CHAPTER 4: Structure of the 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.
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
• This is 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.
Rutherford’s experiment and observations
In this experiment, fast-moving alpha (α)-particles were made to fall
on a thin gold foil. His observations were:
• A major fraction of the α-particles bombarded towards the gold
sheet passed through it without any deflection, and hence most
of the space in an atom is empty.
• Some of the α-particles were deflected by the gold sheet by very
small angles, and hence the positive charge in an atom is not
uniformly distributed.
• The positive charge in an atom is concentrated in a very
small volume.
• Very few of the α-particles were deflected back, that is only
a few α-particles had nearly 180o angle of deflection. So
the volume occupied by the positively charged particles in an
atom is very small as compared to the total volume of an
atom.
Rutherford’s model of an atom
Rutherford concluded the model of the atom from the α-particle
scattering experiment as:
(i) There is a positively charged centre in an atom called the
nucleus. Nearly all the mass of an atom resides in the nucleus.
(ii) The electrons revolve around the nucleus in well-defined orbits.
(iii) The size of the nucleus is very small as compared to the size of
the atom.
Drawbacks of Rutherford’s model
• He explained that the electrons in an atom revolve around the
nucleus in well-defined orbits. Particles in a circular orbit would
experience acceleration.
• Thus, the revolving electron would lose energy and finally fall
into the nucleus.
• But this cannot take place as the atom would be unstable and
matter would not exist in the form we know.
Bohr’s Model of an atom
Bohr came up with these postulates to overcome the objections raised
against Rutherford’s model:
• Electrons revolve around the nucleus in stable orbits without
emission of radiant energy. Each orbit has a definite energy and
is called an energy shell or energy level.
• An orbit or energy level is designated as K, L, M, N shells.
When the electron is in the lowest energy level, it is said to be in
the ground state.
• An electron emits or absorbs energy when it jumps from one
orbit or energy level to another.
• When it jumps from a higher energy level to lower energy level,
it emits energy while it absorbs energy when it jumps from
lower energy level to higher energy level.
Orbits
Orbits are energy shells surrounding the nucleus in which electrons
revolve.
Atomic Number
The number of protons found in the nucleus of an atom is termed as
the atomic number. It is denoted by the letter ‘Z’.
Mass number
Protons and neutrons are present in the nucleus, so the mass number is
the total of these protons and neutrons.
