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of an atom
(i) The central part, nucleus, consists of protons and
(ii) The negatively charged extranuclear electrons revolve around the
of protons, neutrons and electrons
the same as the mass of H2 atom
th the mass of proton or H2 atom.
close to the mass of H2 atom
The number of protons present in the nucleus of an atom
of an element is called the atomic number.
Atomic number (Z) = Number of protons = Number of
mass number (A)
The total number of protons and neutrons present in the
nucleus of an atom of an element is called the atomic mass number.
Atomic mass number (A) = Atomic number (Z) + Neutron
The number of neutrons present in the nucleus of an atom
of an isotope of an element is called the neutron number.
N = A - Z
The atoms of the same element which differ in the number
of neutrons present in the nuclei of their atoms are called isotopes.
The atoms of the same element having the same atomic number but
different atomic mass number are called isotopes.
(i) Isotopes of hydrogen -
(ii) Isotopes of oxygen -
The atoms of different elements which have the same atomic
mass number but different atomic numbers are called isobars.
defect (D m)
The difference between the total mass of the nucleons
(protons and neutrons) present in the nucleus of an atom and the
actual mass of the nucleus is called the mass defect of the nucleus.
D m = [ZmH
+ (A - Z)mn] - M
mH = mass of an atom of hydrogen
M = isotopic mass of an atom
The amount of energy required to break the nucleus of an
atom into its constituent nucleons is called binding energy of the
B.E. = Dm × 931 MeV
D m ´ 931 MeV
Binding energy affects the stability of the nucleus.
The phenomenon of spontaneous disintegration of unstable
nuclei of certain heavy elements with the emission of some
radioactive radiation is called radioactivity.
It is not affected by external factors like temperature, pressure,
catalyst and the state of existence, i.e. whether it is an element or
in a combined state.
of a, b and g - rays
2) charge and 4 a.m.u. mass
(-1) charge and
and zero mass
to 1/100th the velocity of light
of the velocity of light
as the velocity of light
can hardly pass through 0.02 cm thick Al sheet.
than a-rays; can pass through
0.2 cm thick Al sheet.
high; can pass through 100 cm thick Al sheet.
in an electric field
the negative plate
the positive plate