4.2. Structure of the Atom http://www.ck12.org
Using this information, Millikan calculated the charge of an electron to be 1.5924× 10 −^19 coulombs. A coulomb (C)
is the SI unit for electric charge, where 1 coulomb = 1 ampere×1 second. (Remember, an ampere is an SI base unit
for electric current.) Today, the accepted value for the charge of an electron is 1.602176487× 10 −^19 C. Despite the
relatively simple apparatus with which it was determined, Millikan’s value was within 1% of the currently accepted
value. Combining this value with information from J. J. Thomson’s experiments, Millikan was also able to calculate
the mass of an electron. The currently accepted value is 9.10938215× 10 −^31 kg.
Discovery of the X-ray
Right around the time that Thomson was formulating his model of the atom, a scientist named Wilhelm Conrad
Roentgen was studying the behavior of electricity in discharge tubes. These were partially evacuated gas-filled
tubes which would conduct an electric current, similar to the Crookes tube used in the cathode ray experiments. He
devised an experiment in which he covered a discharge tube with black cardboard, and several feet away, he placed a
sheet of paper that had been chemically treated with a compound called barium-platinum cyanide. When he applied
voltage to the discharge tube, he noticed the screen several feet away became fluorescent, emitting light. This was
peculiar, because the tube had been completely covered by material that would block the escape of any cathode rays.
Roentgen continued to explore this phenomenon. He moved the screen further away, he turned the screen around,
and he placed objects between the screen and the discharge tube. In all cases, the screen still fluoresced when the
discharge tube was turned on. Then Roentgen had his wife place her hand atop a photographic plate, and the rays
were shone towards the plate. After developing the plate, he observed an image of his wife’s hand that “showed
the shadows thrown by the bones of her hand and that of a ring she was wearing” (Wilhelm Conrad Roentgen -
Biography).Figure4.7 is an image of what he saw.
FIGURE 4.7
The skeleton of Roentgen’s wife’s hand,
as captured on the photographic plate.
What do you think the dark spot in the
picture might be caused by?This was the first "roentgenogram" ever taken. He interpreted this to mean that another ray, other than the cathode
rays, was being produced that could penetrate and travel through objects at a distance. He called these X-rays, and
he received the Nobel Prize in Physics in 1901 for his brilliant work. Modern day X-rays that one might receive
for a medical exam operate on the same principles that Roentgen discovered. Several major discoveries followed
shortly after Roentgen’s discovery of X-rays. Just two months later, in 1896,radioactivitywas discovered by a
Frenchman named Henri Becquerel. Becquerel discovered that certain substances, like uranium salts, emit charged
particles. Following this work, Marie and Pierre Curie began to study the behavior of various radioactive substances
in 1897. In fact, Madame Curie coined the word “radioactivity.” Their work resulted in the discovery of mass
changes in radioactive elements, which later became known as radioactive decay. They also identified two new
radioactive elements, which later became known as polonium and radium. The Curies were awarded the Nobel Prize