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
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
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
The energy E of the quantum is related to
the frequency ν by E = hν.