Physics of the Atom
1.1 Describe the work done in establishing the modern view of the atom- Including Thomson,
Rutherford, Bohr, and Chadwick.
1.2 Describe the Geiger-Marsden experiment- Establish the nuclear structure of the atom.
Consider that the nucleus contains protons and neutrons of approximately equal mass.
History of the Atom, Key figures
John Dalton, J.J. Thomson, Sir Ernest Rutherford, Neils Bohr, and James Chadwick.
Atoms were initially estimated to be indivisible particles that matter is made up of. It was
believed that if you continued to divide matter into smaller particles, you would reach a
point where it could no longer be divided.
John Dalton proposed the atomic theory in 1803. He proposed that atoms could not be divided
into smaller particles (which we now know is not true) and that atoms of the same elements are
identical. He also proposed that atoms always combine to form compounds in fixed (whole)
numbers. It was also called the billard ball model (pool ball).
Thomson (1897) discovered the subatomic particle electron by using cathode rays.
A cathode ray is a stream of tiny, negatively charged particles called electrons that travel in
a straight line through a vacuum or low-pressure gas inside a sealed tube called a cathode
ray tube. Inside the tube, there’s a metal plate called the cathode (negative) and another
plate called the anode (positive). When electricity is passed through the tube, the cathode
releases electrons in a straight line called the cathode ray which releases a pale green light
on the side of the anode. He was also able to demonstrate the deflecting of the beam by
placing +ve plates and -ve plates on opposite ends of the middle section of the cathode tube.
The green beam would deflect towards the +ve plate.
This showed Thomson that there existed even smaller particles than atoms, with a negative
charge present in all atoms. He then proposed that the atom was a positively charged particle
while the electrons were smaller negatively charged particles scattered within the atom. Similar
to a plum pudding.
Rutherford in 1909 designed an experiment to test Thomson’s theory. This is called the GeigerMarsden experiment (named after two of Rutherfords assistants).
The experiment was designed with a detection screen (fluorescent screen), an alpha-particle
emitter and a gold foil in the middle. A beam of alpha particles (positively charged particles) was
directed at the gold foil. Most of the particles went through the foil but a few particles got
scattered and even deflected back towards the source.
This showed that Thomson’s theory was incomplete as there had to be a positively charged
nucleus in the atom that caused the deflection and scattering of the positively charged beam
(alpha particle). This also demonstrated that the nucleus is dense and only occupies a small,
central section of the atom, with most of the atom having empty spaces. Rutherford estimated
that the diameter of the nucleus was about 10–4 times the diameter of the atom.
Rutherford also proposed the existence of protons and particles without charge called neutrons.
Bohr suggested that the electrons orbit the nucleus similar to how planets orbit the sun, planetary
model/nuclear model. However, the problem was that as the electrons orbit the positively
charged nucleus, electromagnetic energy should be given off that would cause the electron to be
pulled into the nucleus. This was known as the collapsing atom problem.
Bohr then proposed that electrons move around the nucleus in circular, ‘allowed’ orbits ‘electron
shells’ without losing energy. Each orbit was at a fixed distance from the nucleus with the energy
of the orbit further away from the nucleus being the highest.
Chadwick, in 1932, experimented and confirmed the presence of uncharged particles with
approximately the same mass as protons, the neutrons.
Collapsing atom problem: Imagine an atom like a tiny solar system where electrons orbit the nucleus, kind of like planets around the
Sun.
The problem? According to classical physics, this shouldn’t work. Moving charges (like electrons) are supposed to lose energy by
emitting radiation. If that were true, electrons would spiral into the nucleus in less than a second, and atoms (and everything made of
them) would collapse. But clearly, that doesn’t happen.
The solution came with quantum mechanics. Niels Bohr proposed that electrons can only occupy specific energy levels and don’t lose
energy while in these levels. They only absorb or release energy when jumping between them. Later, Schrödinger’s wave model
showed that electrons don’t even orbit like planets—they exist as probability clouds around the nucleus.
This solved the mystery: electrons stay in stable energy levels, preventing atoms from collapsing.