A NEW METHOD OF TEACHING ATOMS AND MOLECULES Most chemistry texts introduce atoms and molrcules by the decomposition of water. This is the traditional method. I do not know any other reason for using it. I have discovered one serious objection t o it. Many high-school students are puzzled by this reaction when it is the first chemical change presented t o their minds. This trouble is due to the iact that the decomposition of water is not the simplest possible reaction but is complicated by having two distinct factors both of which are unknown t o the novice. The quickest students, of course, will comprehend both factors, but we have slow students as well as quick, and i t is more pedagogical t o disentanglc the factors of this problem and present them t o the beginners one at a time. The two factors are: (1) the atoms unite t o form molecules as in rnercnric oxide and hydrogen chloride, (2) one atom such as oxygen may unite with more than one other atom such as the two hydrogen atoms in the water molecule.
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I t is easy to keep the beginner's attention on the first factor by giving him, in the beginning, nothing but examples of binary compounds such as HC1, NaBr, KI, AgC1, etc., until this idea is familiar. I use hydrogen and chlorine and discuss their union in connection with Avogadro's hypothesis. After it is clear that one molecule of hydrogen splits into two atoms and that one molecule of chlorine also splits into two atoms and that these atoms unite to form two molecules of hydrogen chloride, then it is easy for the beginner to take the next step and comprehend the divalence of oxygen in water and the trivalence of nitrogen in ammonia and the tetravalence of carbon in methane. The following, taken from a pamphlet printed by the printing class in San Diego High School shows how I present these steps to the beginners. This pamphlet "Twenty-one Lessons in Chemistry" I do not use in the place of a chemistry textbook. I use it as a supplement to the ordinary text, giving my classes a lesson from the pamphlet a t points where i t will do the most good. The first lesson, given the day I meet my beginning classes is a series of ten experiments with a burning match. Lesson Two is a discussion of why we study natural science in general and chemistry in particular, an answer to the question "What is chemistry?", definitions of matter, substance, body and properties and a statement and discussion of the law of properties, and the states of matter. Lesson Three discusses the beginnings of chemistry, the phlogiston theory, and the work of Priestly and Lavoisier. Lesson Four discusses elements and compounds, physical and chemical changes. By examples it shows that a "Chemical chunge is a change in which one or more substances are transformed into one or more different substances." "All changes in substances which do not transform them into different substances are called physical changes." The law of definite composition is discussed, stated and illustrated. Problems illustrating the law are solved and exercises suggested. Lesson Five is on some common substances illustrating what has been learned. Lessons Six to Eleven are on the measuring of materials. They discuss gas density, the molecular theory and aqueous tension, the laws of Boyle and Charles, absolute temperature and correction for diierence of levels. The molecular composition of gases is explained and prooed in Lesson Eight. This prepares the beginners for Lesson Twelve in which the relations between atoms and molecules, the topic of this paper, is taught. Here is Lesson Twelve: Avogadro's Hypothesis and Atoms ~vogad;o's Hypothesis In 1811 A. D., an Italian scientist, Avogadro, in attempting to explain the laws of Boyle and Charles and certain other chemical and physical facts, elaborated the hypothesis we call by his name.
Avogadro's hypothesis assumes that equal volumes of any two gases contain equal numbers of molecules under the same conditions of temperature and pressure. If this is correct, of course, i t would follow that a molecule of a dense gas like chlorine would be heavier than a molecule of a light gas like hydrogen. I n f a d , if we assume that all the molecules of chlorine are equal in weight and that all the molecules of hydrogen are also equal in weight, i t would follow from Avogadro's hypothesis that the weights of a molecule of chlorine and of a molecule of hydrogen bear the same ratio to each other as the gas densities of the elements. The weight of a chlorine molecule would therefore be 35.5 times that of a hydrogen molecule, since a liter of chlorine weighs 35.5 times as much as a liter of hydrogen. I n other words 1000 molecules of hydrogen would have the same volume as 1000 molecules of chlorine; although each chlorine molecule is 35.5 times heavier than a hydrogen molecule. We wonder how this is possible until we remember that in a body of gas the molecules do not fill all the space but move about freely within the container, just as a number of rubber balls might bounce about a closed room. 1000 molecules of hydrogen will fill a certain space a t a certain temperature and pressure just as 1000 rubber balls can be bounced in a room of a certain size. Now i t is obvious that the same room would hold 1000 balls of a larger size. We can see from this that 1000 hydrogen molecules might occupy the same volume as 1000 larger chlorine molecules. Atoms-Atomic
Weights-Molecular
Weights
We learned that the gas hydrogen will burn in oxygen t o f o m water vapor. Hydrogen will also burn in chlorine to form the gas called hydrogen chloride, HCl. If hydrogen and chlorine are mixed and kept cool and in the dark, they will not combine a t once, but if heated or exposed to bright light they will explode forming hydrogen chloride. If they are mixed in unequal volumes the gas in excess will be left over but if the volumes of hydrogen and chlorine put into the mixture are equal, all of both gases will combine. This is an interesting fact. Why should they combine in exactly equal volumes? We have learned in the law of definite proportions that two or more elements always combine in definite proportions by weight, for instance 200 g. Hg unite with 16 g. 0. The law of definite proportions applies to the union of hydrogen and chlorine also since 1 g. of hydrogen always unites with 35.5 g. of chlorine, that is t o say, the reacting weight of hydrogen is 1, and of chlorine is 35.5 and the reacting volumes are equal, that is, 1 volume of Hz 1 volume of Clz becomes 2 volumes of HC1. Then, by Avogadro's hypothesis, 1molecule of hydrogen must join with 1 molecule
+
of chlorine to give 2 molecules of hydrogen chloride. This is surprising. We would expect only 1 molecule of hydrogen chloride instead of 2. How is i t possible for two molecules t o be joined together and thus become two molecules of a diierent sort? The following theory has been invented t o explain this. When one hydrogen molecule meets one chlorine molecule they both split in half and the halvesjoin to form two hydrogen chloride molecules. If we represent the hydrogen molecule by a small circle, and the chlorine molecule by a larger circle, we may represent this theory graphically as in Fig. 1.
H2 + C l r - t 2 H C l FIG.1.
These pieces of molecules we call "atoms." The word "atom" by its derivation means "not divided." According to the atomic theory molecules may be divided in chemical changes, but atoms are not divided. We believe that each molecule of hydrogen is made up of two similar atoms, also that each molecule of chlorine is made up of two similar atoms, but that each molecule of hydrogen chloride is made up of two dissimilar atoms, one of hydrogen and one of chlorine. The gas density of hydrogen is 1, that of chlorine 35.5, that is, a given volume of chlorine weighs 35.5 times as much as the same volume of hydrogen. By Avogadro's hypothesis the chlorine molecule must weigh 35.5 times as much as the hydrogen molecule. Then since the hydrogen chloride molecule is composed of 1 hydrogen atom and 1 chlorine atom we would expect this molecule t o weigh one half the sum of the hydrogen molecule and the chlorine molecule. That is, we would expect the gas density of hydrogen chloride t o be one half the sum of the gas densities of 35.5) = 18.25, and experiment shows that hydrogen and chlorine, 'Iz(l 18.25 is the gas density of hydrogen chloride. When we wish t o represent the gas hydrogen we do not write the symbol H, but the formula Hz. Nearly all the elements except the metals, have two atoms t o the molecule, as: CL,0 2 , Nz, etc. The metals such as gold, silver, tin, copper, mercury, sodium, etc., when they are heated up t o the gaseous state are found t o have only one atom to the molecule. Phosphorus has 4 atoms to the molecule (formula Pq) as has antimony (formula Sb4). Sulfur in one of its known states has 8 atoms t o the molecule (formula Ss). If we simply wish to abbreviate the name of an elementary substance we may write the symbol without the small number following it, thus: H, C1, 0, N, P, etc.
+
However, when we write equations the formulas must always be written with the small numbers after them thus: Hn Clz --+ 2HC1
+
The coefficient "2" written before the "HCY indicates that there are 2 volumes after the reaction. When Ha is written without a coefficient, "1" is understood. We may read the above equation thus: "One volume hydrogen unites with one volume chlorine t o form 2 volumes hydrogen chloride. It can also be read thus: 2 parts by weight of hydrogen, plus 71 parts by weight of chlorine becomes 2 X (1 35.5) parts by weight of hydrogen chloride. That is t o say the symbols always stand in an equation for the combining weights of the substances used. These combining weights are also called the molecular weights because they represent the relative weight* of the molecules. The combining weights of the atoms are called the atomic weights. The lightest molecule known is that of hydrogen, and the lightest atom is the hydrogen'atom. The weight of the hydrogen atom is taken as unity, precisely 1.008 in this system of atomic weights and molecular weights, and it is called one microcrith. This weight, of course, is millions of times too small to be handled.
+
The hydrogen atom weighs 1miaocrith (approximately). The oxyaen atom weighs 16 microcriths (exactly). The nitrogen atom wcighs I ? micrncriths (approximately). Thc chlorine atom weighz 35.5 microcriths (approxitnately). The sodium atom weighs 23 mimocriths (approximately). The mercury atom weighs 2W microcriths (approximately). ~
~
These numbers are called the atomic weights of the elements. A complete list of all the elements with their symbols and atomic weights is given in the appendix of most chemistry texts. The weight of the atom of an element in microcriths is its atomic weight. The weight of the molecule of a substance in microcriths is its molecular weight. Only elements have atomic weights but both elements and compounds have molecular weights. The hydrogen molecule has two atoms, each weighing one microcrith and therefore its molecular weight is 2. The chlorine molecule has 2 atoms each weighing 35.5 microcriths and therefore its molecular weight is 2 X 35.5 = 71. Observe that the molecular weight is found by adding together the atomic weights of the atoms in the molecule. Hydrogen molecular wt. = 2 Chlorine molecular w t . = 71 Oxygen molecular wt. = 32 Nitrogen molecular wt. = 28 Hydrogen chloride (HCI) = 36.5 = 1 Mercuric oxide (HgO) = 216 = 200 Sodium chloride (NaCI) = 58.5 = 23
+ 35.6 + 16 + 35.5.
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We have learned that hydrogen is the lightest gas and that i t is taken as unity in measuring gas densities. A certain volume of 0% weighs 16 times as much as the same volume of Ha and therefore the gas density of 0% is 16. We can find the gas density of any gas by weighing a sample of i t and dividing its weight by the weight of the same volume of hydrogen. For example, we find that 22.4 1. of ammonia gas S.T.P. weigh 17 g. and 22.4 1. of hydrogen S. T. P. weigh 2 g.; therefore the gas density of ammonia gas is 17 + 2 = 8.5. By Avogadro's hypothesis a certain volume of any gas has the same number of molecules as an equal volume of hydrogen. Therefore it follows that the gas densities of hydrogen and another gas would have the same ratio as the weights of their molecules. Gas densities
Hn 1 Hz 1 Hs 1 HI 1
Molecular weights
Ex 2 Hz 2 Hn 2 H, 2
HCI 18.25 0 2
16 Cb 35.5 Hg 100
HCI 36.5
0, 32 CIz 71 Hg 200
The molecular weight of hydrogen is 2 times its gas density, therefore it follows from the above that we can find the weight of the molecule of any gas by first finding its gas density by experiment and then multiplying that gas density by 2. RULE: To find the molecular weight of a gas find its gas density by experiment and multiply that by 2. The answer is the weight of a molecule of gas in microcriths. For example, we weigh a sample of water vapor and find that it weighs 9 times as much as an equal volume of hydrogen a t the same temperature and pressure. Its gas density is therefore 9. The weight of the water molecule is therefore 9 X 2 = 18 microcriths. That is to say the molecular weight of water is 18. What is the molecular weight of methane (CH4) if its gas density is S? Ans. 16. COz gas density 22; molecular weight 44. HzS gas density 17; molecular weight 34. Valence We find that one volume of chlorine unites with one volume of hydrogen to form 2 volumes of HCl. Chlorine plus hydrogen becomes hydrogen chloride.
m+m-pqq 1 Volume
+ 1 Volume +
2 Volumes
Experiment shows that 1 volume of oxygen unites with 2 volumes of hydrogen to form 2 volumes of water vapor.
H+IHlipGqiGT 1 Volume 2 Volumes
2 Volumes
If our 1 volume of oxygen has 1000 molecules, then by Avogadro's Theory our 2 volumes of hydrogen must have 2000 molecules, and our 2 volumes of water vapor must also have 2000 molecules.
+
1000 molecules 0% 2000 molecules Ha
--t
2000 molecules H 2 0
That is to say 1 oxygen molecule must unite with 2 hydrogen molecules to form 2 water molecules. We have learned that both oxygen and hydrogen molecules have 2 atoms each, therefore we may let a large circle represent the oxygen molecule and 2 small circles represent the 2 hydrogen molecules and we can represent the reaction graphically as in Fig. 2.
Oxygen differs from chlorine in that its atom is able to hold two hydrogen atoms in the molecule of the compound. We say that the valence of oxygen is 2 and the valence of chlorine and hydrogen is 1. If nitrogen and hydrogen are inclosed together in a container and sparks of electricity passed through the mixture part of the gases unite to form ammonia gas. If this gas is separated from the mixture and decomposed we find that: Ammonia
+
p
2 Volumes
lNitrogen i
f
Hydrogen
become 1 Volume
+
3 Volumes
~
q
+
+
c
The equation is written thus: 2NHs +Na 3Ha. The valence of nitrogen is three in this case since its atom will hold 3 hydrogen atoms in a molecule.
In methane (CH3 the carbon atom holds 4 hydrogen atoms in the molecule. What then is the valence of carbon? Phosphine is PHs. What is the valence of phosphorus? Lime is CaO. What is the valence of Calcium?
