ATOM 



464 



ATOMIC THEORY 



AN ATOLL 



The first illustration presents the surface appearance ; the second, a cross-section showing usual 

 depth and form of walls. 



atoms) of hydrogen to make a molecule of 

 water. The change which we can observe (the 

 disappearance of the gases and the formation 

 of the water) is believed to consist of millions 

 and millions of these little atomic transactions 

 all going on at the same time. See ATOMIC 

 THEORY; ATOMIC WEIGHTS; CHEMISTRY. 



The word atom is derived from two Greek 

 words signifying not divisible. In ordinary 

 chemical actions the atoms are not divided, but 

 the facts discovered in connection with the 

 study of radioactivity have led to the con- 

 clusion that the chemical atoms are really 

 complex systems of smaller particles or of posi- 

 tive and negative electrical charges. J.F.S. 



ATOMIC, a torn' ik, THEORY. In the early 

 part of the nineteenth century, John Dalton, 

 the English chemist who laid the foundation 

 of theoretical chemistry, proposed a theory to 

 . explain the laws of chemical action. This 

 hypothesis, known as the atomic theory, as- 

 sumes first that all chemical elements and com- 

 pounds consist of tiny particles, or atoms; 

 secondly, the atoms of the same element have 

 the same weight; thirdly, the atoms of differ- 

 ent elements have different weights; fourthly, 

 chemical action is due to the union or separa- 

 tion of the atoms of the elements. 



A number of laws have been discovered 

 governing chemical reaction. According to one 

 law, the elements forming any substance are 

 always present in the same proportion by 

 weight. Again, when one element, such as 

 nitrogen, unites with another element, oxygen, 

 in several proportions by weight, yielding dif- 

 ferent compounds, the different proportions of 

 oxygen united with one part of nitrogen are 

 simple multiples of a common factor. All the 

 proportions in which the elements combine 

 with one another can be expressed in terms of 

 a set of numbers, one for each element, and 

 the multiples of those numbers. These num- 

 bers are the atomic weights of the elements. 

 See CHEMISTRY ; ATOM ; ATOMIC WEIGHTS. J.F.S. 



winds, so the lagoon has an opening into the 

 surrounding sea. See CORAL. 



ATOM, at' um, in chemistry, the name given 

 to the smallest particles into which elements 

 are divided by chemical reactions. The small- 

 est particle of any substance is called a molecule 

 (which see). If the substance is an element 

 its molecule may consist of one, two or a larger 

 number of similar atoms. If it is a compound 

 its molecule will consist of two or more atoms 

 one atom at least of each of the elements 

 of which the compound is composed. An 

 example will make this clear. The smallest 

 possible particle of water is a molecule of 

 water. As water, it cannot further be sub- 

 divided, but that molecule contains two ele- 

 ments, neither of which is water, namely, 

 hydrogen and oxygen. If these two could be 

 combined in the smallest possible quantities, 

 two atoms of hydrogen with one of oxygen, the 

 product would be one molecule of water. 



Actually, we cannot see, handle or experi- 

 ment with single molecules or atoms, but the 

 behavior of substances in larger quantities 

 has led to the theory that all substances are 

 really made up of minute molecules far too 

 small to be seen even with a microscope and 

 that these molecules (with the exception of 

 a few, such as those of mercury, zinc and 

 argon) are further subdivided into atoms in 

 undergoing chemical changes such as that by 

 which water is formed from hydrogen and 

 oxygen. 



The molecules of hydrogen consist of two 

 hydrogen atoms each; those of oxygen of two 

 oxygen atoms each. Equal volumes of hydro- 

 gen and oxygen are believed to contain equal 

 numbers of molecules. When we mix two 

 volumes of hydrogen with one volume of 

 oxygen and explode the mixture, both gases are 

 completely used up and only water is left. It 

 is inferred that each of the oxygen molecules 

 separates into two atoms and that each of 

 these atoins combines with a molecule (or two 



