Historical Experiments That Determine the Composition of the Atom
The Electron
English physicist J. J. Thomson made an experiment.
He studied electrical discharges in partially evacuated tubes called cathode-ray tubes.
When high voltage was applied to the tube, a ray (in which he called a
cathode ray because it originated from the negative electrode or
cathode) was produced.
This ray repelled by the negative pole of the applied electric field.
Thomson hypothesized that this ray was a stream of negatively charged particles, known as electrons today.
Millikan’s Oil Drop Experiment
Millikan performed an experiment to determine the charge on the electron.
The fall of charged oil droplets due to gravity are halted by adjusting the voltage between the two plates.
The voltage and the mass of the oil drop can then be used to calculate the charge of the oil drop.
Millikan concluded that the charge on an oil drop is always a whole-number multiple of the electron charge.
Radioactivity
French scientist Henri Becquerel along with other scientists discovered
that certain elements produce high-energy radiation. Becquerel
accidentally discovered that a piece of mineral containing uranium
could produce its image on a photographic plate in the presence of
light.
He concluded that there must have been a spontaneous emission of
radiation by the uranium. Other studies discovered the gamma rays, beta
particles, and alpha particles.
The Nuclear Atom Ernest Rutherford performed an experiment to
test Thomson’s plum pudding model
He directed alpha particles at a thin sheet of foil. He expected the
particles through the foil and few minor deflections.
The experiment proved that many alpha particles passed straight through
while others deflected at large angles and some reflected, never
hitting the detector.
Rutherford concluded that the alpha particles must contain a center of
concentrated positive charge that contains most of the atom’s mass,
known as the nucleus today.
Who I'd like to meet:
1) Radioactivity: the spontaneous disintegration of atomic nuclei.
2) Alpha particle: a particle that consists of two neutrons
& two protons that is identical to the helium nucleus. It is
emitted during certain radioactive transformations.
3) Beta particle: a high-speed election emitted from the nucleus
of an atom during radioactive decay. It is created by the splitting of
a neutron into a proton and an electron.
4) Gamma ray: a high-energy photon emitted after nuclear
reactions or spontaneously from the nucleus of a radioactive atom that
lowers the energy level of the nucleus. These rays do not carry any
charge or mass and share the high-frequency end of the electromagnetic
spectrum with X-rays.
5) Nucleus: the positively charged central region of an atom
which consists of protons and neutrons. The nucleus contains most of
the mass in the atom.
6) Electron: a stable particle that carries a negative 1 charge
and contains a small mass that is a fundamental constituent of matter
and orbits around the nucleus of an atom.
7) Proton: a stable particle that is a part of all atomic nuclei and also carries a positive 1 charge.
9) Neutron: a neutral particle with a zero electrical charge and a mass about the same as that of a proton.
10) Atomizer: a device that converts a liquid into a fine spray.
11) Atomic theory: a theory which states that matter is composed of atoms.
12) Nuclear atom: a model of the atom identified by the presence of a small dense nucleus at its center.
13) Cathode: the negatively charged source of electrons in an electron tube.
14) Cathode ray: a stream of electrons that is emitted from a cathode in a vacuum tube.
15) Cathode-ray tube: a vacuum tube in which a stream of electrons is produced and directed onto a fluorescent screen.
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