Electromagnatic Radiations or Eectromegnetic Waves

Nature of Radiations or Emitted by Electron

When we object moves up and down or vibrates continuously, energy, in the form of a wave, is sent or transmitted (or propagated) by the vibrating object to a distant place. The wave travels at right angle to the vibrating vibratory motion of the vibrating object and away from it. Wave produced by a vibrating object can be represented by a wavy curve. The tops (i.e. maximum) of the wavy curve are called crests and the bottoms (i.e. minima) are called troughs. When a stone is thrown in a pond of water, water molecules start to vibrate up and down and transmit their energy as waves on water surface which are seen traveling towards the bank of pond.

In 1864 James Clark Maxwell showed by that when an electrics current is passed through a circuit (i.e. through a tungsten filament), it radiates energy in the form of waves. The energy radiated is called radiate energy and the waves produced are called electromagnetic waves or electromagnetic radiations. These waves are so called because similar sort of  waves can also be obtained by moving a charged body as well as a magnet to and fro in am genetic field. These waves do not require any medium for propagation or transmission; e.g. light and radio waves (which are electromagnetic waves) from the sun and stars reach the earth by traveling through non-material medium i.e. through empty space. All these waves travel with the speed of light.

The radiations (or wave) emitted by the vibratory motion of an electron are also electromagnetic radiations. These radiations also travel with the speed of light.

Bohr's Atomic Model

According To Rutherford's nuclear atomic model, an atom has a nucleus and negative electrons which are revolving round the nucleus  in the same way as the planets revolve round the sun in the solar system.This model could not say anything as to how and where these electrons were arranged.

This model could also not explain how the spectral lines are produced by the simplest atoms like hydrogen atom when an electron jumps from one orbit to another.

In order to explain why an electron revolving round the nucleus does not lose energy and consequently does not fall into the nucleus and how the spectral lines of the emission spectrum of H-atom are produced when aan electron jumps from one energy level to other, Niels Bohr, a Danish Physicist, in 1913, put forward a new atomic model which is based on Planck's quantum theory of radiation.

Before, we could deal with Bohr's atomic model we should have a clear understanding of the following topics:

• Electromagnetic radiations (or waves) and nature of  radiations emitted by electron.
• Emission spectrum, continuous spectrum and line spectrum of hydrogen atom.
• Plank's quantum theory of radiation, and its application in explaining photoelectric effect and Compton effect.

What Is Mass Number (A)?

We know that the nucleus of an atom contains neutrons and protons. The sum of the number of neutrons (n) and protons (p) present in the nucleus of an atom is called the mass number (A) of that atom, i.e.

Mass Number (A) = No. of protons (p) + No. of neutrons (n)

or, A = p+n

In cases where it is not necessary to differentiate between protons and neutrons, these particles are collectively called nucleons. Thus the mass number (A) of an atom is equal to the total number of nucleons in the nucleus of an atom. Obviously the mass number is a whole number.

Each different variety of atom as determined by the composition of its nucleus is called a nucleotide.

Importance Of Mosley's Law

Mosley's Law has been found to be very useful. The importance of this law is evident from the following points:

1. We have seen that this law has established that the atomic number is always numerically equal to the number of electrons revolving round the nucleus. According to this law it is the atomic number and not atomic weight on which the properties of the elements (both physical and chemical) depend. It thus says that the elements must be arranged in the periodic table in the increasing order of their atomic number and not atomic weight as originally enunciated by Mendeleef. Accordingly, Mosley's law has been used to place elements in their proper order in the periodic table in certain questionable cases, e.g. if the elements are placed according to their increasing atomic weight, K(Z=19) should come before Ar (Z=10) ; Ni (Z=28) should precede Co (Z=27) etc. But Mosley's law says that as per their atomic numbers their order should be reversed; i.e. K should be placed after Ar, Co should come before Ni etc. This fact is further supported by chemical properties.
2. Mosley's law has led to the discovery of many new elements. The elements like technetium (Z=43) have been discovered, promethium (Z=61), hafnium (Z=72), rhenium (Z=75) have been discovered on the basis of the gaps left by Mosley in Mosley's X-ray diagrams. These elements have been subsequently discovered.
3. This law has been helpful in determining the atomic number of rare-earths and has thus helped in fixing their position in the periodic table.

Mosley's Law

Rogenton while making some experiments with a discharge tube in 1895, found that when cathode rays are allowed to fall on a metal target called anti-cathode or target material placed in their path, new type of radiation s are produced. These radiations are called X-rays. These rays are also electromagnetic in character and are comparable to light waves. The stream of cathode rays (i.e. electrons) produced from the cathode converges to a point through a concave surface on the anti-cathode and give rise to X-rays.

What Is Atomic Number?

Atom has a nucleus which can carries positive charge on it. The positive charge carries by the nucleus is called nuclear charge which is equal to the total number of positive charges on the nucleus of the atom of that element. This total number of unit positive charges on the nucleus is called the atomic number of the element. Now since each proton has +1 charge and neutron has no charge, the total number unit positive charge carried by the nucleus is equal to the number of protons present in the nucleus of the atom. Thus atomic number of element can also be defined as the number of protons present in the nucleus. Thus:

'Atomic number of an element is defined as the number of  unit positive charges on the nucleus (nucleus charge) of the atom of that element or as the number of protons present in the nucleus.'

Now since the atom as a whole is electricity neutral, the number of protons (positively charged particles) in the nucleus is equal to the number of electrons (negatively charged particles) revolving in orbits around the nucleus. Consequently atomic number of an element is also equal to the number of electrons revolving in orbits round the nucleus. Thus:

Atomic Number (Z) = No. of unit positive charges on the nucleus (Nuclear charge).
                               = Total number of unit positive charges carried by all protonsnt in the nucleus.
                               = No. of protons in the nuclens (p)
                               = No. of electrons revolving in orbits (e)
or,

Z = p = e

Atomic  number of an element is also defined as the serial number of its position in the periodic table, starting from hydrogen as the first element.