Video Transcript
What proposition did Rutherford’s
nuclear model of the atom introduce over the plum pudding model? (A) That electron shells are of
fixed radii. (B) That electrons are swimming in
a positively charged sphere. (C) That a small dense nucleus is
in the center of the atom. (D) That particles have mass but no
charge. Or (E) that particles have mass and
are of positive charge.
To answer this question, let’s
start by looking at the plum pudding model of the atom. The plum pudding model was proposed
by J.J. Thomson around 1897. In this model, electrons, small
negatively charged particles, are embedded in a sea of positive charge much like the
dried fruit in plum or bread pudding, hence the name plum pudding model.
The plum pudding model was quickly
proven false by the results of the gold leaf or gold foil experiment performed by
Hans Geiger and Ernest Marsden under the guidance of Ernest Rutherford. In this experiment, a beam of 𝛼
particles was directed at a thin sheet of gold leaf. The 𝛼 particle deflection was
monitored using a circular luminescent detection screen that would flash when an 𝛼
particle struck it.
As electrons are very small, the
positively charged mass in the plum pudding model is relatively evenly distributed
throughout the atom, and 𝛼 particles move quickly, they expected that all of the 𝛼
particles would go straight through the gold foil. Instead, they observed that a small
fraction of the particles were deflected back towards the 𝛼 particle source.
To explain these results,
Rutherford proposed a new model of the atom. In Rutherford’s model, the atom is
mostly made up of empty space. There is a small, dense nucleus in
the center about which the electrons orbit much like the planets around the Sun. It’s worth noting that this model
is not drawn to scale. The nucleus occupies significantly
less space in the atom than is shown here.
Rutherford’s model fit with the
observations as 𝛼 particles should travel straight through the mostly empty space
of the atom. 𝛼 particles that passed near the
nucleus would be slightly deflected. And 𝛼 particles that traveled
towards the nucleus would be deflected back towards the source.
From this discussion, we can see
that the proposition that Rutherford’s model of the atom introduced over the plum
pudding model is answer choice (C), that a small dense nucleus is in the center of
the atom.
