Alpha particle

“ Half-lives for alpha decay vary from a fraction of a second to billions of years. But even the shortest half-life for alpha decay is remarkably long compared to the time required for the alpha particle to cross the nucleus. This means that the alpha particle makes a tremendous number of attempts to escape from the nucleus before it actually succeeds. According to the older classical theory the alpha process should never occur, and in fact it occurs with a very small probability. ”

“ The alpha particle is a helium nucleus made up, by this theory, of 4 protons and 2 electrons. If a nucleus loses an alpha particle, its mass number should decline by 4 and its atomic number by 4 - 2, or 2. And, indeed, when uranium-238 (atomic number 92) gives off an alpha particle, it becomes thorium-234 (atomic number 90) .
Suppose a beta particle is emitted. A beta particle is an electron and if a nucleus loses an electron, its mass number is almost unchanged. (An electron is so light that in comparison with the nucleus, we can ignore its mass.) ”

“ The loss of two alpha particles and the loss of a single beta particle in a molecule of lead, causes that molecule to become an isotopal molecule of gold, with an atomic number 79, and the atomic weight of 199. ”

“ P. T. R. Wilson by which the expulsion of alpha particles and beta particles can be made visible and even photographed. The first part of the path of the alpha particles is nearly straight, but as they lose energy, contact with molecules of other substances deflects them in a characteristic manner. The beta particles, on account of their greatly inferior mass, pursue very irregular courses. ”

“ The alpha and beta particles move so swiftly that they are actually able to pass through the structure of the atom and are deflected by the intense forces within the atom. Geiger first drew attention to a very unexpected effect with alpha particles. When a pencil of alpha rays falls on a thin film of gold, for example, the great majority of the particles pass through with little absorption. A few, however, are found to be so scattered that they are turned back through an angle of more than a right angle. ”

“ Rutherford used naturally occurring alpha particles from radium as his projectiles because they were the most effective he could then find. But these natural alpha particles have several drawbacks: they are positively charged, like the nucleus itself, and are therefore more or less repulsed depending on the proton number of the element being bombarded; they do not move fast enough to penetrate the nuclei of heavier elements (those with many protons) ”

“ THE CHEMICAL NATURE OF THE ALPHA PARTICLES FROM RADIOACTIVE SUBSTANCES ”

“ Hence, the main difficulty attending the possible amplification of objects less than about the 1100000 of an inch in diameter is a purely physical one, and depends upon the constitution of light itself.
The various phenomena of chemical physics teach us that matter is not homogeneous, but is made up of infinitesimal particles or atoms, the term atom meaning indivisible particle; and that the term molecule—meaning literally a little mass—refers to an aggregation of two or more atoms. ”

“ The incentive to reform came from two sources. Physics, in the kinetic theory of gases, gave a new demonstration of the truth of Avogadro’s law, and led chemists to realize more clearly than before the distinction between atoms and molecules. Soon it was seen that the molecule was the smallest particle of matter which could exist as such, while the atom was the smallest particle which could take part in any chemical change. The metaphysical atom was really the modern molecule; the chemical atom was a new conception, due to the discoveries of chemistry alone. ”

“ We have shown that the ultimate discrete particles are related to each other through pluralities, positions, paths and changes, and we have now to consider another method of association, for particles of matter are incorporated, and enter into fixed associations with one another by affinity, the nature of which has never been explained, although the association is well known. Every particle of matter under certain conditions seems to be able to choose its associate, and a group of such particles that have mutual affinities become compounded into that which is usually denominated a molecule. ”

“ The α particle enters through a small opening into a metal tube containing a gas at a reduced pressure. The ionization produced by the a particle in its passage through the gas is magnified several thousand times by the movement of the ions in a strong electric field. In this way, the entrance of an α particle into the detecting vessel is shown by a sudden and large deflection of the measuring instrument. By this method, they determined that 3.4 X 1010 α particles are ejected per second from one gram of radium itself and from each of its α ray products in equilibrium with it. ”

“ In this lecture an attempt will be made to give a brief historical account of the development of our knowledge of the α-rays and to trace the long and arduous path trodden by the experimenter in the attempts to solve the difficult question of the chemical nature of the α-particles. α-rays were first observed in 1899 as a special type of radiation and during the last six years there has been a persistent attack on this great problem, which has finally yielded to the assault when the resources of the attack seemed almost exhausted. ”

“ Each of these ultimate physical particles may be composed of two or more atoms (as in the case of water vapor) , but such a compound atom conducts itself as if it were a simple and indivisible atom, as regards the amount of space that separates it from its fellows under given conditions of pressure and temperature. The compound atom, composed of two or more elementary atoms, Avogadro proposed to distinguish, for purposes of convenience, by the name molecule. It is to the molecule, considered as the unit of physical structure, that Avogadro's law applies. ”

“ Such a diagram is seen in Fig. 15. It shows to
relatively correct scale the limiting range of all the alpha-ray
producing members of the uranium and thorium families. We know
that each member of a family will exist in equilibrium amount
within the nucleus possessing the parent element. Each alpha ray
leaving the nucleus will just attain its range and then cease to
affect the mica. Within the halosphere, there must be, therefore,
the accumulated effects of the influences of all the rays. Each
has its own sphere of influence, and the spheres are all
concentric. ”

“ The changing atom may radiate a part of its own mass. The
"alpha"-ray (a-ray) is such a material ray. It is an electrified
helium atom cast out of the parent atom with enormous
velocity—such a velocity as would carry it, if not impeded, all
round the earth in two seconds. All alpha-rays are positively
electrified atoms of the element helium, which thereby is shown
to be an integral constituent of many elements. The alpha-ray is
not of much value to medical science, for, in spite of its great
velocity, it is soon stopped by encounter with other atoms. ”

“ The real value of the new atomic hypothesis, however, did not lie in the two points which Democritus and his followers would have considered essential—namely, the indivisibility of the 'atoms' and the presence of an interatomic vacuum—but in the assumption that, to the extent to which our means of analysis take us, material bodies consist of definite minute masses, each of which, so far as physical and chemical processes of division go, may be regarded as a unit—having a practically permanent individuality. Just as a man is the unit of sociology, without reference to the actual fact of his divisibility, so such a minute mass is the unit of physico-chemical science—that smallest material particle which under any given circumstances acts as a whole. ”

“ The identity of the radiant particle with the component of the atom is inferable. It possesses the requisite properties of mass and resilience, and sufficient linear motion. Whether it should also have attraction imputed to it as inherent depends on whether that property in the molecule, where alone it is observed, is derived from the particle. If not, the latter needs only mass to conserve its deflections and its course under the first law of motion, and resilience to secure its compensatory readjustment in equilibrium. ”

“ It thus seems clear that the dimensions of the nuclei of light atoms are small, and almost unexpectedly small in the case of aluminum when we remember that 27 protons and 14 electrons are concentrated in such a minute region. The view that the forces between nuclei change from repulsion to attraction when they are very close together seems very probable, for otherwise it is exceedingly difficult to understand why a heavy nucleus with a large excess of positive charge can hold together in such a confined region. ”

“ If we define the molecule of any substance as the smallest mass of that substance which retains all its chemical properties, we can start with the extensive generalization of Avogadro and Ampère, that an equal volume of every kind of matter in the state of vapor, and under the same pressure and temperature, contains an equal number of such molecules. The conception of matter as consisting of parts, which are perpetually flying over their microscopic orbits, and producing by their fortuitous concourse all the observed qualities of bodies, is as old as Lucretius. ”

“ The particles of alum are not in close contact, and they have no freedom of motion unless they are dissolved in water, when they become invisible; the water by its chemical power destroys the mechanical aggregation of the solid alum far beyond any operation of levigation. The solid alum has become liquid, like water; the particles are now free to move without let or hindrance from friction. A solution, (from the Latin solvo, to loosen) is obtained. The alum must indeed be reduced to minute particles, as they are alike invisible to the eye whether assisted by the microscope or not. ”

“ That which is accomplished by convention in arithmetic, is accomplished by incorporation in nature. In this manner by combination the quantitative property of number in the particle becomes the classific property or kind in the molecule, and as there is a hierarchy of molecules and every one considered as a unit may become a particle in a higher order, we are compelled to consider it in this double and relative capacity, as one of many and as many in one. A molecule in its internal aspect appears as many ”

“ But when we come to what goes on in “empty space,” or in the “æther,” we find ourselves in difficulties if we adhere to the quantum theory. Consider what happens when a wave of light is sent out by an atom, with only one quantum of action in each period. The wave spreads out [Pg 136] in all directions, growing fainter as it goes on, like a ripple when a stone is dropped into a pond. The evidence that light consists of waves remains quite unshaken; it is derived from the phenomena of interference and diffraction. ”

“ Unfortunately, we don't posses such a theory; therefore we are far from completely understanding the optical properties of resting bodies on the basis of electron theory. Hypothesis I thus completely hangs in the air, it is very much capable of being changed.
In the course of assessing the probability of the various hypotheses, one has to care that ideas concerning the nature of the molecular forces and the quasi-elastic forces are far less clarified, and are far less accessible to experimental verification, than the ideas concerning the constitution of the free negative electrons. ”

“ A free electron gives just as much support as an atom does; it is of much smaller mass, but it moves about a hundred times as fast. The smashing of one silver atom provides 47 free electrons, making with the residual nucleus of the atom 48 particles in all. The aluminium atom gives 13 electrons or 14 particles in all; thus 4 aluminium atoms give 56 independent particles. The change from smashed silver to an equal mass of smashed aluminium only means a change from 48 to 56 particles, requiring a reduction of temperature by 14 per cent. ”

“ Of course the atom itself was in no sense displaced, but Avogadro's law soon made it plain that the atom had often usurped territory that did not really belong to it. In many cases the chemists had supposed themselves dealing with atoms as units where the true unit was the molecule. In the case of elementary gases, such as hydrogen and oxygen, for example, the law of equal numbers of molecules in equal spaces made it clear that the atoms do not exist isolated, as had been supposed. ”

“ And thus, whether we contemplate the infinitely small or the infinitely grand, in every case comes back upon us the wide induction, that the action of matter in atom, in molecule, or in mass, is ever at a distance! Of actual contact there is probably no instance afforded in nature, excepting in the intimate substance of the ultimate atom. ”

“ It is true we have no direct evidence of any elementary atom actually undergoing a change of state; but when we regard the variations produced by electrical influence, the changes of state which arise from the power of heat, and the physical alterations produced by light, it will be difficult to come to any other conclusion than that the particles of matter known to us as ultimate are capable of change, and consequently must be far removed from positively simple bodies, since the real elementary atom, possessing fixed properties, cannot be supposed capable of undergoing any transmutation. ”