Chapter 19: Chemical Bonds

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) Webelements.com indiamart.com = +

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)

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)

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

Atomic Stability http://hyperphysics.phy-astr.gsu.edu/hbase/pertab/imgper/perlewis.gif

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! Webelements.com indiamart.com = +

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?