Chapter 4
Structure of an Atom
A major challenge before the scientists at the end of the 19th century was to reveal the structure of
the atom as well as to explain its important properties.
The elucidation of the structure of atoms is based on a series of experiments.
One of the first indications that atoms are not indivisible, comes from studying static electricity and
the condition under which electricity is conducted by different substances.
Charged Particles in Matter
To understand charged particles in matter, activities like combing dry hair and rubbing a glass rod
with a silk cloth are observed. These activities reveal that when objects are rubbed together, they
become electrically charged. This charge comes from the fact that an atom is divisible and consists
of charged particles.
It was known by 1900 that the atom was an indivisible particle but contained one sub-atomic
particle – the electron.
The existence of electrons was shown by J.J. Thomson.
Since atoms contain negatively charged particles-electrons, they must contain an equal number of
positively charged particles to make the atom electrically neutral.
The existence of protons in the atoms was shown by E. Goldstein in 1886.
A proton has a charge, equal in magnitude but opposite in sign to that of the electron.
Its mass was approximately 2000 times that of the electron.
In general, an electron is represented as 'e– ' and a proton as 'p+ '.
The mass of a proton is taken as one unit and its charge is plus one.
The mass of an electron is considered to be negligible and its charge is minus one.
Electrons can be removed easily, but protons cannot.
Neutron was discovered by J. Chadwick in 1932.
In general, a neutron is represented as 'n'.
Neutron has no charge and it is electrically neutral.
The mass of neutron is nearly equal to that of a proton.
Neutrons are present in the nucleus of all atoms, except hydrogen.
The mass of an atom is therefore given by the sum of the masses of protons and neutrons present
in the nucleus.
The Structure of an Atom
Dalton's atomic theory suggested that the atom was indivisible and could not be broken down into
smaller particles. However, the discovery of subatomic particles inside the atom disproved this
postulate of Dalton's atomic theory.
Thomson's Model of an Atom
J.J. Thomson proposed an atom model resembling a Christmas pudding, with electrons in a
positively charged sphere resembling currants (dry fruits).
, This model can also be compared to a watermelon, where the positive charge is spread throughout,
like the red edible part of a watermelon, and the electrons are studded in the positively charged
sphere, like the seeds in the watermelon.
Thomson proposed that:
(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.
Although Thomson's model explained that atoms are electrically neutral, the results of experiments
carried out by other scientists could not be explained by this model.
Rutherford's Model of an Atom
Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. So,
Rutherford designed an experiment for this.
He selected a gold foil for his experiment because he wanted a layer as thin as possible. This gold
foil was about 1000 atoms thick.
α-particles are doubly-charged helium ions. Since they have a mass of 4 u, the fast-moving
α-particles have a considerable amount of energy.
This experiment, which was expected to deflect α-particles by the sub-atomic particles in the gold
atoms, gave unexpected results.
The following observations were made:
(i) Most of the fast-moving α-particles passed straight through the gold foil.
(ii) Some of the α-particles were deflected by the foil by small angles.
(iii) Surprisingly one out of every 12000 particles appeared to rebound.
Rutherford concluded from the α-particle scattering experiment that:
(i) Most of the space inside the atom is empty because most of the α-particles passed through the
gold foil without getting deflected.
(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom
occupies very little space.
(iii) A very small fraction of α-particles were deflected by 180°, indicating that all the positive charge
and mass of the gold atom were concentrated in a very small volume within the atom.
Based on his experiment, Rutherford put forward the nuclear model of an atom, which had the
following features:
(i) There is a positively charged center in an atom called the nucleus.
(ii) Nearly all the mass of an atom resides in the nucleus.
(iii) The electrons revolve around the nucleus in circular paths.
(iv) The size of the nucleus is very small as compared to the size of the atom.
(v) The radius of the nucleus is about 105 times less than the radius of the atom.
Drawbacks of Rutherford's Model of the Atom
Any charged particles in a circular orbit would undergo acceleration, causing the electron to lose
energy and eventually fall into the nucleus and the atom would collapse.
Structure of an Atom
A major challenge before the scientists at the end of the 19th century was to reveal the structure of
the atom as well as to explain its important properties.
The elucidation of the structure of atoms is based on a series of experiments.
One of the first indications that atoms are not indivisible, comes from studying static electricity and
the condition under which electricity is conducted by different substances.
Charged Particles in Matter
To understand charged particles in matter, activities like combing dry hair and rubbing a glass rod
with a silk cloth are observed. These activities reveal that when objects are rubbed together, they
become electrically charged. This charge comes from the fact that an atom is divisible and consists
of charged particles.
It was known by 1900 that the atom was an indivisible particle but contained one sub-atomic
particle – the electron.
The existence of electrons was shown by J.J. Thomson.
Since atoms contain negatively charged particles-electrons, they must contain an equal number of
positively charged particles to make the atom electrically neutral.
The existence of protons in the atoms was shown by E. Goldstein in 1886.
A proton has a charge, equal in magnitude but opposite in sign to that of the electron.
Its mass was approximately 2000 times that of the electron.
In general, an electron is represented as 'e– ' and a proton as 'p+ '.
The mass of a proton is taken as one unit and its charge is plus one.
The mass of an electron is considered to be negligible and its charge is minus one.
Electrons can be removed easily, but protons cannot.
Neutron was discovered by J. Chadwick in 1932.
In general, a neutron is represented as 'n'.
Neutron has no charge and it is electrically neutral.
The mass of neutron is nearly equal to that of a proton.
Neutrons are present in the nucleus of all atoms, except hydrogen.
The mass of an atom is therefore given by the sum of the masses of protons and neutrons present
in the nucleus.
The Structure of an Atom
Dalton's atomic theory suggested that the atom was indivisible and could not be broken down into
smaller particles. However, the discovery of subatomic particles inside the atom disproved this
postulate of Dalton's atomic theory.
Thomson's Model of an Atom
J.J. Thomson proposed an atom model resembling a Christmas pudding, with electrons in a
positively charged sphere resembling currants (dry fruits).
, This model can also be compared to a watermelon, where the positive charge is spread throughout,
like the red edible part of a watermelon, and the electrons are studded in the positively charged
sphere, like the seeds in the watermelon.
Thomson proposed that:
(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.
Although Thomson's model explained that atoms are electrically neutral, the results of experiments
carried out by other scientists could not be explained by this model.
Rutherford's Model of an Atom
Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. So,
Rutherford designed an experiment for this.
He selected a gold foil for his experiment because he wanted a layer as thin as possible. This gold
foil was about 1000 atoms thick.
α-particles are doubly-charged helium ions. Since they have a mass of 4 u, the fast-moving
α-particles have a considerable amount of energy.
This experiment, which was expected to deflect α-particles by the sub-atomic particles in the gold
atoms, gave unexpected results.
The following observations were made:
(i) Most of the fast-moving α-particles passed straight through the gold foil.
(ii) Some of the α-particles were deflected by the foil by small angles.
(iii) Surprisingly one out of every 12000 particles appeared to rebound.
Rutherford concluded from the α-particle scattering experiment that:
(i) Most of the space inside the atom is empty because most of the α-particles passed through the
gold foil without getting deflected.
(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom
occupies very little space.
(iii) A very small fraction of α-particles were deflected by 180°, indicating that all the positive charge
and mass of the gold atom were concentrated in a very small volume within the atom.
Based on his experiment, Rutherford put forward the nuclear model of an atom, which had the
following features:
(i) There is a positively charged center in an atom called the nucleus.
(ii) Nearly all the mass of an atom resides in the nucleus.
(iii) The electrons revolve around the nucleus in circular paths.
(iv) The size of the nucleus is very small as compared to the size of the atom.
(v) The radius of the nucleus is about 105 times less than the radius of the atom.
Drawbacks of Rutherford's Model of the Atom
Any charged particles in a circular orbit would undergo acceleration, causing the electron to lose
energy and eventually fall into the nucleus and the atom would collapse.
