Chapter 19: Molecules and Compounds

(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.)

Definition: "An is a group of atoms all having the same number of ." There are slightly more than one hundred elements. Avogadro's Hypothesis: "Equal numbers of at the same temperature and volume would exert the same no matter what the of the molecules might be." Avogadro's Hypothesis can be used to demonstrate that atoms and molecules are NOT necessarily the same thing (although sometimes they are). When it was finally realized that molecules often consisted of more than one atom, it became a great puzzle to understand why, for example, two atoms might join to form a molecule, but not then add a third and a forth. None of the fundamental forces seemed to have the property that when two particles are bound together, the force turns off (at least for an additional atom)! Nevertheless, we now understand that all molecules are bound together by the force, but in a peculiar way that follows from wave-particle duality. Two atoms at large distance from one another, appear to each other to be electrically neutral and there is no significant attraction of the atoms. If the atoms are in a gas and randomly bump into one another (not too hard and not too soft), an electron in one will be attracted simultaneously to the nuclei of both atoms by electrical attraction. The electron may then form a new which surrounds both nuclei. This kind of orbital is called a(n) (atomic, molecular?) orbital. If the electron has (more, less?) energy in the molelcular orbital than it had in the atomic orbital, and if the difference in energy escapes as a photon, the electron is stuck in the new orbital and it acts as a glue to bind the two atoms together to form a molecule. We say that a chemical has been formed. Definition: A is a group of identical (atoms, molecules?) each of which is composed of at least two different types of atoms." Examples: Water (H2O), carbon dioxide (CO2), methane (CH4), ammonia (NH3). Definition: A is a loose, physical combination (no chemical bonds) of elements and/or compounds. No chemical bond need be broken to separate a mixture into its compounds. Why doesn't a mixture of oxygen and hydrogen spontaneously unite to form water? Electrons in O2 and in H2 have low energy; they are deep in an energy "well." To form a stable compound, new orbitals have to be formed with even less energy. Before the new orbitals can form, the electrons must be "excited" so that the original oxygen and hydrogen molecular bonds are broken. Hence, one must first add to the mixture of oxygen and hydrogen. This is often done by striking a match. Then, hydrogen plus oxygen yields water. We might also write, H2 + O2 ---> H2O, but this form is not . Write the balanced form for the equation: H2 + O2 ---> H2O. Definition: "A chemical occurs when two materials come in contact and chemical (i.e., orbitals) are made or broken." When electrical current passes through water, chemical bonds are broken and remade. Electrolysis is a chemical reaction. Example (balance it!) H2O ---> H2 + O2. A chemical reaction is balanced when all atoms on the left are accounted for on the right with no extras and none short. Balance the following chemical reactions for practice. If only one molecule of a compound is required, enter an explicit "1" as a coefficient: CH4 + O2 ---> CO2 + H2O Mg + O2 ---> MgO Fe + O2 ---> FeO Fe + O2 ---> Fe2O3

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