Structure of Atom
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Charged particle in matter
Discovery of electron
Discovery of proton
Discovery of neutron
The structure of an atom
Thomson’s model of an atom
Rutherford’s model of an atom
Bohr’s model of an atom
Electron distribution in different orbits
Valency
Atomic number & mass number
Isotopes
Isobars
Introduction
Earlier it was believed that ‘atom’ is indivisible. But due to the discovery of static charge and electricity it
had proven that atom is not indivisible.
The atoms are made up of e-, p+ & n.
The atoms of different element differ in the number of e-,p+ & n.
Discovery of electron
In year 1897 sir J.J Thompson proved the existence of negatively charged particle within an atom that is
called electron.
Experimental setup
Explanation of cathode rays
The gas taken in the discharge tube consists of atoms, and all the toms contain electron. When high
electrical voltage is applied, the electrical energy pushes out some of the electrons from the atom of the
gas. These fast moving electrons from cathode rays.
Explanations of cathode rays
When electric field is applied in cathode ray tube. The rays get bend towards positive terminal of electric
field hence cathode rays are made up of negative charge particles called as ‘electron’.
Discovery of proton
The existence of protons in the atoms was shown by E. Goldstien.
Experimental setup
Explanation of anode rays
Hydrogen gases consist of hydrogen atoms. When high electrical voltage is applied to hydrogen gas, the
electrical energy removes the electron from the atom, then a positively charged particle is formed;
which forms anode rays on canal rays ; because of two reasons
1. Attraction towards negative terminal.
2. Reconciliation here rays moves towards cathode.
The nature of anode rays depends upon the gas filed in the tube.
The positive particles obtained from the H2 gas are the lightest and have the smallest charge. The anode
rays obtained from the H2 gas are made up of the same type of +ve particles. These particles are called
protons.
Discovery of neutron
In year 1932 James Chadwick discovered neutron particles
In an experiment he bombarded 𝛼-particle on a metal plate he found that a inert and inassive particle is
given out that’s means which was electrically neutral that’s why it is known as neutron.
Neutron provides necessary force to bind together electron and proton.
Structure of atom
The discovery of electrons and protons suggested that atoms are divisible and they do have an inner
structure.
Thomson’s Model of Atom
In the year 1903 J.J Thomson proposed this model atomic structure
According to this model:
a. An atom consists of a sphere (a ball) of +ve charge with negatively charged electrons embedded
in it.
b. The +ve & -ve charges in our atom are equal in magnitude, due to which an atom is electrically
neutral.
Drawbacks of Model
a. Thomson was unable to explain his own model.
b. This model was unable to explain the Rutherford experiment.
Rutherford’s Model of Atom
It was a turning point in the physics.
Rutherford was looking far away to neutralize the ∝-particle and in the way got this model.
∝-paricles: +2e & 4mp
Experimental setup
He selected a gold foil because he wanted as thin a larger as possible. This gold foil was about 1000
atom thick.
∝-pareticles are doubly charged helium ions. Since they have a mass of 4u, the fast moving ∝-particles
have a considerable amount of energy.
It was expected that ∝-particles would be deflected by the sub atomic particles in the gold atoms. Since
the ∝-particles were much heavier than the protons, he did not expect to see large deflection.
The result of experiment
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Most of the ∝-particles passed undeviated.
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1
∝-particles get deviated / scattered.
200
1
∝-particle appeared to rebound.
20000
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Conclusions of experiment
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Most of the space inside the atom is vacant
Very few particles ere deflected from their path, indicating that the positive charge of the atom
occupies very little space.
All the positive charge and mass of the gold atom were concentrated in a very small volume
within the atom.
Nuclear model based on experiment
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There is +ve charged centre in an atom called nucleus. Nearly all the mass of atom reside in
nucleus.
The e- revolves around nucleus in wall defined orbits.
Te size of the nucleus is very small as compared to the size of the atom.
Drawbacks of nucleus model
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The orbital revolution of the e- is not expected to be stable.
Any particle in a circular orbit would undergo acceleration, charged particles would radiate
energy.
Thus the revolving electron would lose energy and finally fall into the nucleus.
If this were so, the atom should be highly unstable and hence matter would not exist in the form
that we know.
But we know that atoms are quite stable.
Bohr’s model of atoms
In order to overcome the objections raised against Rutherford model of the atom, here bohr’s put
forward the following postulate about model of atom
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Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
While revolving in discrete orbits the electrons do not radiate energy.
Detailed explanation of model
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An atom is made up of three particles: e-, p+ & n.
The p+ & n are located in a small nucleus at the centre of the atom.
The e- revolves rapidly around the nucleus in fixed circular paths called energy levels on shells.
There is a limit to the number of electrons which each shell can hold.
Each energy level is associated with fixed amount of energy.
There is no change of energy of electrons as long as they keep revolving in the same energy
level, and the atom remains stable.
Electron distribution in different orbits
The distribution electrons into different orbit of an atom was suggested by bohr and bury:
The rules to fill electron in shells are:
a. The maximum no. of e- present in a shell is given by 2n2 where 𝑛̈ is the orbit number are energy
level.
b. The maximum no. of electrons that can be accommodated in the outermost orbit is 8.
c. Electrons are not accommodated in a given shell, unless the inner shell are filled. That is, the
shells are filled in a step wise manner.
Valency
The combining capacity of the atom of other elements, that is their tendency to react and form
molecules with atoms of the same are different elements, was thus explained a on attempt to attain a
fully filled outermost shell on valence shell.
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So, the valence shell is the outermost shell of an atom.
Valency is (combining capacity) the no. of electrons present in the outermost shell of an atom.
Those electrons of an atom which take part in chemical reaction are called valence electron.
The number of electron lost are gained by one atom of an element to achieve the nearest
noble gas e- configuration is known as its electro valency.
Na → Na+ + e−
valency -1
Mg → Mg +2 + 2e− electrovalency-2
Al → Al+3 + 3e−
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electrovalency-3
The number of electrons shared by one atom of an element to achieve nearest inert gas
electron configuration is known as its covalency.
Ex: covalency of
H1
C4
Cl1
O2
(NH2)
N3
Atomic number
The number of proteins in one atom of an elements is known as atomic number of that element.
Atomic no. of an element = no. of protons in one atom of
element.
It tells:
a. No. of protons in one atom of element.
b. No. of electron in one atom of element
Mass number
Mass number is the sum of total no. of proton and total no. of neutron present in the nucleus of an
atom of element
Mass no. = no. of proton + no. of neutron
=
n(P)
+
n(n)
Relationship between mass no. & atomic no.
Mass number = atomic number + number of neutrons
A = Z+ no of 𝑛̈
Isotopes
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Isotopes are atom of the some element having the same atomic number but different mass
number.
The isotopes of an element differ in the no. of neutron in their nuclei.
The isotopes of the some element have same no of protons in their nuclei as their atomic
number is similar.
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Since all the isotopes of an element have identical electronic configuration containing the some
number of valence electron, therefore, all the isotopes of an element show identical chemical
properties.
Since the masses of the isotopes of an element are slightly different, therefore, the physical
properties of the isotopes of an element are slightly different.
Reason for the fractional atomic mass of element
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The fractional atomic masses of element are due to the existence of their isotopes having
different masses.
Example
3
Chlorine: 35
17𝐶𝑙 → 75% → 4
17
35𝐶𝑙
1
→ 29% → 4
So the average atomic masses of chlorine
3
1
= 35 × 4 + 37 × 4 = 35.5𝜇
Radioactive isotopes
The isotopes which are unstable (due to the presence of extra neutron in their nuclei) and various types
of radiations are called radioactive isotopes (radio isotopes).
The radiation could be in the form of ∝ −, 𝛽−, 𝑎𝑛𝑑 𝜏 rays.
Application
a.
b.
c.
d.
Used as fuel in nuclear reactions.
Used in cancer treatment.
In industry to defect leakages in underground pipeline.
Top determine the presence of tumors and blood clots.
Isobars
Isobars are atoms of different element having different atomic number but the same mass number.
Isobars have some number of nucleons in nuclei of an atom of different elements.