Chapter Review:

• 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.