5. Periodic Table

By the middle of the 19th century several chemists recognized that similarities in the chemical properties of various elements implied a regularity that might be illustrated by arranging the elements in a tabular or periodic form. The Russian chemist Dmitry Mendeleyev proposed a chart of elements called the periodic table, in which the elements are ar­ranged in rows and columns so that elements with similar chemical properties are grouped together. According to this arrangement, each element was assigned a number (atomic number) ranging from 1 for hy­drogen to 92 for uranium. Because not all the elements were known at the time of Mendeleyev, blank spaces were left in the periodic table, each of which corresponded to a missing element. Further research, aided by the arrangement of the known elements in the chart, led to the discovery of missing elements. Elements of higher atomic number have correspondingly heavier atomic weights; this fact could have been pre­dicted from Prout's hypothesis.

6. Size of Atom

Curiosity about the size of the atom and its weight tantalized hun­dreds of scientists for a long period during which lack of adequate in­struments and proper techniques prevented them from obtaining satis­factory answers. Subsequently, a variety of ingenious experiments was devised to determine the size and weight of the various atoms. The lightest of all atoms, hydrogen, has a diameter of 1 Ч 10^-8 cm (0.00000001 cm) and weighs 1.7 Ч 10^-24 (the fraction of a gram repre­sented by 17 preceded by 23 zeros and a decimal point). An atom is so small that a single drop of water contains more than a million million million atoms.

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7. Radioactivity

That the atom is not a solid bit of matter, incapable of further subdivi­sion, became evident with the discovery of radioactivity. In 1896 the French physicist Antoine Henri Becquerel found that certain substances, such as uranium salts, give off penetrating rays of mysterious origin. Only a year earlier the German scientist Wilhelm Conrad Roentgen, had announced the discovery of X rays, which can penetrate sheets of lead. The French scientists Marie Curie and her husband Pierre Curie contrib­uted further to an understanding of radioactive substances. As a result of the research of the British physicist Ernest Rutherford and his contempo­raries, it was shown that uranium and some other heavy elements, such as thorium and radium, emit three different kinds of radiation, initially called alpha (6), beta (b), and gamma (r) rays. The first two, which were found to consist of electrically charged bits of matter, are now called al­pha and beta particles. Gamma rays eventually were identified as elec­tromagnetic waves, similar to X rays but of shorter wavelengths. .

8. Rutherford Nuclear Atom

Recognition of the nature of radioactive emissions enabled physicists to penetrate into the mystery of the atom. Far from being a solid bit of matter, the atom was found to consist mostly of space. At the center of this space is an infinitesimally small core called the nucleus. Rutherford established that the mass of the atom is concentrated in its nucleus. He also proposed that satellites called electrons travel in orbits around the nucleus The nucleus has a positive charge of electricity; the electrons each have a negative charge. The charges carried by the electrons add up to the same amount of electricity as resides in the nucleus, and thus the normal electrical state of the atom is neutral.