CLASS: IX NCERT (CBSE)
Structure of the Atom
• Atom is the smallest particle of an element, which maintains its identity throughout chemical and physical changes.
• Atoms are made up of three fundamental particles: electrons, protons and neutrons.
• Discharge tube experiments by William Crookes and J.J. Thomson led to the discovery of electron.
• Goldstein’s experiment with a perforated cathode in the discharge tube led to the discovery of anode rays and the existence of protons.
• The existence of the nucleus in the atom was established by Rutherford by his alpha-particle scattering experiment.
• James Chadwick discovered the neutron.
• Neutrons are neutral particles, protons are positively charged and electrons are negatively charged particles.
• The nucleus contains protons and neutrons collectively called nucleons. It is positively charged. The mass of an atom is concentrated in the nucleus.
• The mass of electrons is negligible compared to the mass of the protons.
• Ordinary hydrogen is the only element that has no neutron in it.
• According to Bohr electrons revolve around the nucleus in fixed orbits called K, L, M, N etc.
• The energy of the shells increases as the distance form the nucleus increases.
• The number of electrons in each orbit can be calculated by the formula 2n2
• The number of protons is the atomic number of an element. It is denoted as Z and is the identity of an element.
• Mass number is defined as the sum of the number of protons and neutrons in the nucleus of the atom. It is denoted as A.
• Valency is the combining capacity of an element.
• Metals have 1, 2 or 3 electrons in their valence shells.
• Non-metals have 4, 5, 6 or 7 electrons in their valence shells.
• All noble gases have 8 electrons in their valence shells (except helium which has two).
• Atoms of the same element having same atomic number but different mass numbers are called isotopes.
• Isotopes show similar chemical properties but different physical properties due to the difference in the number of neutrons.