In 1909, Rutherford and
his assistant Hans Geiger embarked on a research program with the goal of
investigating the internal structure of atoms by using alpha particles as
probes. Rutherford proposed that, based on Thomson's plum-pudding model, most
of the alpha particles in a beam would pass through thin sections of matter
with minimal deflection, while a small fraction would be scattered or deflected
as they interacted with electrons. By studying the scattering patterns of these
alpha particles, Rutherford hoped to gain insights into the distribution of
electrons within atoms.
The experimental setup
used in these investigations, as illustrated in the figure above, involved
detecting the alpha particles by the light flashes they emitted when they
struck a zinc sulfide screen at the end of a telescope. Geiger and Ernst
Marsden, one of Rutherford's students, observed the following phenomena when
they bombarded very thin foils of gold with alpha particles.
Rutherford's pioneering
experimentation using alpha particles as probes to study the inner structure of
atoms led to significant observations. The majority of alpha particles passed
through thin sections of matter with no deflection, while a small fraction
experienced slight deflections. However, a few alpha particles (approximately 1
in every 20,000) suffered serious deflections as they penetrated the foil, and
a similar number of particles failed to pass through the foil and bounced back
in the direction from which they had come. Rutherford's initial prediction and
subsequent explanation of these observations can be found in the figure below.
Rutherford's nuclear
atom theory proposed the existence of positively charged fundamental particles
in the nuclei of atoms. Rutherford himself discovered these particles, known as
protons, in 1919 through studies on the scattering of alpha particles by
nitrogen atoms in air. These protons were released due to collisions between
alpha particles and the nuclei of nitrogen atoms. Around the same time,
Rutherford also predicted the presence of electrically neutral fundamental
particles in the nucleus. In 1932, James Chadwick identified a newly discovered
penetrating radiation as beams of neutral particles, later known as neutrons,
which originated from the nuclei of atoms. Therefore, it has only been in the
last century that the atomic model proposed in the figure below has been
established.
In atomic physics,
several key properties define the fundamental particles of matter that make up
an atom. The atomic number, or proton number (Z), indicates the number of
protons present in a given atom. Since the atom is electrically neutral, the
number of electrons is also equal to Z. The total number of protons and
neutrons in an atom is referred to as the mass number (A), and the number of
neutrons (N) is given by A - Z.
The charges and masses
of protons, neutrons, and electrons are presented in the table below. It is
important to note that electrons carry a negative charge of atomic unit, while
protons carry a positive charge of atomic unit. Neutrons, however, are
electrically neutral. The atomic mass unit (amu), abbreviated as 'u', is
defined as exactly 1/12 of the mass of carbon-12. The masses of protons and
neutrons are only slightly greater than 1 u, whereas electrons are much
lighter, with a mass of only about 1/2000th that of a proton or neutron.
No comments:
Post a Comment