INTRODUCTION
The matter is anything that occupies space and has mass
The matter is made up of small tiny particles which were
recognized early by man.
Dalton, an English Scientist, proposed that matter is made
up of tiny particles.
He termed these particles as ATOMS - indivisible.
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Dalton's Atomic theory:
Postulates of Dalton Atomic theory
The matter is made up of extremely small particles called atoms
Atom are indivisible particles, which can be neither be
created nor destroyed
Atoms of a given element are identical in all respects of mass,
shape, size and chemical reactions Atoms of different elements have different
masses and chemical properties
Atoms combine in a ratio of small whole numbers to form
compounds
The relative number and kinds of atoms are constant in a
given compound
Drawbacks of Dalton's Atomic theory
With the discovery of sub-atomic i.e., protons, electrons, and neutrons, the atoms are no longer considered as indivisible.
Dalton's Atomic theory could not explain why certain
substances, all containing atoms of the same elements, should differ in their
properties. Graphite, Diamond, and charcoal are made of carbon atoms only, but
their properties are quite different.
The Discovery of isotopes indicated that atoms of the same elements
are not perfectly
identical. They differ in their masses.
Dalton said that atoms of different elements have different masses.
This was proved wrong by the discovery of Isobars.
Isobars are atoms of different elements having the same mass.
Charged particles in a matter
* Comb dry hair. Comb then attracts small pieces of paper
* Rub a glass rod with a silk cloth and bring the rod near
an inflated balloon.
From the above examples will we able to conclude that
rubbing two objects it gets electrically charged.
It was known by 1900 that the atom was not a simple,
indivisible particle but contained at least one
subatomic particle - the electron discovered by J.J.
Thomson
E. Goldstein in 1886 discovered the presence of new
radiations in a gas discharge and called them Canal rays
These rays were positively charged radiations which
ultimately led to the discovery of another sub-atomic particle named Proton.
Protons had a charge, equal in magnitude but opposite in
sign to that of the electron
It seemed highly likely that an atom was composed of protons
and electrons, mutually balancing them
charges.
It also appeared that the protons were in the interior of the
atom, for whereas electrons could easily be peeled off but not protons.
STRUCTURE OF AN ATOM
As per Dalton's, Atomic theory atoms are indivisible and indestructible.
Discovery of two fundamental particles [Electrons and protons]
let to the failure of aspect of Dalton’s theory.
How electrons and protons arranged within an atom
For proving this, many scientists proposed many atomic
models.
A scientist named J.J Thomson was the first person to propose
a model for the structure of an atom
Thomson's Model of an Atom
Thomson's model of the atom was proposed by Joseph James Thomson in
1897
Thomson proposed that model of the atom is similar to Watermelon.
The positive charge in the atom is spread all over like the red edible part of the Watermelon, while the electrons are studded in the
positively charged sphere like seeds in a watermelon.
THOMSON PROPOSED: -
(I) An atom consists of a positively charged sphere and
electrons are embedded in it.
(ii) The negative charged [Electron] and positively
charged [Proton] are equal in magnitude.
So, the atoms in the whole are electrically neutral.
Thomson model explained that atom is electrically neutral
but results of experiments
carried by other scientists could not be explained by
Thomson's model
RUTHERFORD MODEL OF AN ATOM:
He was interested in knowing how electrons are arranged
within an atom.
For this, he carried out an experiment named Alpha
scattering experiment or gold foil experiment Rutherford conducted an experiment
by bombarding a thin sheet of gold with a-particles
and studied the trajectory of these particles after their
interaction with the gold foil
Rutherford directed
high energy streams of a-particles from a radioactive source at a thin sheet
(100 nm thickness) of gold
In order to study the deflection caused to the a-particles, he placed a fluorescent zinc sulfide
screen around the thin
gold foil. Rutherford made certain observations that
contradicted Thomson's atomic model.
OBSERVATIONS
1) Most of the fast-moving a-particles passed straight
through the gold foil.
2) Some of the alpha particles were deflected by the foil by
small angles
3) Few particles bounce off the nucleus
CONCLUSIONS
1) Most of the space inside the atom is empty.
2) All the positive charges must be concentrated in a very
small space within the atom called the nucleus.
3) The size of the nucleus is very small as compared to the size of the atom
Rutherford's Gold Foil Experiment
Drawbacks of Rutherford model of the atom
We know that charged objects which revolves in a circular
motion gains acceleration gradually If electrons moving fast in a circular path,
then it will also gain acceleration
Electrons also liberate energy in some form as it is gaining
acceleration
Ultimately all the electron will get over as it is radiating
energy and fall into Nucleus This results in high instability of an atom
But these things do not happen in an atom as in nature atoms
are stable
BOHR MODEL OF AN ATOM
Bohr's model of the atom was proposed by Neil Bohr in 1915. It
came into existence with the modification of Rutherford's model of an atom
·
In an
atom, electrons (negatively charged) revolve around the positively charged nucleus in a definite circular path called orbits or shells.
·
Each
orbit or shell has fixed energy and these circular orbits are known as
orbital shells.
·
The
energy levels are represented by an integer (n=1, 2, 3...) known as the quantum
number. This range of quantum numbers starts from the 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.
·
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.
ATOMIC NUMBER- Number of protons present in an atom
ATOMIC MASS -Number of
protons and neutrons present in an atom
ISOTOPES
Atoms have the same element having the same atomic number but
different atomic mass
ISOBARS
Atoms of different elements which have the same atomic mass but different atomic number
ISOTONES
Atoms with the same number of neutrons are called isotones
ISOELECTRONIC SPECIES
Atoms/ molecules with the same number of electrons are called as
isoelectronic species
ISODIAPHERS
Atoms with the same number of excess neutrons are called isodiapheres
Neutrons:
In 1932, Chadwick discovered another subatomic particle that had no charge which had a mass nearly equal to that of a proton.
This sub-atomic particle was eventually named NEUTRON
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.
In 1932, the physicist James Chadwick conducted an
experiment in which he bombarded Beryllium with alpha particles from the
natural radioactive decay of Polonium.
Neutrons were produced when Beryllium-9 was bombarded with
alpha particles.
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