Atomic Structure of an Element : : Properties of an Elements
Atomic Structure :
- In 1809, Dalton suggested that atom is the smallest particle of the element and it is indivisible.
- But in the beginning of 20th century Rutherford, J.J. Thomson etc. suggested that atom is divisible and made up of electrically charged particles.
- The electron is a fundamental particle of an atom which carries a unit negative charge.
- It was discovered by J.J. Thomson in 1897.
- It is a fundamental particle of an atom carrying a unit positive charge.
- It was discovered by Rutherford and Goldstein in 1886.
- It is a fundamental particle of an atom carrying no charge.
- It was discovered by Chadwick in 1932.
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Properties of Cathode Rays :
- They travel in straight lines
- They cast shadow of solid objects placed in their path.
- They cause mechanical motion i.e., they consists of material particles.
- They carry -ive charge.
- These rays are deflected when magnetic field is applied on them.
- They produce heating effect.
- They cause ionization of gas through which they pass.
- They produce green fluorescence on the glass walls of the discharge, tube as well as on certain other substances such as zinc suplhide.
- They effect the photographic plates.
- They have penetrating power.
- The ratio of charge / mass (i.e., e / m) = 1.76 x 108 coulombs / g.
Properties of Anode Rays :
- They travel in straight lines, however, their speed in much less than that of cathode rays.
- They are made up of material particles.
- They are positively charged.
- They deflect in electric and magnetic field.
- The ratio of charge / mass is found to be different for the particles constituting anode rays when different gases are taken inside the discharge tube.
Properties of Nucleus :
- Nucleus of an atom consists of protons and neutrons which are collectively known as nucleons.
- The forces that bind the electrons to the nucleus are electrical or coulombic in nature.
- Density of nucleus is enormous and is of order of 1014 g/cm.
- Instability of the nucleus is due to high neutron proton ratio.
- The radius of the nucleus is around 5 x 10-13 cm (5 Fermi).
Atomic Number ( Z ) :Atomic number of an Element. = Total No. of protons present in the nucleus = Total No. of Electrons present outside the nucleus. ( or )
Z = p = e
Mass Number ( A )Mass Number = Number of Protons + Number of Neutrons
A = p + n (or) A = z + n
Mass Number =>A
Atomic Number =>ZX
e.g., 2311Na, 1735Cl and so on