UNIT 2 STRUCTURE OF ATOM
Discovery of electrons
In mid 1850s, Michael Faraday began to study electrical discharge in partially evacuated tubes, known as cathode ray discharge tubes. A cathode ray tube is made of glass containing two thin pieces of metal called electrodes sealed in it. When sufficiently high voltage is applied across the electrodes, current starts flowing through a stream of particles moving in the tube from the negative electrode (cathode) to the positive electrode (anode). These were called cathode ray particles. The results of this experiment were:
- The cathode rays start from cathode and move towards the anode.
- These rays are not visible but their behaviour can be observed with the help of certain kind of materials (fluorescent or phosphorescent) which glow when hit by them.
- In the absence of electrical or magnetic field, these rays travel in straight lines.
- In the presence of electrical or magnetic field, the behaviour of cathode rays are similar to that expected from negatively charged particles suggesting that cathode rays consist of negatively charged particles, called electrons..
- The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of gas present in the cathode ray tube.
Discovery of protons and neutrons
Electrical discharge carried out in the modified cathode ray tube led to the discovery of particle carrying positive charge, also known as canal rays. The characteristics of these positively charged particles are:
- Unlike cathode rays, the positively charged particles depend upon the nature of the gas present in the cathode ray tube. These are simply positively charged gaseous ions.
- The charge to mass ratio of the particles is found to depend on the gas from which these originate.
- Some of the positively charged particles carry a multiple of the fundamental unit of electric charge.
- The behaviour of these particles in the magnetic or electric field is opposite to that observed for electron or cathode.
The smallest and lightest positive ion was obtained from hydrogen and was called proton. When electrically neutral particles having a mass slightly greater than that of the protons was emitted, these particles were called neutrons.
Mass of electron= 9.10939 x 10-31 kg, charge on electron= − 1.6022 x 10-19
Mass of proton= 1.67262 x 10-27 kg, charge on proton= + 1.6022 x 10-19
Mass of neutron= 1.67493 x 10-27 kg, charge on neutron= 0
WAVE NATURE OF ELECTROMAGNETIC RADIATION
When electrically charged particle moves under acceleration, alternating electrical and magnetic fields are produced and transmitted in the forms of waves called electromagnetic waves or electromagnetic radiation. The properties of these electromagnetic waves include:
- These oscillating electric and magnetic fields produced by oscillating charged particles are perpendicular to each other and both are perpendicular to the direction of propagation of the wave.
- Unlike sound waves or water waves, electromagnetic waves do not require medium and can move in vacuum.
- There are many types of electromagnetic radiations, which differ from one another in wavelength (or frequency). These constitute the electromagnetic spectrum.
PARTICLE NATURE OF ELECTROMAGNETIC RADIATION
Some observations that could not be explained by the wave nature of electromagnetic radiation were:
- The nature of emission of radiation from hot bodies (black body radiation)
- Ejection of electrons from metal surface when radiation strikes it (photoelectric effect)
- Variation of heat capacity of solids as a function of temperature
- Line spectra of atoms with special reference to hydrogen
These gave rise to Planck’s Quantum Theory. Planck suggested that atoms and molecules could emit (or absorb) only in discrete quantities and not in a continuous manner. The name quantum was given to the smallest quantity of energy that can be emitted or absorbed in the form of electromagnetic radiation. The energy (E) of a quantum of radiation is proportional to its frequency and is given by: E = hν where h is Planck’s constant and has the value 6.626×10-31 Js
According to quantum mechanical model of the atom, the electron distribution of an atom containing a number of electrons is divided into shells. The shells consist of one or more subshells and subshells are assumed to be composed of one or more orbitals, which the electrons occupy. Electrons are filled in these orbitals according to the following two rules:
- Pauli’s exclusion principle: No two electrons in an atom have the same set of four quantum numbers.
- Hund’s rule of maximum multiplicity: Pairing of electrons in the orbital belonging to the same subshell does not take place until each orbital belonging to that subshell has got one electron each i.e., is singly occupied.