details, the principle was the same as that employed at the turn of
the century: you smash things together and look at the fragments
that fly off to see what was inside them. The approach has been
compared to shooting a clock with a rifle and then studying the
pieces that fly off to figure out how the clock worked.
Discussion Questions
A The diagram, showing alpha particles being deflected by a gold
nucleus, was drawn with the assumption that alpha particles came in on
lines at many different distances from the nucleus. Why wouldn’t they all
come in along the same line, since they all came out through the same
tube?
B Why does it make sense that, as shown in the figure, the trajectories
that result in 19◦and 20◦scattering cross each other?
C Rutherford knew the velocity of the alpha particles emitted by radium,
and guessed that the positively charged part of a gold atom had a charge
of about +100e(we now know it is +79e). Considering the fact that some
alpha particles were deflected by 180◦, how could he then use conserva-
tion of energy to derive an upper limit on the size of a gold nucleus? (For
simplicity, assume the size of the alpha particle is negligible compared to
that of the gold nucleus, and ignore the fact that the gold nucleus recoils
a little from the collision, picking up a little kinetic energy.)8.2.4 The structure of nuclei
The proton
The fact that the nuclear charges were all integer multiples ofe
suggested to many physicists that rather than being a pointlike ob-
ject, the nucleus might contain smaller particles having individual
charges of +e. Evidence in favor of this idea was not long in arriv-
ing. Rutherford reasoned that if he bombarded the atoms of a very
light element with alpha particles, the small charge of the target
nuclei would give a very weak repulsion. Perhaps those few alpha
particles that happened to arrive on head-on collision courses would
get so close that they would physically crash into some of the target
nuclei. An alpha particle is itself a nucleus, so this would be a col-
lision between two nuclei, and a violent one due to the high speeds
involved. Rutherford hit pay dirt in an experiment with alpha par-
ticles striking a target containing nitrogen atoms. Charged particles
were detected flying out of the target like parts flying off of cars in
a high-speed crash. Measurements of the deflection of these parti-
cles in electric and magnetic fields showed that they had the same
charge-to-mass ratio as singly-ionized hydrogen atoms. Rutherford
concluded that these were the conjectured singly-charged particles
that held the charge of the nucleus, and they were later named
protons. The hydrogen nucleus consists of a single proton, and in
general, an element’s atomic number gives the number of protons
contained in each of its nuclei. The mass of the proton is about 1800
times greater than the mass of the electron.506 Chapter 8 Atoms and Electromagnetism