Chapter 21: Compounds of Nonmetals

(Press the TAB key to position the cursor at the beginning of the first blank and type what you believe completes the thought. Then press the TAB key for feedback and to move to the next blank. If you miss it, delete all of the word by positioning the cursor with the mouse and backspacing over all of the letters and try again. You should be able to complete this exercise without error before completing this chapter.)

Some of the most important componds in nature, including those essential to life, combine nonmetal atoms with other nonmetal atoms. Examples: H2O (water), CO2 (carbon dioxide), CH4 (methane), NH3 (ammonia). some of the compounds of nonmetals with nonmetals are charge objects (ions): NH4+ (ammonium ion), NO3- (nitrate ion), SO42- (sulfate ion). Why does nature like NO3- but NOT NO3 ? Pattern: The electrons in the atoms of the Noble Gas family have particularly low average energy. There is a tendency for atoms in columns near the Noble Gases to "want to be like the nearest Noble Gas" in order to lower the energy of their electrons. In forming the ionic bond, metals give away valence electrons to nonmetal atoms which accept them. But, there is another way for nonmetal atoms to accomplish the same thing: sharing. When nonmetal atoms share electrons, the shared electrons set up orbitals that surround both atoms and thus serve as a glue to bind the atoms together. This bond is called a(n) bond. It is summarized by the Octet Rule: "Atoms will form covalent bonds such that by sharing electrons, each atom completes a valence shell (or subshell of s = p orbitals) with eight electrons." Characteristics of the covalent bond: Usually gases or ; Usually colorless; (Conductors, nonconductors?) of electricity?; Usually toxic (for simple molecules with just a few atoms, such as NH3). Example: Fluorine forms a (single, double, triple?) covalent bond with another fluorine atom. Each fluorine atom then has a total of (number, how many?) valence electrons. Oxygen forms a (single, double, triple?) covalent bond with another oxygen atom. Each oxygen atom then has a total of (number, how many?) valence electrons. Nitrogen forms a (single, double, triple?) covalent bond with another nitrogen atom. Each nitrogen atom then has a total of (number, how many?) valence electrons. Carbon (does, does not?) form a quadruple covalent bond. At the level of this course it is not possible to predict the formulas of compounds of nonmetals. However, it is possible to test formulas that one guesses or are proposed as possible compounds. There is a recipe that you can follow to see if a proposed compound is one that nature forms and which can be expected to be stable. Example: CO2: How many valence electrons are actually present in CO2? (Read this information from the columns of the Periodic Table.) ; How many valence electrons are needed if each atom in the compound is to have 8 electrons to itself? (Multiple the total number of atoms in the compound by 8.) ; How many electrons must be shared? (The answer to this part of the recipe is the difference between the answers to the first two questions in the recipe.) ; How many electrons does one actually have that are not to be shared? (The answer to this is the difference between the number of electrons you actually have (Question 1) and the number you are going to share (Question 3).) ; Can it be arranged? (Here you draw a diagram. Arrange the atoms as symmetrically as possible. For CO2 that means a carbon in the middle and an oxygen on each side. Now draw in the shared electrons, two-at-a-time until they are used up; then assign the unshared electrons. Shared electrons are placed BETWEEN atoms to represent the bond. Unshared electrons are placed around atoms, but NOT BETWEEN until each has 8, counting the shared electrons already in place. If it comes out even and every atom has 8, you can predict that nature "likes" that compound, that it will exist, and that it will likely be stable.) Additional Rule: "Any extra electrons or missing electrons in the structure that make it a charged ion are to be counted as valence electrons." Example: NO3- How many valence electrons are present? ; How many valence electrons are needed? ; How many valence electrons must be shared? ; How many valence electrons are not shared? ; Can it be arranged (yes, no?)? . Some compounds have both ionic and covalent bonds: NaNO3, Mg(NO3)2, (NH4)2SO4. These compound have the bulk characteristics of the bond: white, crystalline solid, brittle, nonconductors of electricity as a solid, conducting when dissolved, transparent as a solid.

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