Structure of atom
Atom and molecules are the building
blocks of matter. The different matter around us are due the different atoms
and molecules that combine together to form those substance. The atoms of
different matter are different which have different number of electrons,
protons and neutrons. Earlier it was a conception that these small particles
are indivisible, but later through experiments and calculations it was
concluded that atoms are also divisible and they have structural body.
The components which form an atom are:
electrons, protons, and neutrons, these smaller particles are called subatomic
particles. Among these subatomic particles electrons are negatively charged,
protons are positively charged and neutrons are neutral, that means neutrons
are charge less.
Protons and neutrons are present at the
centre of atom, this centre is called nucleus and the electron is outside the
nucleus.
In an atom the number of electrons and
protons are equal thus the overall charge of an atom is zero, on comparing the
mass the proton and neutron carries the total mass of an atom in the nucleus
and the electron which travels in a circular path around the nucleus
contributes nothing to the mass of atom as its mass is almost negligible.
Presence of charged particles around us
When we rub inflated balloon in dry
hair, balloon is attracted towards woollen cloth. When we rub glass rod with
silk cloth, glass rod attracts an inflated balloon. These two experiments prove
that balloon when rubbed with dry hair get electrically charged such that when
bought in contact with woollen cloths both attract each other. Thus we can say
that charged particles are present in the atoms.
Characteristics of electron, proton and
neutron
Characteristics of electron:
I.
Mass of electron: Absolute mass
of electron is 9×10-28grams. It is 1/1840 of mass of one atom of hydrogen.
II.
Charge of an electron:
Absolute charge of an electron is 1.6 × 10-19 coulomb negative
charges. 1.6 × 10-19 coulomb
is the smallest charge carried by any particle, so it is also called one unit
charge.
Characteristics of proton:
I.
Mass of proton: Absolute mass
of proton is 9×10-24grams. It is the
mass of one atom of hydrogen.
II.
Charge of a proton: Absolute
charge of an proton is 1.6 × 10-19 coulomb positive charges. This is
equal and opposite charge to that of an electron. It is the smallest positive
charge carried by any particle, so it is also called one unit charge.
Characteristics of neutron:
I.
Mass of neutron: Absolute mass
of electron is 9×10-24grams. It is equal to the mass of a proton
II.
Charge of a neutron: neutron
has no charge, it is electrically neutral.
Discovery of electron, proton, and
neutron:
Discovery of electron:
The existence of electron in an atom
was discovered by J.J.Thomson.
Experiment:
A discharge tube was taken with gas in
it at a very low pressure.
Electricity at high voltage was passed
through it.
Observations:
Stream of minute particles were given
out by the cathode (negative electrode through which current is passing).
These streams of particles are called
cathode rays because they come out of cathode.
Since the particles move out and away
from cathode (negatively charged), it was concluded that these particles are
negatively charged.
Since all different kind of gases emit
cathode rays so it was concluded that all atoms contain negatively charged
particles.
The mass and charge of these particles
does not depend on the gas.
Later these negatively charged
particles were called electrons.
Discovery of proton:
The existence of proton in an atom was discovered
by E.Goldstein. the presence of electrons already proved that there are some
positive particles present in an atom that keep an atom electrically neutral.
Experiment:
A discharge tube was taken with gas in
it at a very low pressure.
Electricity at high voltage was passed
through it.
Observations:
Stream of heavy particles were given
out by the anode (positive electrode through which current is passing).
These streams of particles are called
anode rays because they come out of anode.
Since the particles move out and away
from anode (positively charged), it was concluded that these particles are positively
charged.
The mass and charge of these particles
depend on the gas used in the discharge tube
Later these positively charged
particles were called protons.
Discovery of neutron:
The existence of neutron in an atom was
discovered by James Chadwick.
After the discovery of protons and
electrons, the charges present in atom and the neutral behaviour of atom was
explained, but the extra mass of atom other than the mass of proton was still
to be explained. This problem was solved by the discovery of another subatomic
particle. This particle was neutron which had mass but was charge less.
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