Chapter 20 Chemical bonds

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

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.

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

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- http://science360.gov/obj/tkn-video/9517826e-609b-4dab-aca3-ad5fd95a510d

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 http://science360.gov/obj/tkn-video/22f01ba8-2f23-4b9c-b3ca-62e12f3a73f7

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