Subatomic Particles and the Structure of the Atom

1. Section 4.2 - Structure of the Nuclear Atom Introduction • A simple but important device first used by scientists, the cathode-ray tube, would achieve its greatest fame as the picture tube of the common TV set. • Today, cathode-ray tubes are found in TVs, computer monitors, and many other devices with electronic displays. • Scientist’s observations provided important evidence about the structure of atoms.

2. 2. Subatomic Particles • Most of Dalton’s theory is accepted today. • Now, however, it is known that atoms are divisible. • Three kinds of subatomic particles are electrons, protons, and neutrons. • Electrons • In 1897, the English physicist J. J. Thomson (1856–1940) discovered the electron. • Electrons are negatively charged subatomic particles. • Thomson performed experiments that involved passing an electric current through gases at low pressure sealed in a glass tube with electrodes on either end. Cathode-ray tubes are found in TVs, computer monitors, and many other devices with electronic displays.

3. Subatomic Particles (cont.) Cathode Ray Tube • The result was a glowing beam, or cathode ray, that traveled from the cathode (-) to the anode (+). • The cathode ray could be deflected by electrically charged plates. • Thomson concluded that a cathode ray is a stream of high speed electrons. • To test his hypothesis, Thomson set up an experiment to measure the ratio of charge of an electron to its mass. • He found that the ratio was constant, independent of the gas used. • Concluded that electrons must be parts of atoms of all elements. A cathode ray is deflected by a magnet. A cathode ray is deflected by electrically charged plates.

4. Subatomic Particles (cont.) • U.S. physicist Robert A. Millikan (1868-1953) carried out experiments to find the quantity of charge carried by an electron. • Called the oil drop experiment • Ionized oil drops were sprayed, their terminal velocity was measured in order to determine mass of the oil drops based on density and gravitational field. • A magnetic field was introduced, once the oil drops were suspended, magnetic field and gravity were balanced, which determined charge. • Using mass-to-charge ratio of an electron, he calculated the mass of the electron. • An electron carries exactly one unit of negative charge, and its mass is 1/1840 the mass of a hydrogen atom

5. 4.2 Subatomic Particles (cont.) • Protons and Neutrons • If cathode rays are electrons given off by atoms, what remains of the atoms that have lost the electrons? • Four ideas about matter and electric charges: • Atoms have no net electric charge, they are electrically neutral. • Electric charges are carried by particles of matter • Electric charges always exist in whole-number multiples of a single basic unit, there are no fractions of charges. • When a given number of negatively charged particles combines with an equal number of positively charged particles, an electrically neutral particle forms.

6. Subatomic Particles (cont.) • In 1886, Eugen Goldstein (1850–1930) observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays, he called them anode rays. • He noticed that when the cathode was perforated a glow was emitted at the cathode end. • He concluded that they were composed of positive particles whose identity was dependent on the gas inside the tube. • The e/m ratio was calculated for different gases and it was discovered that the positively charged subatomic particles were protons.

7. Subatomic Particles (cont.) • In 1932, the English physicist James Chadwick (1891–1974) confirmed the existence of yet another subatomic particle: the neutron. • Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton. • J.J. Thomson and others supposed the atom was filled with positively charged material and the neutrons and electrons were evenly distributed throughout. • Called the “plum pudding” model Table 4.1 summarizes the properties of electrons, protons, and neutrons.

8. 3. The Atomic Nucleus • In Thomson’s atomic model, known as the “plum-pudding” model, electrons were stuck into a lump of positive charge, similar to raisins stuck in dough. • This model of the atom turned out to be short-lived, however, due to the work of Ernest Rutherford (1871–1937). Ernest Rutherford’s Portrait

9. The Atomic Nucleus (cont.) • Rutherford’s Gold-Foil Experiment • In 1911, Rutherford and his coworkers at the University of Manchester, England, directed a narrow beam of alpha particles at a very thin sheet of gold foil. • According to prevailing theory, the alpha particles should have passed easily through the gold, with only slight deflection due to the positive charge thought to be spread out in the gold. • However, the great majority of alpha particles passed straight through the gold atoms, without deflection. • Only a small fraction of the particles bounced off at very large angles. Rutherford’s Gold-Foil Experiment

10. The Atomic Nucleus (cont.) • The Rutherford Atomic Model • He proposed that the atom is mostly empty space due to lack of deflection. • He concluded that all the positive charge and almost all of the mass are concentrated in a small region called the nucleus. • The nucleus is the tiny central core of an atom and is composed of protons and neutrons. • In Rutherford’s model, called the nuclear atom, the protons and neutrons are located in the nucleus. • The electrons are distributed around the nucleus and occupy almost all the volume of the atom.

11. END OF SECTION 2