Question

a) State the postulates stated by Neils Bohr. [2 Marks]

b)Define the terms (1) isotope, (2) isobar giving one example in each case. [ 2 Marks]
c)What were the three major observations Rutherford made in the gold foil experiment? Also, explain the conclusion Rutherford made after the experiment.
[3 Marks]

Answer 1

a) Main postulates of Bohr's model of an atom are:
i. In atoms, the electron revolve around the
nucleus in stationary circular paths called
orbits or shells or energy levels. [0.5 Mark]

ii. While revolving around the nucleus in an
orbit, an electron neither loses nor gains
energy. [0.5 Mark]

iii. An electron in a shell can move to a higher or
lower energy shell by absorbing or releasing
a fixed amount of energy. [0.5 Mark]

iv. The orbits or shells are represented by
the letters K,L,M,N,… or the numbers,
n= 1,2,3,4. [0.5 Mark]

b)
(1)isotopes can be defined as the different atoms of the same element having same atomic number but different mass numbers. Example: ₁H¹, ₁H², ₁H³ . [1 Mark]

(2) Isobars can be defined as : the atoms belonging to the different elements with same mass numbers but different atomic numbers. Example: ${}_1$${}_9{K}^4$${}^0$ , ${}_2$${}_{0}C{a}^4$${}^0$ . [1 Mark]

c) The three major observations Rutherford made in the gold foil experiment were:
i. Major fraction of alpha particles pass through the foil undeflected. [0.5 Mark]
ii. A few alpha-particles were scattered by very small angles. [0.5 Mark]
iii. A very small number of particles were deflected back. [0.5 Mark]

The observations made by Rutherford led him to conclude that:
i. Since most of the alpha particles passed through the gold foil without getting deflected, so, most of the space inside the atom was empty. [.5 Mark]

ii.Very few particles deflected from their path; this indicated that the positive charge of the atom occupies very little space.
[.5 Mark]

iii.A small fraction of alpha particles bounced back by 180 degrees, this indicated that the entire positive charge and mass of the atom was concentrated in a very small volume within the atom. [.5 Mark]