Class9 Most Important Questions

--- Structure of an Atom 1. Isotopes and Isobars Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Example: Carbon-12 (6 protons, 6 neutrons) and Carbon-14 (6 protons, 8 neutrons). Isobars are atoms of different elements that have the same mass number but different atomic numbers. Example: Argon-40 (18 protons, 22neutrons) and Calcium-40 (20 protons, 20 neutrons). 2. Bohrs Atomic Model and Its Limitations Postulates: 1. Electrons revolve in yxed, circular orbits around the nucleus without radiating energ3. 2. Electrons can onl3 occup3 certain orbits or energ3 levels. W. qhen electrons move between orbits, energ3 is emitted or absorbed in discrete Zuantities. Limitations: 1. It could not explain the spectrum of atoms with more than one electron. 2. It could not explain the phenomenon of yne spectral lines or the Teeman effect. W. It was unable to incorporate the wave nature of electrons, which was later explained b3 Zuantum mechanics. W. khomsons Atomic Model and Its Shortcomings khomsons Model (Plum-Pudding Model): khe atom is a positivel3 charged sphere with electrons embedded within it li"e Rplums in a pudding.R Shortcomings: 1. It could not explain the results of Nutherfords gold foil experiment.