# Structure of atom II

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Isotopes: These are the atoms of the same element having the same atomic number but different mass number. eg 1H1,1H2,1H3

Isobars: Isobars are the atoms of different elements having the same mass number but different atomic number. eg 18Ar40 , 20Ca40

Isoelectronic species:

• These are those species which have the same number of electrons.
• Electromagnetic radiations: The radiations which are associated with electrical and magnetic fields are called electromagnetic radiations.
• When an electrically charged particle moves under acceleration, alternating electrical and magnetic fields are produced and transmitted.
• These fields are transmitted in the form of waves. These waves are called electromagnetic waves or electromagnetic radiations.
a. Oscillating electric and magnetic field are produced by oscillating charged particles. These fields are perpendicular to each other and both are perpendicular to the direction of propagation of the wave.
b. They do not need a medium to travel. That means they can even travel in vacuum.

a. Wavelength:
It may be defined as the distance between two neighbouring crests or troughs of wave as shown. It is denoted by λ.

b. Frequency (ν):
It may be defined as the number of waves which pass through a particular point in one second.

c. Velocity (v):
It is defined as the distance travelled by a wave in one second. In vacuum all types of electromagnetic radiations travel with the same velocity. Its value is 3 X 108m sec-1
It is denoted by v.

Wave number:
Wave number  is defined as the number of wavelengths per unit length
Velocity = frequency x wavelength c = νλ

Planck's Quantum Theory: The radiant energy is emitted or absorbed not continuously but discontinuously in the form of small discrete packets of energy called ‘quantum’. In case of light , the quantum of energy is called a ‘photon’The energy of each quantum is directly proportional to the frequency of the radiation, i.e. E α υ or E= hυ where h = Planck’s constant = 6.626 x 10-27 Js. Energy is always emitted or absorbed as integral multiple of this quantum. E = nhυ Where n = 1,2,3,4,.....

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