NCERT Solutions for Class 9th Science
Chapter 4. Structure of The Atom
- Atoms and molecules are the fundamental building blocks of matter.
- The existence of different kinds of matter is due to different atoms constituting them.
- The electron is identified by J.J. Thomson.
- Electrons, protons and nutrons are sub atomic particles.
- E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays.
- Electrons carry Negative (-) charge.
- Protons carry Positive (+) charge.
- Nutrons carry No any charge.
- J.J. Thomson was the first one to propose a model for the structure of an atom.
- The electrons revolve around the nucleus in well-defined orbits.
- The size of the nucleus is very small as compared to the size of the atom.
- Protons and Nutrons are situated in nucleus of atom.
- Orbits or shells of an atom are represented by the letters K, L, M, and N.
- J. Chadwick discovered another sub atomic particles named nutron.
- The number of protons present in nucleus of an atom is known as atomic number. i.e Atomic mass = total number of protons in nucleus os an atom.
- The sum of the masses of protons and neutrons present in the nucleus is known as Atomic Mass.
- The maximum number of electrons present in a shell is given by the formulla 2n2.
- The electrons present in the outermost shell of an atom are known as the valence electrons.
- The number of valence electrons present in outer most shell of an atom is known as its valency.
- The mass number is defined as the sum of the total number of protons and
neutrons present in the nucleus of an atom.
- The atoms of the same element, having the same atomic number but differentmass numbers is called its isotopes.
- Protium, Deuterium and tritium are isotopes of Hydrogen.
- Many elements consist of a mixture of isotopes. Each isotope of an element is a pure substance.
- The chemical properties of isotopes are similar but their physical properties are different.
- An isotope of uranium is used as a fuel in nuclear reactors.
- An isotope of cobalt is used in the treatment of cancer.
- An isotope of iodine is used in the treatment of goitre.
- Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.