Atomic Structure - AQA Combined Science Trilogy - PDF

**[INTRODUCTION]** The structure of matter that shapes the world around us has been a subject of study since long time. The first contribution in this regard came from Dalton who postulated that matter is made of atoms, which are indivisible. The word atom comes from a Greek word Atoms which m...

**[INTRODUCTION]** The structure of matter that shapes the world around us has been a subject of study since long time. The first contribution in this regard came from Dalton who postulated that matter is made of atoms, which are indivisible. The word atom comes from a Greek word Atoms which means \'no cut. J.J. Thomson proposed a structure for the atom, which was modified by Rutherford and later by Neil's Bohr. *Dalton\'s Atomic Theory* All elements are consists of very small invisible particles. called atoms. Atoms of same element are exactly same and atoms of different element are different. *Thomson\'s Atomic Model* Every atom is uniformly positive charged sphere of radius of the order of 10 m, in which entire mass is uniformly distributed and negative charged electrons are embedded randomly. The atom as a whole is neutral. This was also known as plum-pudding model of an atom. P4) Quiz 1 -- Answers -- AQA Combined Science Trilogy - Elevise *Limitations of Thomson\'s Atomic Model* 1\. It could not explain the origin of spectral series of hydrogen and other atoms. 2\. It could not explain large angle scattering of α- particles. *Rutherford\'s α-Ray Scattering Experiment* An α particle is helium nucleus containing 2 protons and 2 neutrons. Therefore, an alpha particle has 4 units of mass. This experiment was done by Rutherford and his collaborators, Geiger and Marsden and is shown in fig  *Conclusion* A typical graph of the total number of a-particles scattered at different angles, in a given interval of time, is shown in Fig. The dots in this figure represent the data points. Many of the a-particles pass through the foil. It means that they do not suffer any collisions. Only about 0.14% of the incident α -particles scatter by more than 1º; and about 1 in 8000 deflect by more than 90º. Rutherford argued that, to deflect the a-particle backwards, it must experience a large repulsive force. This force could be provided if the greater part of the mass of the atom and its positive charge were concentrated tightly at its centre. This led Rutherford to postulate that entire positive charge of the atom must be concentrated in a tiny central core of the atom. This tiny central core of each atom was called atomic nucleus.  **[RUTHERFORD\'S MODEL OF AN ATOM]** An atom is composed of positively charged particles. Majority of the mass of an atom was concentrated in a very small region. This region of the atom was called as the nucleus of an atom. It was found out later that the very small and dense nucleus of an atom is composed of neutrons and protons. Atoms nucleus is surrounded by negatively charged particles called electrons. The electrons revolve around the nucleus in a fixed circular path at very high speed. These fixed circular paths were termed as \"orbits.\" An atom has no net charge or they are electrically neutral because electrons are negatively charged and the densely concentrated nucleus is positively charged. A strong electrostatic force of attractions holds together the nucleus and electrons. The size of the nucleus of an atom is very small in comparison to the total size of an atom. **[ELECTRON ORBITALS]** The electrostatic force of attraction, Fe between the revolving electrons and the nucleus provides the requisite centripetal force (Fc) to keep them in their orbits. Thus, for a dynamically stable orbit in a hydrogen atom, The total energy of the electron is negative. This implies the fact that the electron is bound to the nucleus. **[LIMITATIONS OF RUTHERFORD ATOMIC MODEL]**  **[BOHR'S ATOMIC MODEL]** Niels Bohr (1885 1962) made certain modifications in this model by adding the ideas of the newly developing quantum hypothesis. Postulates of Bohr\'s model:- 1\. Every atom consist of central core called nucleus, in which entire positive charge and almost entire mass of atom are concentrated. A suitable no. Of electrons revolve around the nucleus in circular orbits. The centripetal force for revolution is provided by electrostatic force of attraction between electron and the nucleus. 2\. Electron revolves around the nucleus only in those orbits for which the angular momentum is some integral multiple of h/2π where h is the Planck\'s constant. Thus the angular momentum (L) of the orbiting electron is quantised. i.e. L= nh/2π 3\. An electron might make a transition from one of its specified non-radiating orbits to another of lower energy. When it does so, a photon is emitted having energy equal to the energy difference between the initial and final states. The frequency of the emitted photon is then given by, hv = E~i~ -- E~f~ *Radii of Bohr\'s stationary orbits* *Velocity of electron in Bohr\'s stationary orbits*  *Frequency of electron in Bohr\'s stationary orbits* *Total energy of electron in Bohr\'s stationary orbits*  **[DE BROGLIE'S VIEW OF 2^ND^ POSTULATE]** **[LIMITATIONS OF BOHR'S MODEL]**  **[BIBLIOGRAPHY]** - - Class 12 NCERT Physics vol.2