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(Gr. a privative, and temno, cut; indivisible). Primarily, the smallest particle of matter which can exist; the ultimate and smallest division of matter, in physics, sometimes the smallest particle to which a substance can theoretically be reduced; in chemistry, the smallest particle of matter that can exist in combination with other atoms building up or constituting molecules. Two opposite doctrines of the constitution of matter were held by the ancient philosophers. One was that matter was infinitely divisible without losing its distinctive and individual properties. This is the doctrine of continuity or homoeomery. Anaxagoras is given as the founder of this view of the constitution of things. According to it any substance, such as wood or water, can by no process of subdivision, however far it might be carried, be made to be anything but a mass of wood or water. Infinite subdivision would not reach its limit of divisibility. Democritus and others held that there were ultimate particles of matter which were indivisible, and these were called atoms. This is the doctrine of atomacity, upheld by Epicurus, and enlarged on by Lucretius in his "De Rerum Natura". The early atomists held that the atoms were not in contact, but that voids existed between them, claiming that otherwise motion would be impossible. Among the moderns, Descartes and Spinoza adhered to continuity. Leibniz upheld atomicity, and Boscovich went to the last extreme of the theory, and defined atoms as centres of force denying them the attribute of impenetrability.

Molecule and atom

Modern science holds that matter is not infinitely divisible, that there is an ultimate particle of every substance. If this particle is broken up that particular form of matter will be destroyed. This particle is the molecule. It is composed of another division of matter called the atom. Generally, probably always, a molecule consists of several atoms. The atoms unite to form molecules and cannot exist except as constituents of molecules. If a molecule of any substance were broken up, the substance would cease to exist and its constituent atoms would go to form or to enter into some other molecule or molecules. There is a tendency to consider the molecule of modern science as identical with the atom of the old philosophers but the modern atomic theory has given the molecule a different status from that of the old-time atom. Atom, as used in natural science, has a specific meaning based upon the theory of chemistry. This meaning is modified by recent work in the field of radioactivity, but the following will serve as a definition. It is the smallest particle of an element which can exist in a compound. An atom cannot exist alone as such. Atoms combine with each other to form molecules. The molecule is the smallest particle of matter which can exist without losing its distinctive properties. It corresponds pretty closely to the old Epicurean atom. The modern atom is an entirely new conception. Chemistry teaches that the thousands of forms of matter upon the earth, almost infinite in variety, can be resolved into eighty substances, unalterable by chemical processes and possessing definite spectra. These, substances are called elements. The metals, iron, gold, silver and others, sulphur, and carbon are familiar example of elements. A mass of an element is made up of a collection of molecules. Each molecule of an element as a rule is composed of two atoms. Elements combine to form compound substances of various numbers of atoms in the molecule. Water is an example of a compound substance, or chemical compound. Its molecule contains three atoms, two atoms of hydrogen, and one atom of oxygen. If a quantity of these two elements were mixed, the result would be a mechanical mixture of the molecules of the two. But if heat, or some other adequate cause were made to act, chemical action would follow and the molecules, splitting up, would combine atom with atom. Part of a molecule of oxygen--one atom--would combine with part of two atoms of hydrogen--two atoms. The result would be the production of a quantity of molecules of water. Each water molecule contains one atom of oxygen and two atoms of hydrogen. The splitting-up of the elemental molecules into atoms is synchronous with their combining into molecules, so that an atom never exists alone. The molecules of the elements, oxygen and hydrogen, have disappeared, and in their places are molecules of water. There are about eighty kinds of atoms known, one kind for each element, and out of these the material world is made.

Invariability of composition

The invariability of composition by weight of chemical compounds is a fundamental law of chemistry. Thus water under all circumstances consists of 88.88% of oxygen and 11.11% of hydrogen. This establishes a relation between the weights of the atoms of hydrogen and oxygen in the water molecule, which is 1:8. Oxygen and hydrogen are gaseous under ordinary conditions. If water is decomposed, and the vases collected and measured, there will always be two volumes of hydrogen to one of oxygen. This illustrates another fundamental law--the invariability of composition by gaseous volume of chemical compounds. From the composition by volume of water its molecule is taken as composed of two atoms of hydrogen and one of oxygen, on the assumption that in a given volume of any gas there is the same number of molecules. As there are two atoms in the molecules of both of these elements, the above may be put in a more popular way thus: the atoms of hydrogen and oxygen occupy the same space. The ratio spoken of above of 1:8, is therefore the ratio of two atoms of hydrogen to one of oxygen. It follows that the ratio of one atom of hydrogen to one atom of oxygen is 1:16. The numbers 1 and 16 thus determined, are the atomic weights of hydrogen and oxygen respectively. Strictly speaking they are not weights at all only numbers expressing the relation of weight. Atomic weights are determined for all the elements, based on several considerations, such as those outlined for the atoms of oxygen and hydrogen. Thus the term atom indicates not only the constituents of molecules but has a quantitative meaning, the proportional part of the element which enters into compounds. The sum of the weights of the atoms in a molecule is the molecular weight of the substance. Thus the molecular weight of water is the sum of the weights of two hydrogen atoms, which is two, and of one oxygen atom, which is sixteen, a total of eighteen. If we divide the molecular weight of a compound into atomic weight of the atoms of any element in its molecule, it will give the proportion of the element in the compound. Taking water again, if we divide the molecular weight, 18, into the weight of the atoms of hydrogen in its molecule, 2, we obtain the fraction 2/18, which express the proportion of hydrogen in water. The same process gives the proportion of oxygen in water as 16/18.

Every element has its own atomic weight, and the invariability of the chemical composition by weight is explained by the invariability of the atomic weights of the elements. Tables of the atomic weights of the elements are given in all chemical text-books. The relations of the atomic weights to each other are several. The atom of lowest weight is the hydrogen atom. It is usually taken as one, which is very nearly its exact value if oxygen is taken as sixteen. On this basis one quarter of the other elements will have atomic weights that are whole numbers. This indicates a remarkable simplicity of relationship of weights, which is carried out by the close approach of the rest of the elements to the same condition, as regards their atomic weights. The range of the atomic weights is a narrow one. That of hydrogen is 1.008 — that of uranium 238.5. The latter is the heaviest of all. Between these all the other atomic weights lie. Many of the elements resemble each other in their chemical relations. It might appear that those nearest to each other in atomic weight should be of similar properties. This is not the case. If the elements are written down in the order of their atomic weights, beginning with the lightest and ending with the heaviest, it will be found that the position of an element in the series will indicate pretty clearly its properties. The elements will be found to be so arranged in the list that any element will be related as regards its chemical properties to the element eight places removed from it. This relationship may be thus expressed: the properties of an element are a periodic function of its atomic weight.

Mendeléeff's table

This relation is called Mendeléeff's Law, from one of two chemists who independently developed it. The elements may, as before said, be written down in the order of their atomic weights, but in eight vertical columns. Along the top line the eight elements of lightest atomic weights are written in the order of their weights, followed on the second line by the next eight, also in the order of their atomic weights. This arrangement, obviously, when carried out brings the elements eight atomic weights apart, into vertical columns. It will be found that all the elements in any vertical column are of similar chemical properties. When Mendeléeff made out his table it was supposed that several elements were as yet undiscovered. The table also brought out clearly certain numerical relations of the atomic weights. These together with other factors caused him to leave blank spaces in his table, which none of the known elements could fill. For these places hypothetical elements were assumed, whose general properties and atomic weights were stated by him. One by one these elements have been discovered, so that Mendeléeff's Law predicted the existence of elements later to be discovered. These discoveries of predicted elements constitute one of the greatest triumphs of chemical science. Up to within a very recent period the atom was treated as the smallest division of matter, although the possibility of the transmutation of the elements in some way, or in some degree, has long been considered a possibility. It was conjectured that all the elements might be composed of some one substance, for which a name, protyle, meaning first material, was coined. This seemed to conflict with the accepted definition of the atom, as protyle indicated something anterior to or preceding it. The idea rested in abeyance, as there was little ground for building up a theory to include it. Recent discoveries have resuscitated this never quite abandoned theory; protyle seems to have been discovered, and the atom has ceased to hold its place as the ultimate division of matter.


The most recent theory holds that the atom is composite, and is built up of still minuter particles, called corpuscules. As far as the ordinary processes of chemistry are concerned the atom remains as it was. But investigations in the field of radioactivity, largely physical and partly chemical, go to prove that the atom, built up of corpuscules as said above, depends for its atomic weight upon the number of corpuscules in it, and these corpuscules are all identical in nature. In these corpuscules we have the one first material, or protyle. It follows that the only difference between atoms of different elements is in the number corpuscules they contain. Any process which would change the number of corpuscules in the atoms of an element would change the element into another one, thus carrying out the transmutation of elements. So far one transmutation is accepted as effected experiments in radioactivity go to prove that some elements, notably radium project particles of in conceivable minuteness into space. These particles have sometimes one-half the velocity of light. They are called corpuscules. The corpuscule is sometimes defined as a particle of negative electricity, which, in the existing state of electrical knowledge, is a very imperfect definition. They are all negatively electrified, and therefore repel each other. The condition of equilibrium of groups of such particles, if held near to each other by another external force has been investigated by Prof. J.J. Thomson, and his investigations establish a basis for a theory on the constitution of atoms. Thus, assume an atom to consist of a number of corpuscules, not touching each other, negatively electrified so that they repel one another, and held within the limits of the atom by what may be termed a shell of attractive force. Professor Thomson shown that such particles, under the conditions outlined above, arrange themselves into groups of various arrangement, the latter depending on their number. If the number of particles in a group be progressively increased, a periodic recurrence of the groupings will occur. Assume a group of five particles. These will form a group of definite shape. If more particles are added to the group, the first additions will cause the five group to disappear, other groups taking the place, until the number reaches fifteen, when the original grouping of five will reappear, surrounded by the other ten particles. On adding more particles, the five and ten group disappear, to be succeeded by others, until the number of thirty is reached. At this point the original five group and the ten group reappear, with a new group of fifteen. The same recurrence of groupings takes place with forty-seven and sixty seven particles. This gives the outlines of an explanation of the periodic law. If any number of particles be taken they will show groupings, characteristic of the number, and subject to periodical reappearance of groupings is exactly comparable to the phenomena of the periodic law. It is the reappearance of the similar properties at periodic intervals. The corpucular theory also accounts for the variation of the elements in atomic weight. Corpuscles are supposed to be all like, so that the weight of an atom would depend on how many corpuscules were require to form it. Thus an atom of oxygen would contain sixteen times as many corpuscules as would an atom of hydrogen, weighing only one-sixteenth as much. The weight of an atom of hydrogen has been approximately calculated as expressed by the decimal, 34 preceded by thirteen ciphers, of a gram. This means that thirty-four thousand millions of millions of atoms of hydrogen would weigh in the aggregate one gram. These calculations are based on determination of the electric charge of corpuscules. Corpuscles are calculated as being one-thousandth of the mass of an atom of hydrogen. Professor Oliver Lodge gives the following comparison: if a church of ordinary size represent an atom, a thousand grains of sand dashing about its interior with enormous velocity would represent its constituent corpuscules. When atoms unite to form molecules, they are said to saturate each other. Elements vary in the saturating power of their atoms. The saturating power is called atomicity or valency. Some elements have a valency of one, and are termed monads. A monad can saturate a monad. Others are termed dyads, have a valency of two, two monads being required to saturate one dyad, while one dyad can saturate another dyad. Valencies run on through triads, tetrads, pentads, hexads, heptads, and octads, designating valencies of three, four, five, six, seven, and eight respectively.

About this page

APA citation. Sloane, T. (1907). Atom. In The Catholic Encyclopedia. New York: Robert Appleton Company.

MLA citation. Sloane, Thomas O'Conor. "Atom." The Catholic Encyclopedia. Vol. 2. New York: Robert Appleton Company, 1907. <>.

Transcription. This article was transcribed for New Advent by Joseph P. Thomas.

Ecclesiastical approbation. Nihil Obstat. 1907. Remy Lafort, S.T.D., Censor. Imprimatur. +John M. Farley, Archbishop of New York.

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