STRUCTURE OF AN ATOM

LESSON OBJECTIVES Dalton’s theory Discovery of fundamental particles Thomson’s model of an atom Rutherford’s model Concept of atomic number and mass number Drawback of Rutherford’s model

DALTONS ATOMIC THEORY - 1800 ATOMS CAN NEITHER BE CREATED NOR DESTROYED ATOMS ARE FURTHER DIVIDED INTO SUB –ATOMIC PARTICLES ELECTRONS PROTONS AND NEUTRONS All matter is made of atoms. Atoms of an element are identical. Each element has different atoms. Atoms of different elements combine in constant ratios to form compounds. Atoms are rearranged in reactions. His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

DICOVERY OF CATHODE RAYS ( ELECTRONS ) J DICOVERY OF CATHODE RAYS ( ELECTRONS ) J.J THOMSON According to Michael faraday when chemical substances in aqueous or molten form in the presence of electricity deposition or accumulation of substances occurs which is electrolysis gases are poor conductors of electricity at normal pressures but by decreasing pressure they can behave as electrolytes

A cathode ray tube is made of glass containing two pieces of metal called electrodes When sufficient high voltage and pressure is applied across the electrodes , current start flowing through a stream of particles moving in the tube These are cathode rays which are –ively charged These rays are no visible but their behavior can be observed with the help of certain kind of materials (television tube )

PROPERTIES OF CATHODE RAYS In the absence of electrical or magnetic field , these rays travel in straight lines They are material particles as they produce mechanical motion in a small paddle wheel The characteristics of cathode rays don’t depend upon the Material of electrodes and nature of the gas present in cathode ray tube They are deflected from their path by electric and magnetic fields.

e/m ratio of electron By using cathode tube and applying electrical and magnetic field perpendicular to each other as well as to the path of electrons * magnitude of –ive charge is greater , greater is interaction and greater is deflection *lighter the particle greater deflection *deflection increases with increase in voltage and strength of magnetic field * refer figure 2.2 from text book page.no 28 draw figure and write the values of e/m and e and m values of electron from textbook (pg.28,29)

DISCOVERY OF ANODE RAYS ( PROTONS ) E DISCOVERY OF ANODE RAYS ( PROTONS ) E.GOLDSTEIN Here in this experiment perforated cathode is used where some rays are observed behind the perforated cathode *charge / mass ratio depend on the gas from which they originate

PROPERTIES OF ANODE RAYS They travel in straight line They are deflected by electric and magnetic field The nature of anode rays depends upon the nature of gas e/m ratio of anode rays is not constant and depends on the gas present in the discharge tube Neutron was discovered by chadwick by bombarding a thin sheet of beryllium by alpha particles when electrically neutral particles having a mass slightly greater than that of proton was emitted 9 4 12 1 Be + He → C n 4 2 6 0

particles Symbol Unit charge Unit mass Charge in Coulomb Mass Proton Neutron Electron Negligible

J.J Thomson plum pudding model Atom possess spherical shape in which positive charge is uniformly distributed Electrons are embedded into it in such a manner as to give the electrostatic arrangement It is not consistent with the results of new experiments Electron Positive sphere

Rutherfords nuclear model of an atom

Rutherford’s Experiment - Results A beam of  particles aimed at thin gold foil. Most of the particles passed through – most of the space in atom is empty A few came back – presence of concentrated mass at centre Others deflected at various angles – repulsions between two positively charged particles

RUTHERFORD ATOMIC MODEL : Atom consist of two parts: Nucleus: Almost the whole mass of the atom and positive charge is concentrated in this small region (b)Extra nuclear part:this is the space around the nucleus in which electrons are revolving at high speeds in fixed path © Electrons and nucleus are held together by electrostatic forces of attraction

Rutherfords drawbacks :

Rutherfords drawbacks According to electromagnetic theory of Maxwell ,charged particles when accelerated should emit radiations So an electron in an orbit emit radiation ,the energy carried by radiation comes from electronic motion So orbit will thus continue to shrink ,but this does not happen Rutherford model cannot explain the stability of an atom How electrons are distributed around the nucleus Atomic mass n atomic number