NEET Physics Notes Optics -Atoms and Nuclei -Theory of Atoms
Theory of Atoms
Atom is the smallest particle of an element which contains all properties of element. Molecule is a single atom or a group of atoms joined by chemical bonds. It is the smallest unit of a chemical compound that can have an independent existence. Nuclei refers to a nucleus of an atom, having a given number of nucleons. It is a general term referring to all known isotopes-both stable and unstable of the chemical elements. Thus, O16 and O17 are different nuclides.
Scattering of a-particles
In 1911, Rutherford successfully explained the scattering of a-particles on the basis of nuclear model of the atom.
Number of a-particles scattered through angle θ is given by
where, K is the kinetic energy of a-particle and Z is the atomic number of the metal.
Rutherford’s Model of the Atom
On the basis of scattering of a-particles, Rutherford postulated the following model of the atom
- Atom is a sphere of diameter about 10-10 m. Whole of its positive charge and most of its mass is concentrated in the central part called the nucleus.
- The diameter of the nucleus is of the order of 10-14 m.
- The space around the nucleus is virtually empty with electrons revolving around the nucleus in the same way as the planets revolve around the sun
- The electrostatic attraction of the nucleus provides centripetal force to the orbiting electrons.
- Total positive charge in the nucleus is equal to the total negative charge of the orbiting electrons.
Rutherford’s model suffers from the following drawbacks
- stability of the atomic model
- nature of energy spectrum