of water that is still water.
One drop of water may contain millions of water molecules!

5. Combining Atoms into Molecules
An oxygen molecule is always made up of 2 oxygen atoms.
If the atoms are separated, they will no longer be oxygen. O C
A carbon dioxide molecule is always made up of 2 oxygen atoms and one carbon atom.
If the atoms are separated, they will no longer be carbon dioxide.
Any time more than one atom is joined together, you have a molecule.

6. Compounds
compound = atoms of two or different elements combined to form a special kind of molecule O H
A water molecule is made up of both oxygen and hydrogen atoms.
Since there are two different kinds of atoms in a water molecule, it is called a compound. O
An oxygen molecule is not a compound because it is made up of only one kind of atom: oxygen.

7. Compound or Molecule? O O H O C Ca N

8. Properties of Compounds
The properties of a compound are different from the properties of the elements that make up the compound. Na 11
Sodium Cl 17
Chlorine
NaCl
Sodium Chloride + =
silvery metal
reacts violently with water
produces heat
greenish-yellow
poisonous gas
white crystals
table salt
We eat it!

10. More About Compounds c + a + t = cat + + =
Compounds are always made up of the same kind and number of elements.
Just like words are always made up of the same number and kind of letters. c + a + t =
cat O O O Ca C Ca + + C =
Calcium Carbonate O

11. Compounds & Formulas Each compound has its own formula.
Formulas are written using the element’s symbol and, sometimes, a number.
Calcium carbonate has one calcium atom, three oxygen atoms, and one carbon atom. O Ca C O O
Calcium carbonate’s formula is written as CaCO3.
The number 3 after the O in the formula means that this compound has three oxygen atoms.
If there is no number after an element’s symbol, it means there is only one atom of that element.

12. Why is This Important?
Scientists study compounds to find new ways of combining atoms and molecules.
This is how newer, faster, lighter, stronger materials are developed.
We can thank hard-working scientists for all the useful items that make our lives easier!

13. Any Questions?