Freshman Chemistry 20A Lecture Series 1 Chemistry and the Periodic Table The point of this course is to learn how to think about chemical structure the periodic table and the relevance of these things to the world around us Consider the periodic table The elements at the far right are essentially inert The elements at the far left are amongst the most reactive of all elements Yet the difference between the two elements is that the ones on the far right are characterized by 1 less electron and 1 less proton than the corresponding element 1 row down on the far left If you don t know what an electron or a proton is just wait a bit We ll get to that Let s take one of the simplest reactions possible 2Na Br2 2NaBr There are several things to note about this reaction First Na is a covalent solid and it is a metal which means that it is very shiny like a silvered mirror In fact in terms of its electrical conductivity it is one of the best metals there is Recall also that by nature of its position at the far left of the periodic table it is a very reactive element Br2 is a molecule and it is a liquid until 58oC above which it is a gas and it is red in color It turns out that Br2 is only slightly reactive on a relative scale that would compare the reactivity of various elemental molecules NaBr is an ionic solid which implies that it is not a metal but rather an insulator In fact it is one of the best insulators that there is In addition it is nearly completely inert or non reactive and it is clear and colorless So we combine a highly reactive covalent metallic solid with a quasi reactive red liquid or gas and we get an inert product that is an ionic insulating clear colorless solid Let s look at another reaction Na H2O NaOH H2 In this reaction we are taking a reactive metallic shiny covalent solid and reacting it with a clear colorless molecular liquid how is the liquid held together and producing a colorless white reactive ionic solid and a slightly reactive colorless molecular gas We have only written down two apparently simple and common chemical reactions and we have already introduced a tremendous amount of complexity Other questions that we haven t brought up yet include How fast are these reactions Do they proceed to completion etc With this complexity already introduced how can we ever hope to understand truly complicated chemical systems such as living organisms or the earth s atmosphere or the chemical processes that are involved in fabricating electronics or plastics etc In this class we will only begin to shed light on these issues and we will do it slowly one step at a time We will first introduce the language of chemistry and we will follow this with a highly simplified description of what makes up an atom This description will necessarily involve some quantum mechanics starting with Chapter 15 We will discuss the types of experiments that have led to various descriptions of chemistry including spectroscopy which is the interaction of light with matter After we discuss the atom we will move onto the structure of simple molecules and how simple models may be used with caution to predict such structures Finally we will increase the complexity of the systems we are trying to describe and include organic molecules polymers and transition metal complexes although we may not quite get to the transition metals For now let s back up a bit and just try to understand the language of the reactions we have already mentioned and discuss why they were written they way they were First let s return to the reaction 2Na Br2 2NaBr What we are saying when we write this reaction down is that 2 units of sodium atoms react with 1 unit of bromine gas to produce two units of sodiumbromide The numbers in front of the chemical species are intended to balance the chemical reaction We could have just as easily written the reaction as Na Br2 NaBr Notice that this works out just as well if we are trying to conserve mass In fact that is just what we are trying to do when we balance a chemical equation In all chemical reactions mass is conserved This is a principle that was first stated by Lavoisier early in the last century Notice that we did something similar when we wrote down our second reaction of the day Na H2O NaOH H2 However this reaction is a little more complicated 2 reactants to produce 2 products Balancing equations never involves more than algebra but nevertheless the algebra can get a little complicated Molecular Units Moles and the Periodic Table When we say that 1 unit of sodium plus 1 unit of water reacts what do we mean by units Chemists use all types of units to describe quantities of chemicals Below we have listed a few Mass Grams g milligrams mg 0 001 g Volume liters l 0 2 gallons roughly milliliter ml 0 001 l kilograms kg 1000 g 2 2 pounds microliter l 10 6 l However when we are dealing with reactions such as those described above we need to work in units that are related to the number of molecules or atoms that are reacting or are produced We could certainly just say that 1 atom of sodium reacts with molecule bromine to produce 1 molecule of sodiumbromide However this is not very much material For example 1 atom of sodium only weighs 4 x 10 23 grams The most sensitive mass scales measure only a few nanograms 10 9 grams or so and so even those scales are 14 orders of magnitude away from being able to measure what happens in this reaction 10 23 grams is so very small that it is incomprehensible to most people including me Thus we would like to work with amounts that are comprehensible The weight of a mole of molecules is such an amount 1 mole of things is 6 022x1023 of those same things so 1 mole of sodium atoms weights about 23 grams This is much better 6 022x1023 is called Avogadro s number and is represented in shorthand by NA How did someone ever come up with the definition of a mole as 6 022x1023 It turns out that 1 mole of carbon atoms weighs 12 grams exactly It is good to remember that 1 mole of things is a tremendously large number of things 1 mole of seconds is longer than the age of the universe 1 mole of grams is approximately the mass of water on the earth We will find that many things in chemistry are characterized by masses charges velocities etc that are of a scale that makes them very difficult to comprehend Things like NA are used to make such physical quantities a little more manageable Scientific notation Notice that above we have written that 1 l 10 6 l We could