Video Transcript
What proposition did Bohr’s orbital
model of the atom introduce over Rutherford’s nuclear model? (A) That a small dense nucleus is
in the center of the atom. (B) That electrons are swimming in
a positively charged sphere. (C) That particles have mass but no
charge. (D) That electron shells are of
fixed radii. Or (E) that particles have mass and
are of positive charge.
Let’s start by looking at
Rutherford’s nuclear model. Rutherford suggested that the atom
is mostly made of empty space occupied by the electrons and that there is a small
and very dense, positively charged nucleus in the center of the atom. This figure is not drawn to scale,
as the model indicated that the atom was mostly empty space and the nucleus occupied
a very small amount of it. His model suggests that the nucleus
of the atom is composed of positively charged particles that he called protons.
The nuclear model of the atom posed
some immediate problems. The problem with the model had to
do in part with electrostatic attraction. These are the attractive forces
that occur between oppositely charged particles. As negatively charged electrons
orbit a positively charged nucleus, they should lose energy and eventually spiral
into the nucleus, causing the atom to collapse.
In addition to this, Rutherford’s
model suggested that the electrons in the atom should release energy in a continuous
spectrum, which would create a rainbow. But when scientists looked at the
spectrum produced by excited gaseous atoms, they observed a series of lines of
different colors of light. These emission line spectra that
were produced by these excited atoms were unique for each element.
To explain this, Niels Bohr
proposed a new model for the atom. He proposed that atomic nuclei are
surrounded by electrons that are essentially confined to discrete energy levels as
they orbit the nucleus. These orbits on which the electrons
are confined are referred to as electron shells or orbitals, giving the model its
name. These electron shells were of
discrete energy levels that had fixed radii proportional to their energy.
Therefore, the proposition that
Bohr’s orbital model of the atom introduced over Rutherford’s nuclear model is
answer choice (D), that electron shells are of fixed radii.
