WebLecture: Atoms, Molecules, and the Mole
Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 2: Sections 6-10.
2.6 Molecules, Compounds, and Formulae. Pay attention to definitions: Molecules are any combination of atoms held together by various kinds of bonds. They range from discrete units to ions to metal lattices. The active part of certain organic molecules is called a functional group; this is the part that will form bonds with other organic molecules to make long chains or polymers. You may remember these from biology (hydroxyl, carbonyl, carboxyl, amine).
There are four common representations of molecules.
2.7 Ions and Ionic Compounds. Ions are simply atoms or molecules that have lost or gained electrons. Ionic compounds are held together by the attraction between the oppositely-charged components. The force between components depends on the number of charges involved.
Naming conventions allow us to talk about ions and molecules without having to spell out the formulae.
2.8 Molecular Compounds. Many substances form discrete molecules, as opposed to crystalline or metallic lattices. We can break down molecular compounds into discrete units that have all the chemical properties of the substance. Again, we use naming conventions that allow us to talk about chemicals without resorting to spelling out formulae.
2.9 Atoms, Molecules, and the Mole. Atoms and molecules interact individually, according to the kinetic molecular theory. But they are so small that we cannot measure reactions between individual atoms; we have to look at measurable masses of the components of a reaction, which means that we need a way to translate grams into numbers of atoms. We determine the molar mass of compounds by adding up the atomic masses of the components.
2.10 Determining Compound Formulas. A common method of chemical analysis allows us to use the principle of constant composition to go from the mass of the compound to its formula by looking at an intermediate amount: the percentage of the mass of a sample that is a given element. The proportions of the masses tell us something indirectly about the proportions of the atoms in the molecule. The percentage composition can't tell us directly, because atoms of different elements have different masses. We have to convert percentage composition to molar masses, then we can use the molar masses to determine the empirical formula of the compound. This still doesn't tell us the molecular formula, but it does give us the proportions of the elements in the compound.
Be sure that you work through all the examples and understand them, and read the problem-solving tips! The more foundation work you lay now, the less trouble you will have later on.
If you've never seen Coulomb's Law before (p. 78), don't be confused by the format here; it is really a simple relationship that says the force between two charges is equal to the product of the charges (q1*q2) divided by the square of the distance between them, and multiplied by a constant that depends on the units we use to measure the charge and distance: Fe= kQq/r2. It's similar to the inverse square law that governs gravity and for the same reason: we have a point source for the force.
Homework problems: Please visit the Moodle for the current assignment and posting instructions. Do all the homework problems assigned and check the forum for your posting assignment.
References: Illustrated Guide to Home Chemistry Experiments [Required text], or Home School Chemistry Kit Manual which comes with the Home Scientist Chemistry Kit CK101 set and is available online at The Home Scientist.
You may find some of the basic lab skills described in Lab Techniques 1: weighing samples, filtering solutions, and cleaning equipment and Lab Techniques 2: Reading thermomenters, handling hot equipment, using the Q test useful as well.
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