1. Chapter 2 Section 2 What are chemical formulas?
Objectives:
Interpret chemical symbols.
Recognize common chemical formulas of some familiar compounds.

2. Key Terms
chemical symbol: shorthand way of writing the name of an element
chemical formula: formulas that shows the elements that make up a compound
subscript: number in a chemical formula that shows how many of each atom are in one molecule of a compound

3. Chemical Symbols
Each element has its own chemical symbol. The chemical symbol is used to represent one atom of that element.
In 1813, a swedish chemist named Jons Jakob Berzelius suggested using the first letter of the name of an element as its chemical symbol. When more than one element has the same first letter, a second letter is added. That second letter is a lowercase letter.

4. Chemical Symbols CHEMICAL SYMBOLS Element Symbol Oxygen O Hydrogen H
Nitrogen N
Carbon C
Aluminum Al
Helium He
Iron Fe
Lead Pb
Sodium Na
Mercury Hg
Figure 2-7

5. Chemical Symbols
Some elements do not use the first letters of their English names as their symbols. The symbols for these elements may come from the names of the elements in a different language.
Look at Figure 2-7 for some examples. The chemical symbol for sodium is Na. This comes from the Latin name for sodium, which is natrium.

6. IDENTIFY:
Why do some elements have unusual letters for their symbols?

7. Chemical Formulas
We just learned that a chemical symbol is a shorthand way of writing the name of an element. Chemical symbols are more often used to write chemical formulas. Sodium chloride is the chemical name for table salt. The chemical formula for sodium chloride is NaCl. By looking at the formula, you can see that table salt is made up of sodium and chlorine. The chemical symbol for sodium is Na. For chlorine, it is Cl.

8. 2. ANALYZE:
How are chemical symbols and chemical formulas related?

9. Subscripts
Hydrogen peroxide is a compound made from hydrogen (H) and oxygen (O). The chemical formula for hydrogen peroxide is H2O2.
The number 2 after the H and the O is called a subscript. Subscripts are written slightly below the line.

10. Subscripts
The chemical formulas for some compounds are shown in figure 2-8. The chemical formula for the compound iron sulfide is FeS2. This chemical formula shows that a molecule of this compound contains two atoms of sulfur (S) and one atom of iron. When there is no subscript, it means that there is only one atom of that element in the molecule.

11. CHEMICAL FORMULAS FOR COMMON COMPOUNDS
Water
H20
Carbon monoxide CO
Carbon dioxide
CO2
Iron Sulfide
FeS2
Calcium carbonate
CaCO3
Aluminum oxide
AL2O3
Ammonium hydroxide
NH4OH
Silica
SiO2
Sodium phosphate
NaPO4
Hydrogen peroxide
H2O2
Figure 2-8

12. 3. INTERPRET
What do subscripts show?

13. Classwork CHECKING CONCEPTS Chemical symbols are shorthand for ___.
Subscripts are written slightly ___ the line.
A chemical symbol stand for one ___ of an element.
The first letter of a chemical symbol is always a ___ letter.

14. Classwork THINKING CRITICALLY
5. ANALYZE: Why do you think scientists need shorthand ways of writing elements and compounds?

15. Classwork INTERPRETING VISUALS
Use Figure 2-8 to answer the following questions.
6. CALCULATE: How many atoms make up one molecule of carbon dioxide?
7. ANALYZE: How does a molecule of carbon monoxide differ from a molecule of carbon dioxide?
8. IDENTIFY: How many different elements are in calcium carbonate?