SOME FUNDAMENTAL PHYSICS AND CHEMISTRY 31 



the quantity of its elements but also in their structural arrangement. 

 The molecules are completely separable from one another; in a solution 

 of sugar in water, the molecules of sugar float singly, and in air the 

 molecules of oxygen or of nitrogen are free from other molecules. 



The molecules of many substances are in turn composed of atoms. 

 These are defined as the smallest divisions of matter that may exist, 

 either singly or in combination. Some molecules consist of only one 

 atom, as in the gas helium. In such substances there is no distinction 

 between molecule and atom. In oxygen, however, the molecule is 

 composed of two atoms. Here the atoms have properties veiy dif- 

 ferent from those of the molecules ; the atoms enter into chemical reactions 

 much more readily than do the molecules. 



Protons, Neutrons, and Electrons. — Even the atoms are not the 

 ultimate units in the structure of matter, for they are made up of protons, 

 neutrons, and electrons. These entities may be spoken of as particles, 

 though they may be such only in a very special sense. The astounding 

 feature of these units is that they are the same in all kinds of matter. 

 The protons of an atom or molecule of oxygen are exactly like the protons 

 of chlorine. Similarly the neutrons are everywhere the same, in all 

 elements, and the electrons are the same in all. 



The protons have mass, and each of them bears a positive electric 

 charge. This positive charge is a unit which is the same in all protons. 

 Neutrons have mass, practically identical with that of the protons, but 

 they carry no electric charge. Electrons are units of negative electric 

 charge; their mass is negligible. Atoms and molecules of all substances 

 are made up of these units. The mass (weight) of an atom is dependent 

 almost entirely on the protons and neutrons it contains, while its volume 

 is determined mostly by the electrons. These relations will be made 

 clear by an examination of the structure of the atom in several elements. 



Structure of the Atom. — An atom of any substance consists of a 

 central nucleus, around which one or more electrons are distributed. The 

 nucleus of an atom contains one or more protons, and usually one or 

 more neutrons. Since the protons bear positive electric charges, the 

 nucleus of an atom is always positively charged. How great a charge 

 it carries depends on how many protons it contains. Both protons and 

 neutrons contribute to the mass of the nucleus, but only the protons 

 furnish the charge. This positive charge of the nucleus is balanced by 

 the negative charges of the surrounding electrons. There are as many 

 electrons around the nucleus as there are protons in it, so that the atom 

 is neutral. 



Structure of the Elements. — With this knowledge of the fundamental 

 similarity of all matter let us return to the elements. The number of pro- 

 tons and neutrons in the nucleus varies considerablv, as does also the 



