CBSE Class 9 Chemistry Structure of the Atom
Model of An Atom
Credit for the discovery of electron goes to J.J.Thomson, proton to E.Goldstein and neutrons to J. Chadwick.
J.J. Thomson proposed that electrons are embedded in a positive sphere.
The mass of positively charged proton and neutral neutron is one unit each.
The mass of negatively charged electron is about 1/1840 times the mass of a proton.Discovery of Nucleus
During Rutherford's gold foil, he observed that most of the alpha particles passed through the gold foil without any deflection, few were deflected from their path and a small fraction rebounded.
Rutherford's alpha-particle scattering experiment led to the discovery of the atomic nucleus.
The stability of the atom could not be explained by this model.
Niels Bohr's proposed that electrons are distributed in different shells with discrete energy around the nucleus.
Maximum number of electrons that can be accommodated in the outermost shell is 8.
Valency is the combining capacity of an atom.Distribution of Electrons in Orbits
Distribution of Electrons in OrbitsParameters of an Atom
Atomic number of an atom is the number of protons in the nucleus while the atomic mass of an atom is the sum of protons and neutrons in the nucleus.
An atom is neutral as the number of protons is equal to the number of electrons
Isotopes are atom of same element, with different number of neutrons. Isobars are atom of different elements with same mass number.
