UNIT 2 STRUCTURE OF ATOM
SUBATOMIC PARTICLES 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:
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:
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:
PARTICLE NATURE OF ELECTROMAGNETIC RADIATION Some observations that could not be explained by the wave nature of electromagnetic radiation were:
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 6.626x10-31 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:
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