```     atom of an element has the same number                     As the mass number is equal to the
of electrons and protons:                                   number of protons plus the number of
neutrons:
    Number of neutrons = Mass Number -
Number of protons = A – Z
    Number of protons = Number of
electrons = atomic number = Z (placed as
a left subscript)
    Mass number (A) is equal to the number
of protons and neutrons in the isotope
(placed as a left superscript).
This is an example of an atomic symbol.
Isotopes                Isobars                    Isotones                 Isomers
The atoms having         Nuclides having the       Atoms of different        The atoms having
same atomic number       same mass number          elements         having   same atomic numbers
but different atomic     but     having      the   different          mass   and      the    atomic
mass number.             different                 number and different      masses, but their
Proton/Atomic             atomic number but         structures           are
number.                   same neutron number       different.
1                      40                             3
Ex: Hydrogen 1     H,    Ex: Argon      18 Ar ,    Ex:     Tritium     1 H,  Ex:              Allene
2                            40                4
Deuterium    1 H,        Potassium        19K,     Helium 2 He (both                      , Propyne
3                        40
Tritium   1 H            Calcium    2 C            have 2 neutrons)
Niels Bohr Model of the Atom                                    The quantity h is known as the Planck’s
constant, h = 6.626 X 10-34 Js.
Niels Bohr model (in 1913) of the atom is very
small, positively charged nucleus which                     Louis de Broglie"s Concept of Matter
contains proton and neutron surrounded by                   Waves
negatively charged electrons. This negatively
charged electrons travel in circular orbits                 Lewis de-Broglie (in 1924) proposed that
around the nucleolus. This atomic model is                  matter waves, the matter has dual nature.
more similar to the solar system. But in this
When the matter is moving it shows the wave
atomic model, electrostatic force providing
properties (like interference, diffraction etc.)
attraction force rather than gravity force.
are associated with it and when it is in the state
Planck’s Quantum Theory                                     of rest then it shows particle properties.
    Max Planck suggested that the energy of                It states that wavelength (λ) of electron is
light is proportional to its frequency, also           inversely proportional to its momentum (p).
showing that light exists in discrete quanta                h    h
of energy.                                                  
p mv
    The energy E of the quantum is related to
the frequency ν by E = hν.
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