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Essentially, the structure of an atom comprises protons, neutrons and electrons.
These basic components provide the mass and charge of the atoms. The nucleus
comprises protons and neutrons, with the electron orbiting around that.
Introduction to the Structure of an Atom
Atoms
Atoms are the building blocks of matter. It is the smallest unit of matter that is
composed of three sub-atomic particles: the proton, the neutron and the electron.
Cathode Ray Experiment
J. J. Thomson discovered the existence of electrons.
He did this using a cathode ray tube, which is a vacuum-sealed tube with a cathode and
anode on one end that created a beam of electrons travelling towards the other end of
the tube.
The air inside the chamber is subjected to high voltage, and electricity flows through the
air from the negative electrode to the positive electrode.
The characteristics of cathode rays (electrons) do not depend upon the material of
electrodes and the nature of the gas present in the cathode ray tube.
The experiment showed that the atom was not a simple, indivisible particle and contained
at least one subatomic particle – the electron.
Apparatus of the experiment
Electrons
Electrons are the negatively charged sub-atomic particles of an atom.
The mass of an electron is considered to be negligible, and its charge is -1.
, The symbol for an electron is e–
Electrons are extremely small.
They are found outside the nucleus.
Thomson’s Model of an Atom
According to Thomson,(i) An atom consists of a positively charged sphere and the
electrons are embedded in it. (ii) The negative and positive charges are equal in
magnitude. So, the atom as a whole is electrically neutral
The first model of an atom to be put forward and taken into consideration.
He proposed a model of the atom be similar to that of a Christmas pudding/watermelon.
The red edible part of the watermelon is compared with the positive charge in the atom.
The black seeds in the watermelon are compared with the electrons which are embedded
on it.
Radioactivity
Radioactivity
Radioactivity is the term for the process by which an unstable nucleus of an atom loses
energy by giving out particles.
It does so by giving out particles such as alpha and beta particles.
This process is spontaneous.
An atom is unstable if the nucleus has an imbalance, meaning a difference in the protons
and neutrons.
T
Rutherford Model
Rutherford’s Experiment and Observations
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