Modern Physics - SS1 Physics Past Questions and Answers - page 5

The Bohr model of the atom, proposed by Niels Bohr, suggests that electrons orbit the nucleus in specific energy levels or orbits. The model incorporates the idea of quantized energy, where electrons can only exist in certain discrete energy states. Electrons closer to the nucleus have lower energy levels, while those farther away have higher energy levels. This model explains the stability of atoms by stating that electrons in the innermost orbit have the lowest energy and are more tightly bound to the nucleus. As a result, they are less likely to be lost or gained during chemical reactions.

Isotopes are different forms of the same element that have the same number of protons but different numbers of neutrons. The atomic number of an element remains the same for all isotopes, indicating the number of protons, but the atomic mass varies. Isotopes have different mass numbers, which represent the total number of protons and neutrons in the nucleus. Due to the difference in the number of neutrons, isotopes can have slightly different physical properties and different atomic masses.