Atomic Structure — Question 2

(i) Electrons — Experimental proofs leading to the discovery of Electrons were given by William Crookes in 1878 and Sir J.J. Thomson in 1897.

• William Crookes found that when an electric discharge is passed through a tube containing a gas at low pressure (0.01 mm of Hg), blue rays were emitted from the cathode and were termed cathode rays.
• Sir J.J. Thomson studied the characteristics and constituents of cathode rays and found that:
  • Cathode rays travel in straight lines from cathode to anode and cast a shadow of the object placed in their path.
  • They are deflected by magnetic and electric field [attracted towards positively charged plate].
  • They have kinetic energy and raise the temperature of a metallic object on which they fall.

These findings lead to the conclusion that Cathode rays consist of negatively charged particles now called electrons.

(ii) Protons : Goldstein discovered protons.

• As atoms are electrically neutral there must be equal positively charged particles in atom. This led to discovery of the protons.
• He used a perforated disc as cathode and found positively charged rays travelling in opposite direction to cathode rays.
• These rays consisted of positively charged particles which led to the discovery of protons.

(iii) Atomic Nucleus : Lord Rutherford discovered atomic nucleus in 1911.

• He directed alpha particles towards a metallic sheet.
• The deflection of the alpha particles was observed and concluded that there is a nucleus in the centre of the atom.

(iv) Neutrons : James Chadwick discovered neutron in 1932.

• He established that an atom contains protons and electrons.
• Electrons have negligible mass, hence the mass is concentrated in the nucleus. ∴ Atomic mass = mass of protons.
• However, in the case of helium - there are two protons in it's nucleus and hence atomic mass should be equal to twice the mass of each proton i.e., 2 x 1.008 but it's atomic mass is calculated experimentally and found to be 4.003 a.m.u.
• It was therefore proved that, in the nucleus of an atom, there is other particle called a neutron which has no electrical charge but is almost of an equal mass as the protons.
• This confirmed the existence of neutrons