The attraction between positively charged nucleus and negatively charged electrons keeps the electrons within the atom.Negatively charged electrons rotate around the nucleus in orbits (or shells) The convention of indicating the atomic number and mass number of an element Atomic number.A = Z + N is called the atomic mass number.The nucleus contains N number of chargeless neutrons.The nucleus contains Z number of positively charged protons.Symbol of the element A X Mass number Z 40 Ca 23 Na 1 H 20 11 1 Protons = ? Protons = ? Protons = ? Electrons = ? Electrons = ? Electrons = ? Neutrons = ? Neutrons = ? Neutrons = ? Inside the atomThe fundamental particles Part 1: Atomic structure (contd) Inside the atom Negatively charged electrons orbiting around extremely small positively charged solid nucleus Bohr (1913)Electrons can occupy only ‘allowed’ orbits having stable energy levels Progressive changes in the understanding of the atom The scientist Niels Bohr further improved this model by stating that electrons can only occupy ‘allowed’ orbits whose energy levels are stableĭalton (1808) The smallest indivisible particle of an element Thomson (1897) negatively charged electrons embedded in a positively charged solid sphere Rutherford (1911) Planetary model. Atomic radius is 10-8cm while the radius of the nucleus is only 10-13 cm i.e 100000 times smaller than atomic radius.Negatively charged electrons revolve around the nucleus in circular or elliptical orbits just like the planets which orbit around the sun.The mass and positive charge of an atom are concentrated in the centre (called as nucleus).Nucleus Electron Rutherford’s planetary model of the atom only one particle took a U turn and deviated by almost 180 degrees.nine particles were scattered in various directions.about 19990 passed through the foil without deviation.Alpha particles: Minute positively charged particles.Rutherford fired alpha particles at high velocity on a thin gold foil.Would this solid part be smaller or larger compared to the size of the atom? Only one alpha particle seems to have collided against a solid part of the atom and come right back.If positive particles were distributed uniformly inside the atom would only nine alpha particles have been scattered ?.If the atom was solid would most alpha particles have gone through the foil without deviating ?.Thin gold foil Alpha ray Polonium Alpha particle detector screen Box lined with Lead Rutherford’s experiment Negatively charged electrons are embedded in the atom like seeds of a water melon The scientist Ernest Rutherford answered these questions.Electrons are microscopic constituents of atoms.Considering their deviation these particles must benegatively charged.Cathode rays are a stream of particles (electrons) not light rays. ![]() Why are electrons the only particles coming out of atoms ?.How are positive charges distributed in the atom ?.How do we know that the atom is solid ?.J.J.Thomson 1856 - 1940 Thomson’s atomic model It was observed that the ray emanating from the cathode would deviate under the influence of electric field between the positive and negative plates The scientist J.J.Thomson answered these questions.Experiments using electric discharge tube and cathode ray tube helped in discovering the internal structure of an atom. ![]()
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