Structure of the atom, Discovery and charged particles of matter

STRUCTURE OF THE
ATOM
Chapter 4

CONTENT
1. Charged Particles in the Matter: Electrons, Protons and Neutrons
2. The structure of an Atom
o THOMSON’S MODEL OF AN ATOM
o RUTHERFORD’S MODEL OF AN ATOM and Drawbacks of Rutherford’s model of the
atom
o BOHR’S MODEL OF ATOM
3. How are Electrons Distributed in Different Orbits (Shells)?
4. Valency
5. Atomic Number
6. Mass Number
7. Isotopes
8. Isobars

The Structure of an Atom
Discovery of Electron:
• J.J. Thomson discovered the existence of electrons.
• Thomson during his Cathode ray experiment (1897)
proved the existence of a negatively charged particle
called electron that exists inside the atom.
• He did by Cathode Ray Experiment and his experiment
showed that the atom was not a simple, indivisible particle
but contains at least one sub-atomic particle-the Electron.
• J.J. Thomson was awarded the Nobel prize in Physics in 1906
for his work on the discovery of electrons.

The Structure of an Atom:
Discovery of Proton:
• E. Goldstein in 1886, carried out an electrical
discharge in the modified cathode ray tube.
• He discovered the new radiations in a gas
discharge and named them as canal rays.
• These rays originated from the anode and
hence were known as the anode rays.
• These rays were positively charged radiations
which ultimately led to the discovery of another
sub-atomic particle.
• This sub-atomic particle had a charge, equal in
magnitude but opposite in sign to that of the
electron. Its mass was approximately 2000 times
as that of the electron.
• It was given the name of proton.

The Structure of an Atom:
Discovery of Proton:
• In 1919, Ernest Rutherford discovered a new sub-atomic particle
which was later named as proton.
• A proton has a unit positive charge.
• It is a fundamental particle of atom and is represented by p+.
• It carries a charge of 1.6 × 10-19
C and has a mass 1.67 ×10-27
kg.
(mass of electron 9.1×10-31
kg )
• Its mass is 1836 times to that of the electron.

THOMSON’S MODEL OF AN ATOM:
• Thomson’s Atomic Model was proposed by J.J. Thomson in 1904.
• It was the first scientific model to describe the internal structure of an
atom.
• Thomson's atomic model, also known as the plum pudding model.
• Thomson during his Cathode ray experiment (1897) proved the
existence of a negatively charged particle called electron that exists
inside the atom. And after complaining about his result he prosed the
Thomson atomic model (also called as Plum Pudding Model).
• J.J. Thomson was awarded the Nobel prize in Physics in 1906 for his work
on the discovery of electrons.

The structure of an Atom
THOMSON’S MODEL OF AN ATOM
Observations of Thomson’s Atomic
Model:
• Thomson’s Atomic Model, also called the Plum Pudding Model
states that an atom can be compared to a plum pudding
where electrons are like dry fruits in a sphere of positive
charge, representing the pudding.
• We can also compare this with a watermelon where the
positive charge is comparable to the edible part of the
watermelon, and electrons are like seeds that are
embedded in that sphere.
The diagram shows that positive
charge is spread throughout the
atom and negative charge is
embedded in the atom maintaining
the electrical neutrality of the atom.

The structure of an Atom:
Postulates of Thomson’s Atomic Model:
 An atom consists of a positively charged sphere and the
electrons are embedded in it.
 The negative and positive charges are equal in
magnitude. So, the atom as a whole is electrically
neutral.

Limitations of Thomson’s Atomic Model:
Thomson’s atomic model had several limitations that were eventually overcome by further
research and the experiments of various scientists later on.
 It fails to explain the finding of Rutherford’s Gold Foil experiment
The model could not explain the results of the gold foil experiment conducted by Rutherford, which
showed that most of the mass and positive charge of an atom is concentrated in a small, dense
nucleus at the Centre of the atom. But, the model assumed that the positive charge was
distributed uniformly throughout the atom.
 It fails to explain the existence of the neutrons that were discovered by Chadwick.
 The model did not explain the existence of Isotopes.
 The model could not explain the phenomenon of atomic spectra, which is the emission
and absorption of specific wavelengths of light by atoms.

Rutherford's Model of an Atom:
• Rutherford Atomic Model was proposed by Ernest Rutherford in 1911.
• It is also called the Planetary Model of the Atom.
• It introduced the concept of a dense, positively charged nucleus at the
center of an atom, with electrons orbiting around it, forming the basis for
modern atomic theory.
• According to Rutherford’s Atomic Model, the positively charged particles and
the majority of the mass of an atom were said to be concentrated in a small
volume. He referred to this area of the atoms as the nucleus.
• Another idea put forward by Rutherford’s model of an atom was that an
atom’s nucleus should be surrounded by negatively charged electrons.
Rutherford also suggested that the electrons circle the nucleus at the speed
of light. He called these elliptical paths orbits.

Rutherford Gold Foil Experiment:
• In this experiment, fast moving alpha(α)-particles were
made to fall on a thin gold foil.
• He selected a gold foil because he wanted as thin a
layer as possible. This gold foil was about 1000 atoms
thick.
• α-particles are doubly-charged helium ions. The fast-
moving α-particles have a considerable amount of
energy as they have a mass of 4 u.
• It was expected that α-particles would be deflected by
the sub-atomic particles in the gold atoms. Since the α-
particles were much heavier than the protons, he did
not expect to see large deflections.

Observations of Rutherford's Atomic
Model:
• A large percentage of alpha particles travelled through the gold film
without being deflected, indicating that the majority of space in an atom is
empty. As a result, an atom’s main portion must be empty.
• The positive charge in an atom is concentrated in a relatively small volume
and is not dispersed evenly. When bombarded, the gold foil only deflected
a small number of alpha particles. They experienced extremely slight
angles of deflection. So he arrived at the stated conclusion.
• Very few alpha particles had deflected back or at large angles. In addition,
relatively few particles had 180o deflected. As a result, he came to the
conclusion that the positively charged particles only occupied a small
portion of an atom’s overall volume.
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.

Features of Rutherford's Atomic
Model:
• There is a positively charged center in an atom called the
nucleus.
• Nearly all the mass of an atom resides in the nucleus.
• The electrons revolve around the nucleus in circular paths.
• Since electrons are positively charged and the tightly packed
nucleus is positively charged, an tom has no net charge or is
electrically neutral.
• The nucleus and electrons are held together by a strong force of
attraction.
• The size of the nucleus is very small as compared to the size of the
atom. Experimentally, it was found that the radius of the nucleus is
about 105
times less than the radius of the atom.

Conclusions of Rutherford's Atomic Model:
• Most of the space inside the atom is empty because most of
the α-particles passed through the gold foil without getting
deflected.
• Very few particles were deflected from their path, indicating that
the positive charge of the atom occupies very little space.
• A very small fraction of α-particles were deflected by 180 o
C,
indicating that all the positive charge and mass of the gold
atom were concentrated in a very small volume within the
atom.

Drawbacks of Rutherford's Atomic
Model:
• Rutherford’s Model does not obey the Maxwel theory of electrodynamics.
If an electrically charged particle revolves around the positively charged
nucleus, which is not anticipated to be stable.
• A charged particle in rapid motion along a circular route, would lose energy
continually and eventually collapse into the nucleus. This causes an atom to
be unstable, whereas we know that atoms are extremely stable.
• Because it merely postulated the existence of protons in the nucleus, the
Rutherford Model could not resolve the problem of atomic mass.
• Rutherford’s Atomic Model doesn’t explain the arrangement of electrons in
the atom, which makes this model incomplete in this regard.

BOHR’S MODEL OF ATOM:
• Bohr theory modified the atomic structure model by explaining that electrons
move in fixed orbitals (shells) and not anywhere (in between the shells).
• He also explained that each orbit (shell) has a fixed energy.
• Rutherford explained the nucleus of an atom and Bohr modified that model into
electrons and their energy levels.

Postulates of Bohr’s Model of an Atom
• The electron is able to revolve in certain stable orbits known as discrete orbits of
electrons around the nucleus without radiating any energy.
• Only certain special orbits known as discrete orbits of electrons, are allowed inside the
atom.
• Note: These stable orbits are called stationary orbits and are attained at certain discrete distances
from the nucleus. The electron cannot have any other orbit in between the discrete ones.
• While revolving in discrete orbits the electrons do not radiate energy.
• The electrons in an atom move from a lower energy level to a higher energy level by
gaining the required energy and an electron moves from a higher energy level to lower
energy level by losing energy.
• The energy levels are represented by an integer (n=1, 2, 3…) known as the quantum number.
This range of quantum number starts from nucleus side with n=1 having the lowest energy
level. The orbits n=1, 2, 3, 4… are assigned as K, L, M, N…. shells and when an electron
attains the lowest energy level, it is said to be in the ground state.

Structure of the atom, Discovery and charged particles of matter

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