Man's physical universe

xanabras

572 MAN IS MASTERING HIS MATERIAL WORLD

that atoms have an open structure similar to that of our solar system.

Each atom is then considered to consist almost entirely of unoccupied

space, which electrons or alpha particles could pass through without

hitting anything, just as the stars move rapidly through space for

millions of years without meeting another star.

Ernest Rutherford found that the majority of alpha particles would

pass through very thin metal foils without changing their direction,

but that some alpha particles would be more or less deflected, while

others would rebound from the foil. A relatively heavy mass, positively

charged, must have caused these deflections in direction. Rutherford

calculated that all of the positive charges in a gold atom must be concentrated

into a space having a diameter of about one hundredthousandth

that of the whole atom in order to produce the deflection

which he observed. Rutherford therefore concluded that atoms are

composed of a very small positively charged nucleus surrounded by

electrons distributed at intervals throughout the rest of the space

within the atom. The atom is, therefore, not a solid unit of matter

but a portion of space in which the nucleus maintains its attraction

for a number of electrons equal to the charge on the nucleus.

Moseley AflSxed the Atomic Numbers of the Elements.

Sir W. H. Bragg found that the surface of a crystal could be used to

reflect X rays. The X rays produced when cathode rays in a vacuum

tube strike any target are composed of radiations of many different

frequencies. These rays are diff^racted, or sorted out, by the crystal,

in much the same way that a prism or diffraction grating sorts out

visible light into its component colors.

H. G. J. Moseley found that in passing from one element to another

in the periodic table all of the lines in the X-ray spectrum are shifted.

Thus it became possible to determine the position of an element in the

periodic table by measuring the shift in the X-ray spectrum. Starting

with hydrogen, the elements are numbered according to this shift, and

the number so obtained is called the atomic number.

Moseley's atomic numbers were obtained by experiment and in

themselves gave no definite information concerning the internal

structure of atoms.

The Mass Spectrograph Revealed the Existence of Many Isotopes.

F. W. Aston (1877- ), of the Cavendish Laboratory in England,

modified and greatly improved Thomson's original mass spectograph.

With the mass spectograph one may determine the atomic weights of

the elements. One half of the elements have had their atomic weights

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