(i) Isotopes have varied applications, three of them are as follows:
1. Isotopes of hydrogen are used in nuclear reactors.
2. An isotope of carbon, Carbon-14 is used to determine the age of fossils.
3. An isotope of phosphorus, Phosphorus-31 is used as a tracer to identify the nutrients exchange in plants.
(ii) The main ideas of Bohr's atomic model are as follows:
1. Electrons revolve around the nucleus of an atom in fixed paths called orbits or shells.
2. Electrons in each shell have definite energy and so they are also called energy levels.
3. As long as an electron revolves in a particular orbit, its energy remains constant.
4. The energy of shells increases as the distance from the nucleus increases.
5. The shells around the nucleus can be numbered from near the nucleus as 1, 2, 3, 4, 5... or represented by the letters K, L, M, N, O, and so on.