1. Chapter 19: Chemical Bonds

2. Section 1: Stability in Bonding
Elements sometimes combine with other elements to form compounds
Compound: a new substance formed when two or more elements chemically combine
Example: solid metal sodium (Na) chemically combines with toxic, yellow chlorine (Cl) gas to form table salt (sodium chloride, NaCl)
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3. Chemical Formulas
A new compound has very different properties than the elements that make it up
NaCl (sodium chloride) is the chemical formula, or a shorthand way of writing the name of the compound
These are chemical formulas:
SiO2: silicon dioxide (sand)
H2O: dihydrogen monoxide (water)
C12H22O11: sucrose (table sugar)
HC2H3O2: acetic acid (vinegar)

4. Chemical Formulas
A chemical formula tells which elements a compound contains and the exact number of atoms of each element in a unit of that compound
Notice that this number is a subscript (meaning, “written below”)
How many of each atom are there?
SiO2: silicon dioxide (sand)
H2O: dihydrogen monoxide (water)
C12H22O11: sucrose (table sugar)
HC2H3O2: acetic acid (vinegar)

5. Atomic Stability Why do elements combine to form compounds anyway?
When the compound formed is more stable than as separate atoms
Exception: the noble gases (group 18). Why?
Noble gases have their outer energy level filled with electrons—very stable
Elements want to be like the noble gases
As a result, atoms will gain, lose or share their outer energy level electrons with other atoms to become chemically stable

6. Atomic Stability

7. Sodium Chloride Revisited
Sodium and chlorine are unstable by themselves
Sodium want to lose an electron to be like neon (Ne)
Chlorine wants to gain an electron to be like argon (Ar)—it works out perfectly!
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8. Review Questions, p. 578 Q1-4 Answer these questions for tomorrow:
What happens to the properties of elements when atoms form compounds?
Describe all that you can about the compound BaF2.
Why are some elements stable by themselves while others are more stable in compounds?
In what 3 ways can a chemical bond form between atoms?