12
R. N. Mohapatra
explain light emission from hydrogen using discreteness. Naturally, old guard
physicists like Thompson, Lorentz, and Rayleigh were very skeptical of Bohr’s
idea, but it turned out to be decisive in later years, leading to the birth of a
whole new subject called quantum mechanics.
Soon afterwards, Bohr’s model of the hydrogen atom received confirmation
from observations using other atoms and acquired firmer footing. This picture
was supplemented by the exclusion principle for electrons suggested by
Wolfgang Pauli in 1924, which said that no two electrons could occupy the
same space-time point or same energy and momentum. This is known as
the Pauli exclusion principle and it helped explain the atomic structure from
hydrogen and helium to heavier atoms. Soon it would also play an important
role in understanding the chemical properties of various elements.
The protons and electrons were slowly taking their well deserved place in the
pedestal of physics as two fundamental building blocks of matter. Is that then
the end of the story? Fortunately not quite. People were pouring into details
of what actually was going on inside the atom and discovering new things that
would guide the future direction of physics and provide many new revelations.
2.2 Add the Neutron
As Ernest Rutherford had speculated, there is more mass in the core of the
atomic nuclei than only protons can account for. The number of protons is
after all fixed by the fact that atoms are neutral, and therefore must contain an
equal number of protons and electrons to cancel each other’s electric charge.
Also, Rutherford’s argument made it clear that whatever else is there in the
atomic nucleus in addition to protons must be electrically neutral to keep
the atom as a whole neutral. Proton and electron were discovered due to
their motion in electric fields resulting from their electric charge. The fact
that the remaining heavy stuff had no electric charge made it impossible to
use electric field methods to uncover its nature. In 1932, James Chadwick,
working in the Cavendish laboratory, bombarded Beryllium nucleus with
alpha particles and found a new kind of radiation which was not deflected by
electric field, which meant that it is electrically neutral. This kind of radiation
was also observed by Bothe and Becker but they concluded that the emanating
radiation was gamma rays (or energetic photons), which was the only neutral
particle known at the time but the photon was too light to satisfy the energy–
momentum conservation in this reaction. In fact, it was the use of the principle
of energy–momentum conservation that helped Chadwick conclude that the
new electrically neutral particle that was emitted in the Beryllium plus alpha
R. N. Mohapatra
explain light emission from hydrogen using discreteness. Naturally, old guard
physicists like Thompson, Lorentz, and Rayleigh were very skeptical of Bohr’s
idea, but it turned out to be decisive in later years, leading to the birth of a
whole new subject called quantum mechanics.
Soon afterwards, Bohr’s model of the hydrogen atom received confirmation
from observations using other atoms and acquired firmer footing. This picture
was supplemented by the exclusion principle for electrons suggested by
Wolfgang Pauli in 1924, which said that no two electrons could occupy the
same space-time point or same energy and momentum. This is known as
the Pauli exclusion principle and it helped explain the atomic structure from
hydrogen and helium to heavier atoms. Soon it would also play an important
role in understanding the chemical properties of various elements.
The protons and electrons were slowly taking their well deserved place in the
pedestal of physics as two fundamental building blocks of matter. Is that then
the end of the story? Fortunately not quite. People were pouring into details
of what actually was going on inside the atom and discovering new things that
would guide the future direction of physics and provide many new revelations.
2.2 Add the Neutron
As Ernest Rutherford had speculated, there is more mass in the core of the
atomic nuclei than only protons can account for. The number of protons is
after all fixed by the fact that atoms are neutral, and therefore must contain an
equal number of protons and electrons to cancel each other’s electric charge.
Also, Rutherford’s argument made it clear that whatever else is there in the
atomic nucleus in addition to protons must be electrically neutral to keep
the atom as a whole neutral. Proton and electron were discovered due to
their motion in electric fields resulting from their electric charge. The fact
that the remaining heavy stuff had no electric charge made it impossible to
use electric field methods to uncover its nature. In 1932, James Chadwick,
working in the Cavendish laboratory, bombarded Beryllium nucleus with
alpha particles and found a new kind of radiation which was not deflected by
electric field, which meant that it is electrically neutral. This kind of radiation
was also observed by Bothe and Becker but they concluded that the emanating
radiation was gamma rays (or energetic photons), which was the only neutral
particle known at the time but the photon was too light to satisfy the energy–
momentum conservation in this reaction. In fact, it was the use of the principle
of energy–momentum conservation that helped Chadwick conclude that the
new electrically neutral particle that was emitted in the Beryllium plus alpha