Question

Write short notes on:

(a) Rutherford’s scattering experiment.

(b) Isotopes.

Answer 1

(a) Rutherford's scattering experiment: In Rutherford’s experiment, a beam of alpha particles were directed onto a thin sheet of gold and a zinc sulphide screen was used as a detector at the focus of the microwave. The screen and the microscope could be moved around to study any deflected alpha ray at any given angle.

Observation: When the alpha rays were passed through the gold sheet, it was observed that many of the rays passed through the sheet without getting deflected. There were few alpha rays that got deflected by some angles and a few came back to the source of alpha rays but, overall very few rays were deflected at angles greater than 90 degree.

Conclusion: All protons are present inside the nucleus, which is situated at the centre of the atom.

• Electrons reside outside the nucleus and revolve around the nucleus in well-defined orbits.

• The size of the nucleus is very small in comparison to the size of an atom. As per Rutherford’s calculations, the size of the nucleus is 105 times smaller than an atom.

• As the mass of the electron is negligible in comparison to the mass of the proton, almost all the mass of the atom is concentrated in the nucleus.

(b) Isotopes: Isotopes are defined as atoms of the same element having the same atomic number, but different mass numbers. These atoms contain an equal number of protons and electrons, but a different number of neutrons. For example, Carbon element shows isotopes because this atom exists in three forms, each form differing in its mass number. There are three principal isotopes of carbon with atomic masses 12, 13 and 14.

For any element, the atomic number is written as the subscript and the atomic mass is written as the superscript. Thus, for an element X with atomic number Z and atomic mass A, we have

A_ZX

We know that isotopes of an element have the same atomic number. Therefore, the isotopes of carbon will be denoted by the same symbol that is C.

Therefore, the isotope of carbon with atomic mass 12 will be written as ¹²₆C.

The isotope of carbon with atomic mass 13 will be written as ¹³₆C.

The isotope of carbon with atomic mass 14 will be written as ¹⁴₆C.