1. Chapter 20
Chemical bonds

2. Sec. 1: Stability in Bonding
Atoms from different elements can combine to form compounds
When atoms combine, the compound has different properties than the elements
Ex. Sodium Chloride (table salt) is made of sodium & chlorine
Sodium is a soft metal that reacts violently with water
Chlorine is a poisonous greenish-yellow gas

3. Formulas Ex. NaCl has 1 atom of sodium & 1 atom of chlorine
Chemical Formula—tells what elements a compound contains and the exact number of the atoms of each element
Ex. NaCl has 1 atom of sodium & 1 atom of chlorine
Ex. H2O has 2 atoms of hydrogen and 1 atom of oxygen.
A subscript (small # written below) is written after a symbol to tell how many atoms of that element are in the compound.

4. Atomic stability
Atoms combine to form a compound that is more stable than the separate atoms
Noble gases are already stable and do not form compounds
Their outer energy level is full of electrons.
Atoms can gain, lose, or share electrons to get a full valence electron shell
Chemical Bond—the force that holds atoms together in a compound

5. Sodium and chlorine Sodium has 1 valence electron
Chlorine has 7, so it needs 1 more electron to be stable
If sodium give its 1 valence e- to chlorine, they are both stable with 8 valence e-

6. Water molecule
In water, each hydrogen atom needs 1 e- to fill its outer shell
Oxygen needs 2 e- to fill its outer shell
To be stable, the atoms share electrons

7. Ionic Bonds Covalent Bonds Types of bonds
Contain a metal & a nonmetal (at least 1 of each)
Electrons are gained or lost
Ex. Salt, NaCl
Covalent Bonds
Contain 2 or more nonmetals or hydrogen
Electrons are shared
Ex. Water, H2O