Chemistry— Section 4.2
Rutherford's Model
"Rutherford concluded that the plum pudding model was incorrect because it could not explain the results of the gold foil experiment. Considering the properties of the alpha particles and the electrons, and the frequency of the deflections, he calculated that an atom consisted mostly of empty space through which the electrons move. He also concluded that almost all of the atom's positive charge and almost all of its mass were contained in a tiny, dense region in the center of the atom, which he called the nucleus. The negatively charged electrons are held within the atom by their attraction to the positively charged nucleus. Rutherford's nuclear atomic model is shown in Figure 15. Because the nucleus occupies such a small space and contains most of an atom's mass, it is incredibly dense. If a nucleus were the size of the dot in the exclamation point at the end of this sentence, its mass would be approximately as much as that of 70 automobiles! The volume of space through which the electrons move is huge compared to the volume of the nucleus. A typical atom's diameter is approximately 10,000 times the diameter of the nucleus.
Explain the direction of a cathode Ray in a cathode Ray rube.
A cathode ray is composed of a stream of electrons that originate at the cathode and flow through the tube to the anode. Because the electrons have a negative charge, they are attracted to the anode, which has a positive charge.
Complete Model of Atom
All atoms are made up of the three fundamental subatomic particles—the electron, the proton, and the neutron. Atoms are spherically shaped, with a small, dense nucleus of positive charge surrounded by one or more negatively charged electrons. Most of an atom consists of fast-moving electrons traveling through the empty space surrounding the nucleus. The electrons are held within the atom by their attraction to the positively charged nucleus. The nucleus, which is composed of neutral neutrons (hydrogen's single-proton nucleus is an exception) and positively charged protons, contains all of an atom's positive charge and more than 99.97% of its mass. It occupies only about one ten-thousandth of the volume of the atom. Because an atom is electrically neutral, the number of protons in the nucleus equals the number of electrons surrounding the nucleus.
Proton and Neutron
By 1920, Rutherford had refined the concept of the nucleus and concluded that the nucleus contained positively charged particles called protons. A proton is a subatomic particle carrying a charge equal to but opposite that of an electron; that is, a proton has a charge of 1+. In 1932, Rutherford's coworker, English physicist James Chadwick (1891-1974), showed that the nucleus also contained another subatomic neutral particle, called the neutron. A neutron is a subatomic particle that has a mass nearly equal to that of a proton, but it carries no electric charge. In 1935, Chadwick received the Nobel Prize in Physics for proving the existence of neutrons.
Rutherford's Experiment
In the experiment, a narrow beam of alpha particles was aimed at a thin sheet of gold foil. A zinc-sulfide-coated screen surrounding the gold foil produced a flash of light when struck by an alpha particle. By noting where the flashes occurred, the scientists could determine if the atoms in the gold foil deflected the alpha particles. Rutherford was aware of Thomson's plum pudding model of the atom. He expected the paths of the massive and fast-moving alpha particles to be only slightly altered by a collision with an electron. And because the positive charge within the gold atoms was thought to be uniformly distributed, he thought it would not alter the paths of the alpha particles, either. A few of the alpha particles were deflected at large angles. Several particles were deflected straight back toward the source. Rutherford likened the results to firing a large artillery shell at a sheet of paper and the shell coming back at The" Excerpt From Chemistry Thandi Buthelezi, Laurel Dingrando, Nicholas Hainen & Cheryl Wistrom https://itunes.apple.com/us/book/chemistry/id491195682?mt=13 This material may be protected by copyright
Plum Pudding Model
J.J. Thomson's plum pudding model of the atom states that the atom is a uniform, positively charged sphere containing electrons
Mass of an Electron
Knowing the electron's charge and using the known charge-to-mass ratio, Millikan calculated the mass of an electron. The" Excerpt From Chemistry Thandi Buthelezi, Laurel Dingrando, Nicholas Hainen & Cheryl Wistrom https://itunes.apple.com/us/book/chemistry/id491195682?mt=13 This material may be protected by copyright.
Cathode Ray
Radiation that originates from the cathode and travels to the anode of a cathode ray tube. — stream of negatively charged particles
Atom
Smallest particle of an element that retains the properties of that element
The Oil-Drop Experiment
The next significant development came in the early 1910s, when the American physicist Robert Millikan (1868-1953) determined the charge of an electron using the oil-drop apparatus shown in Figure 10. In this apparatus, oil is sprayed into the chamber above the two parallel charged plates. The top plate has a small hole through which the oil drops. X-rays knock out electrons from the air particles between the plates and the electrons stick to the droplets, giving them a negative charge. By varying the intensity of the electric field, Millikan could control the rate of a droplet's fall. He determined that the magnitude of the charge on each drop increased in discrete amounts and determined that the smallest common denominator was 1.602 × 10 -19 coulombs. He identified this number as the charge of the electron. This charge was later equated to a single unit of negative charge noted 1−; in other words, a single electron carries a charge of 1
Summarize how JJ Thompson discovered the electron.
Unable to measure the particle's mass directly, English physicist J. J. Thomson (1856-1940) began a series of cathode-ray tube experiments at Cambridge University in the late 1890s to determine the ratio of its charge to its mass. Charge-to-mass ratio. By carefully measuring the effects of both magnetic and electric fields on a cathode ray, Thomson was able to determine the charge-to-mass ratio of the charged particle. He then compared that ratio to other known ratios. Thomson concluded that the mass of the charged particle was much less than that of a hydrogen atom, the lightest known atom. The conclusion was shocking because it meant there were particles smaller than the atom. In other words, Dalton had been incorrect—atoms were divisible into smaller subatomic particles. Because Dalton's atomic theory had become so widely accepted and Thomson's conclusion was so revolutionary, many other scientists found it hard to accept this new discovery. But Thomson was correct. He had identified the first subatomic particle—the electron. He received a Nobel Prize in 1906 for this discovery.
Explain how the cathode ray tube was discovered.
While working in a darkened laboratory, English physicist Sir William Crookes noticed a flash of light within one of the cathode-ray tubes. A green flash was produced by some form of radiation striking a zinc-sulfide coating that had been applied to the end of the tube. Further work showed that there was a ray (radiation) going through the tube. This ray, originating from the cathode and traveling to the anode, was called a cathode ray.
Scanning Tunneling Microscope
an instrument used to generate images of individual atoms a fine point is moved above a sample and the interaction of the point with the superficial atoms is recorded electronically
What experimental results led to the conclusion that electrons were part of all forms of matter?
changing the type of electrode or the type of gas did not affect the ray produced.
