Atomic structure and the periodic table

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ATOMIC STRUCTURE and the PERIODIC TABLE ROBERT L.EBEL Edison Institute High School, Dearborn, Michigan

T

HERE are two chief purposes in all scientific endeavor. The one is to secure accurate kuowledge. The other is to interrelate the various items of that knowledge through the establishment of progressively unifying principles. The degree of maturity of any science may be quite accurately judged by the extent to which the tested knowledge within its domain has been organized and integrated. To one who has had experience in teaching hoth chemistry and physics as elementary courses on the secondary level, i t is clearly apparent that the older science, physics, exhibits the greater maturity. We must admit that much of what we teach in general chemistry in secondary schools is still purely descriptive and empirical, not interpretive and theoretical. There are those who contend that the adolescent mind is better adapted to the understanding of simple descriptions of natural phenomena than to the comprehension of organized theoretical knowledge. They suggest that the content of our science courses should stress

descriptions a t the expense of theory. If we follow their suggestion, however, we may no longer rightly call our courses "science." For they do not provide the sort of essential experience in rigorous, abstract thinking which educators had in mind when they made a year's course in science a requirement for high-school graduation. They do not reveal to the student the beauty and power of organized knowledge which has made this age truly an age of science. Therefore, those of us who are interested in teaching chemistry as a science should lose no opportunity to advance its integration, not only because such integration will give the beginning pupils a truer concept of the science of chemistry, but also because it will simplify the student's problem of understanding and remembering the facts taught. I t is obvious that the concept which has the greatest integrating potentialities for chemistry is that of atomic structure. When we clearly understand the structure, motions, and forces within the atom we will have a

basis for a unified explanation of all chemical phenomena. Rapid progress has been made in understanding the atom, and that progress has been reflected in many textbooks of elementary chemistry. The purpose of this article is to suggest a way in which the periodic table and its interpretation may be clarified by a closer correlation with the fundamental concepts of atomic structure. As commonly taught a t present, the periodic law is related only incidentally to atomic structure. But since the law deals essentially with the properties of

SYNOPSIS O F ATOMIC STRUCTURE

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the atom, and since the properties of the atom are determined by atomic structure, the relationship between periodic law and atomic structure should be more than mere incidental correlation. Atomic structure can be used to explain, in part, a t least, the why of the periodic law. Furthermore, there are a t least two complications in the usual periodic table which present difficulties t o the beginning student. The first complication is the presence of the long periods, which necessitates the placement of dissimilar elements in the same group and the introduction of two families in a group. The second complication is the presence of the rare earth elements, which are placed in a separate table and lamely explained away as having no place in the periodic table. Atomic structure can be made the basis of a revised periodic table which avoids these complications. If we arrange the atoms in the order of their increas-

ing atomic numbers we observe that each atom has one more extranuclear electron than the one preceding it. Paying particular attention to this single electron which distinguished any atom from the one preceding it, and which might therefore be called the diierentiating electron, we see that its placement seems to follow certain general tendencies which may be expressed in the following laws. The Law of Capacity.-The maximum number of electrons which can normally occupy any given orbit is given by the simple formula 2 X nZ,where n is the orbit number, counting outward from the nucleus. That is, for the K orbit, n is 1; for the L orbit, n is 2, and so forth. The Law of Syrmnetry.--The orbital electrons tend to be distributed symmetrically among the various orbits. The greatest number of electrons is usually found in an intermediate orbit, with decreasing numbers in the orbits to the outside and to the inside. The Law of Consistency.-For atoms of successively increasing atomic number, the additional electron tends to be placed in the same orbit as the one preceding i t until the orbit is filled to capacity, or until a critical point is reached, and the symmetry of the atom wiU be best preserved by adding electrons to another orbit. Critical points are reached with two, eight, or eighteen electrons in the orbit. Perhaps a clearer notion of the operation of these laws may be obtained from a study of the diagram following. The orbit letters are given along the top line, the electron numbers a t the left. The position of each atomic number on the body of the chart indicates the placement of the differentiating electron for that atom. Thus, for sodium, whose atomic number is 11, the differentiating electron is the first electron in the M orbit. The law of capacity is illustrated in orbits K, L, M, and N , each of which is completely filled. The v-shaped solid lines indicate the law of symmetry. And the law of consistency may be observed by following the atomic numbers in numerical sequence. Theie are three distinct levels on the chart. The distinguishing characteristic of these three levels is found in the placement of the differentiating electron. In the first, upper level, the electron difference occurs in the outer shell of electrons. In the second level, the electron difference occurs in the electron shell second from the outside. In the third level, the electron diierence occurs in the electron shell third from the outside. Since the outer shell of electrons is the most effective one in determining the properties of an atom, with the inner shells progressively less effective, we would expect to find that there are great differences in the properties exhibited by elements in the first level, less diierence on the second level, and least of all on the third level. Our expectations are verilied when we examine the properties of the elements which appear a t each level. On the first level we find the representative elements, including metals, non-metals, inert elements, liquids,

THE PERIODIC TABLE

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8

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16

17

18

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6

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10

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The Rclnld Metals (Leudl 2)

orbits

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12

13

14

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Thc Rare Eorrha (Lewd 3) Elacirons 19

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20

21

22

Ce R Nd I1 58

59

60

61

23

24

25

26

27

28

29

30

31

32

Sm Eu Gd Tb Dy Ho Er' Tm Yb Lu 62 63 64 65 66 67 68 69 70 7 1

and gases. On the next level we find the related metals, easily distinguishable from each other, but

decidedly similar in their metallic properties and in the variety of valences they usually exhibit. On the lowest level we find the rare earth elements, whose resemblance to each other is so great that chemists have experienced much difficulty in separating and identifying them. On the basis of these distinctions in atomic structure, the following periodic table has been constructed. It is a t once apparent that this arrangement eliminates the complications of long periods and rare earths mentioned earlier. The elements are logically grouped on the basis of their atomic structures. Through the use of this table, in conjunction with the preceding chart, i t has been possible to develop rather effectively these two principles. 1. That the structures of the atoms are periodic functions of their atomic numbers. 2. That the properties of an atom are determined by its atomic structure. Taken together these two statements are equivalent to, but more significant than, the usual statement of the periodic law.