2. Atoms & Elements

Which of the following atoms has the greatest number of protons?

A charged particle moves between two electrically charged plates, as shown here.

(a) What is the sign of the electrical charge on the particle?

Which experiment and subsequent observation led to the discovery that atoms contain negatively charged particles, now known as electrons? (LO 2.10–2.12) (a) Oil is sprayed into a chamber and the speed at which the oil droplets fall is measured with and without an applied voltage. X rays in the chamber knock electrons out of air molecules. The electrons stick to the oil pro-ducing an overall negative charge on the drops. Adjusting the voltage changes the speed at which the negatively charged oil droplets fall. (b) When a high voltage is applied across metal electrodes at opposite ends of a sealed glass tube, a cathode ray is produced. The cathode ray is repelled by a negatively charged plate. (c) A radioactive substance emits alpha particles, which are directed at a thin gold foil. Most of the alpha par-ticles pass through the foil, but a few alpha particles are slightly deflected and some even bounce back toward the radioactive source. (d) The mass of different elements in a pure chemical com-pound are measured. Different samples of the compound always contains the same proportion of elements by mass.

An unknown particle is caused to move between two electrically charged plates, as illustrated in Figure 2.7. You hypothesize that the particle is a proton. (b) Would it be deflected by a smaller or larger amount than the b rays?

An unknown particle is caused to move between two electrically charged plates, as illustrated in Figure 2.7. You hypothesize that the particle is a proton. (a) If your hypothesis is correct, would the particle be deflected in the same or opposite direction as the b rays?

Determine whether each of the following statements is true or false. If false, correct the statement to make it true: (d) The protons in the nucleus of the helium atom are held together by a force called the strong nuclear force.

(a) Thomson's cathode-ray tube (Figure 2.4) and the mass spectrometer (Figure 2.11) both involve the use of electric or magnetic fields to deflect charged particles. What are the charged particles involved in each of these experiments?

What affects the magnitude of the deflection of the cathode ray in Thomson's experiment?

Label the following statements about J. J. Thomson's cathode-ray tube experiments shown in Figure 2.6 as true or false. (b) A cathode ray is a stream of charged particles.

Label the following statements about J. J. Thomson's cathode-ray tube experiments shown in Figure 2.6 as true or false. (c) The cathode ray is deflected away from a positively charged plate.

Label the following statements about J. J. Thomson's cathode-ray tube experiments shown in Figure 2.6 as true or false. (f) By measuring the deflection of the cathode ray beam caused by electric fields of known strength, the charge-to-mass ratio of the electron was calculated.