Atomic Structure PDF

Atomic Structure Atomic Structure 1.  Atom y Each element is composed of smallest Definition particles known as ‘ATOM’. y Atom the name is derived from Greek word...

Atomic Structure Atomic Structure 1.  Atom y Each element is composed of smallest Definition particles known as ‘ATOM’. y Atom the name is derived from Greek word Each element is composed of Atom means - ‘Not to be cut’. smallest particles called ‘ATOM’. Concept Ladder Democritus was the person who first suggested the existence of ATOM & coined the name ATOMOS means Not to be cut or Indivisible. DALTON’S THEORY OF ATOM J. Dalton developed his famous theory of atom in 1803. The main postulates of Rack your Brain y Atom was considered as a samllest, dense and hard indivisible particle. y Each element consists of a specific type of Why and how do atoms combine atoms. together to form compound y The properties of elements differ because of atoms? difference in kinds of atom contained in them. y This theory provides a satisfactory basis for law of chemical combinaiton. Limitations of Dalton’s Theory y Dalton’s theory fails to explain why atoms of Concept Ladder different kinds should differ in valency and mass etc. y The discovery of isobars and isotopes showed Atoms is indestructible, that atoms of same elements have different i.e., it cannot be created or destroyed. Atomic Structure atomic masses (isotopes) and atoms of different kinds may have same atomic masses (isobars). 2. y The discovery of various sub-aomic particles like protons, electrons, X-rays etc. during late Concept Ladder 19th century lead to idea that the atom was no longer an indivisible and smallest particle of the matter. R.S. Mullikan measured the charge on an electron by DISCOVERY OF FUNDAMENTAL PARTICLES oil drop experiment. The y Atom consist of several sub-atomic particles charge on each electron is like neutron, proton, electron, neutrino, –1.602 × 10–19 C. positron etc. Out of these particles, electron, proton and the neutron are called fundamental subatomic particles. Rack your Brain (1) Electron y Electron discovered by J.J. Thomson (1897) and it is negatively charged particle. Why, cathode rays do not depend Cathode Ray Experiment upon the nature of gas or the y William Crookes in 1879 studied the electrical cathode meterial used in discharge discharge in partially evacuated tubes called tube? as cathode ray discharge tube. y Discharge tube is made of glass, about 60 cm long containing two thin pieces of metals known as electrodes, sealed in it. This is Atomic Structure called crooke’s tube. Negative electrode is called as cathode and positive electrode is called anode. 3.  y When a gas enclosed at low pressure (~10–4 atm) in discharge tube is subjected to a high Rack your Brain voltage (~10,000 V), invisible rays originating from the cathode and producing a greenish Why do cathode rays produce glow behind the perforated anode on the glass fluorescence? wall coated with phosphorescent material ZnS is observed. These rays Properties y Cathode rays travel in straight line. Previous Year’s Question y Cathode rays produce mechanical effect, as they can rotate the wheel that placed in their Cathode rays have path. [AIPMT] y Cathode rays consist negatively charged (1) Mass only particles called as electron. (2) Charge only y Cathode rays travel with high speed. (3) No mass and charge y Cathode rays can cause fluorescence. (4) Mass and charge both y Cathoderays heat the object on which they fall due to transfer of kinetic energy to the object. Concept Ladder y When cathode rays fall on heavy metals, X-rays produced. y Cathode rays possess ionizing power that is Order of specific charge they ionize the gas through which they pass. e e e   1 is called excited state. Š For the nth shell = (n - 1) the excited state Š Excitation energy: Energy required to excite an electron from its ground state to any excited state is called excitation energy. Š Wave Mechanical Model of An Atom: WAVE MECHANICAL MODEL OFAN ATOM: The Dual Nature of Matter (The Wave Nature of Electron) De-Broglie Equation (Dual nature of matter and radiation): h h   mc p h h   mv 2m(K.E.) If a charged particle Q is accelerated through potential difference V from rest then De-broglie wavelength is Atomic Structure h  2mQV 62. Š The circumference of the nth orbit is equal to n times the wavelength of the electron. 2prn = nl. Š Heisenberg’s Uncertainity Principle: It is impossible to obtain simultaneously both position and velocity (or momentum) of a microscopic particle with absolute accuracy. h h h x.p  or m x.v  or x.v  4 4 4m Š Quantum Mechanical Model: The Schrodinger Equation:  2   2   2  82m 82m    or 2   (E  V )   0 x2 y 2 z2 h2 h2 Š Quantum Numbers: (i) Principal quantum number (n): Number of orbitals present in nth shell = n2. The maximum number of electrons which can be present in a principal energy shell is equal to 2n2. (ii) Azimuthal quantum number (l): Number of orbitals in a given subshell = 2l + l Maximum number of electrons in particular subshell = 2 × (2l + l) Orbital angular momentum L  h (  1)   (  1) 2 (iii) Magnetic quantum number (m): It describes the orientations of the subshells. It can have values from –l to +l including zero, i.e., total (2l + l) values. (iv) Spin quantum number (s): It describes the spin of the electron. It has values +1/2 and –1/2. (+) signifies clockwise spinning and (–) signifies anticlockwise spinning. Š Shape of The Orbitals: Nodal plane and Nodal surface :- The space where probability of finding an e– is zero. Nodal plane = l ; Nodal surface = n - l – 1 Š Stability of Completely Filled and Half-filled Subshells: Symmetrical distribution of electrons: Due to small shielding, the electrons are pulled closer to the nucleus and this decrease in energy leads to stability. Exchange energy: Electrons with the same spin have a tendency to exchange their positions when they are present in the degenerate orbitals of a subshell. The energy released during this exchange is called exchange energy Atomic Structure 63.