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Chapter 2: Structure of Atom (CBSE Class 11)
Introduction
- The first atomic theory of matter was proposed by John Dalton, in 1808, called Dalton’s Atomic Theory. According to Dalton’s atomic theory, the atom is the ultimate particle of matter and is indivisible.
- Dalton’s atomic theory was able to explain the law of conservation of mass, the law of constant composition and the law of multiple proportions.
- Dalton’s atomic theory failed to explain the results of many experiments, for example, “Why does a glass rod or ebonite get electrically charged when rubbed with silk or fur?”
Discovery of Sub-atomic Particles
A large number of subatomic particles have been discovered so far but only electron, proton, and neutron are of great importance among them and hence, are called fundamental particles.
Atom
Atomic structure is the study of the structure of the atom. The word atom is derived from the Greek word ‘atomio’ which means indivisible and indestructible.
The main facts related to the atom are as follows:
- An atom is the building block of matter.
- An atom is made mainly from three subatomic particles, viz. electron, proton and neutron.
- The charge-to-mass ratio (e/m) is determined by using discharge tube and applying electric and magnetic field. Deviation of particle takes place when electric and magnetic field is applied to it.
- Millikan oil drop model is used to measure charge of the electron and it was found that electrical charge is always an integral multiple of charge on electron.
- q = ne, where, q = total charge, e = charge of one electron = 1.602 ́ 10-19 C and n = positive integer = 0, 1, 2, 3.
Electron
Electron was discovered as a result of study of cathode rays by JJ Thomson. It was named by Stony.
It carries a unit negative charge (−1.6 × 10−19 C).
The mass of an electron is 9.11 × 10−31 kg, and the mass of one mole of electrons is 0.55 mg. Some of the characteristics of cathode rays are:
- These travel in a straight line away from the cathode and produce fluorescence when they strike the glass wall of the discharge tube.
- These cause mechanical motion in a small pinwheel placed in their path.
- These produce X-rays when strike with metal and are deflected by electric and magnetic fields.
Charge-to-Mass Ratio of Electron
In 1897, British physicist JJ Thomson measured the ratio of electrical charge (e) to the mass of an electron (me ) by using a cathode ray tube and applying electrical and magnetic fields perpendicular to each other as well as to the path of electrons. Thomson argued that the amount of deviation of the particles from their path in the presence of an electrical or magnetic field may vary as follows:
- If greater the magnitude of the charge on the particles, greater is the deflection.
- The mass of the particle, lighter the particle, greater the deflection.
- The deflection of electrons from its original path increase with the increase in the voltage. By this Thomson determined the value e/ me as 1.758820 × 1011 C kg −1.