CBSE Class 9 Science,
Structure of the Atom
: Chapter notes
Main topics covered in CBSE Class 9 Science,
Structure of the Atom: Chapter Notes, are:
- Sub Atomic Particles of an Atom and Their Discovery
- Thomson’s Model of atom
- Rutherford’s Model of Atom
- Bohr’s Model of Atom
- Arrangement of electrons in an atom:
- Orbit
- Valency
- Atomic number
- Mass number
- Isotopes
- Isobars
Key notes for Chapter- Structure of the Atom,
are:
Introduction:
Dalton’s
atomic theory in suggested that an atom was indivisible. However, the discovery
of two fundamental particles named as electrons and protons, inside the atom,
led to the failure of Dalton’s atomic theory.
Fundamental
particles of an atom:
Three
particles; electron, proton and neutron from which an atom is consisted of, are
called fundamental particles of an atom or sub atomic particles.
Discovery
of Electron:
By J.
J. Thomson in 1897.
He
carried a cathode ray experiment in which observed a stream of negatively
charged particles coming out of cathode towards the anode. These particles were
named as electrons.
z
Discovery
of Proton:
By
Ernest Goldstein in 1886.
He
observed in the same gas discharge tube, with different situations that the
anode emitted positive particles which he named as Canal Rays. His experiment
led to the discovery of proton.
Discovery
of Neutron:
By J.
Chadwick in1932.
Neutron
is present in the nucleus of all atoms.
1.
Thomson’s Model
of Atom (by Joseph James Thomson in 1897)
Thomson’s
model of an atom proposed that:
- An atom is a uniform sphere of positive charges (due to presence of protons) as well as negative charges (due to presence of electrons).
- Atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude.
This
model is also known as water melon model, plum pudding model and apple pie
model.
Limitations
of Thomson’s Model:
- It failed to explain how protons and electrons were arranged in atom so close to each other.
2.
Rutherford’s Model of Atom (By Ernest Rutherford in 1909)
This
was based on an experiment in which α – particles were bombarded on a thin gold
foil.
Observations
of Rutherford’s α-particle scattering experiment are:
(a)
Most of the α – particles passed without any hindrance.
(b)
Some of the α – particles deflected from their original path at a noticeable
angle.
(c)
Very few of the α – particles bounced back at their original path.
Postulates
of Rutherford’s Model of Atom:
(a)
Most of the part in an atom is empty.
(b)
There is a positively charged center in atom, which contains nearly the whole
mass of atom. The centre is called nucleus.
(c)
The size of nucleus is very small compared to an atom.
(d)
Electrons revolve round the nucleus.
Drawbacks
of Rutherford Model
(a)
According to Rutherford’s Model, electron revolves round the positively charged
nucleus which is not expected to be stable. But a charged particle in an
accelerated motion along a circular path would continuously undergo loss of
energy and finally would fall into nucleus. This makes an atom unstable while
atoms are quite stable.
(b)
Rutherford model could not solve the problem of atomic mass of atom as it
proposed only the existence of protons in the nucleus.
3.
Bohr’s Model of Atom (By Neils Bohr in 1913)
This
model of atom states that:
- An atom consists of heavy positively charged nucleus. The whole mass of the atom is concentrated in the nucleus.
- The electrons in an atom revolve around the nucleus in definite circular paths called orbits or energy level.
- Each energy level is associated with definite amount of energy.
- The change in energy takes place when electron jumps from one energy level to another energy level.
Arrangement
of electrons in an atom:
The
arrangement of electrons in various shells (energy levels) of an atom of the
element is known as Electronic configuration.
The
Maximum number of electrons that could be put in a particular shell, i.e.,
energy levels, was given by Bohr and Bury.
According to Bohr-Bury Scheme:
According to Bohr-Bury Scheme:
- The maximum number of electrons that can be accommodated in any energy level is given by 2n2where n = 1, 2, 3, 4, …. (for K, L, M, N…..)
- The maximum number of electrons in the outermost orbit will be 8 electrons even if it has capacity to accommodate more electrons.
- Electrons, are not accommodated in a given shell. Unless earlier shells are filled, that is stepwise filling of shells is followed.
Orbit
The
circular path around the nucleus is called orbit, energy level or shell. Energy
level are represented by English alphabets: K, L, M, N, …. and so on.
Therefore,
- 1st orbit is denoted by K
- 2nd orbit is denoted by L
- 3rd orbit is denoted by M, and so on.
Distribution
of Electrons in a Orbit or Shell
The
distribution of electrons in an orbit can be obtained by using formulae 2n 2
where ‘n’ is number of that orbit.
For
example:
Number
of electrons in K-shell i.e. in 1st orbit
Here n
= 1
Therefore,
2n2 = 2 × 12 = 2
Thus,
maximum number of electrons in K-shell, i.e. 1st shell = 2
Number
of electrons in L-shell, i.e. in 2ndorbit
Here n
= 2, therefore,
2n2 =
2 × 22 = 8
Thus,
maximum number of electrons in L-shell = 8
In
similar way maximum number of electrons in any shell can be calculated.
Valence
Electrons
The
electrons present in the outer most shell of an atom are known as valence
electrons. These electrons determine the valency of an atom.
Valency
Valency
is the combining capacity of an atom, i.e. their tendency to react and form
molecules with atoms of the same or different elements.
For
the atoms having valence electrons less than or equal to 4, valency is same as
that of the number of valence electrons in that atom.
For
example, valency of Magnesium (2, 8 , 2) = 2
If
number of valence electrons exceed 4, then valency = 8 – Number of valence
electrons.
For
example, valency of Oxygen (2, 6) = 8 – 6 = 2.
Valency
of atoms with 8 valence electrons is zero as they have fully filled valence
shell and cannot gain or lose electrons to form, molecules or compounds.
Atomic
number (Z)
Atomic
number of an element is equal to the number of protons present in the nucleus
of an atom.
Since
an atom is electrically neutral, thus number of protons and number of electrons
are equal, therefore:
Atomic
number (z) = number of protons = number of electrons.
Mass
Number (A)
The total number of the protons and neutrons present in the nucleus of an atom is called mass number. It is denoted by A.
The total number of the protons and neutrons present in the nucleus of an atom is called mass number. It is denoted by A.
The
protons and neutrons together are called nucleon.
The
number of neutrons present in the nucleus of an atom is rperesented by N.
Isotopes
Atoms
of the same element having same atomic number but different mass numbers
are known as Isotopes.
For example:
Applications
of Isotopes
- Isotope of uranium is used as a fuel in nuclear reactor
- Isotope of cobalt is used in treatment of cancer
- Isotope of iodine is used in treatment of goitre.
Isobars
Atoms
of different elements having same mass numbers are known as Isobars,
For
example:
Potassium,
K and Argon. Ar both have the mass numbers equal to 40.
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