Structure of atom (part 2)

BOHR'S MODEL            OF ATOM

 The Bohr's model of the atom was proposed by Neil Bohr in 1915.

It came into existence with the modification of Rutherford's model of atom .

And it is called morden model of the atom it is based on quantum mechanics.

The Bohr's model contains some errors ,but it is important because it describes most of the accepted features of atomic theory without all of the high level math of the morden version .

The Bohr  explained that a nucleus (positively charged) is sorrunded by (negatively charged) electrons .

Main points of the Bohr's model :- 

1. Electron orbit the nucleus in orbits that have a set size and energy .

2.  The energy of the orbit is related to its size the lowest energy is found in the smallest orbit .

3.  Radiation is absorbed and emitted when electron moves from one orbit to another .

 4. The Bohr model is a planetary model  (simillar to the planets orbiting the sun ).  

Two developments played a major role in the formulation of Bohr's model of atom . These were :-

(1). Dual character of the electromagnetic radiation which means that particle like properties, 

(2). Experimental results regarding atomic spectra .

Hydrogen's Bohr Model : - the Bohr model of hydrogen is the only one that accurately predicts all the electron energies.

The Neil Bohr atomic model is utilized to explain the working and structural formation of a hydrogen atom and its energy levels. 

Each orbital shell indicates a quantum number 'n' associated with corresponding energy levels. 'eV' represents the unit amount of each energy level. The maximum energy level an orbital shell can have is the ionization energy of 13.598 eV. 

 1. Bohr model of the hydrogen atom first proposed the planetary model but later an assumption concerning the electron was made .

2.  The total energy revolving in orbit is obtained by summing up its kinetic and potential energy.

3.  when electrons move between the different energy levels in a hydrogen atom, a spectrum is produced. This means that during the transition of electrons to different energy levels, the transmission of light appears.  

 4.There are four different wavelengths present in a visible spectrum of light in a hydrogen atom. These refer to the excited state of electrons that transition down to a minimum of n=2 and are described by the Balmer equation.

Equations Included In The Bohr Model of Hydrogen Atom :-

  The difference between the two energy levels 

  (assuming E1, E2) and Planck's constant corresponds to the energy absorbed or emitted. 

  ΔE = E2-E1 = hv 

 Where, ∆E is the energy that is emitted or absorbed .  

   'h'  is Planck's constant 

    'v' is the frequency of electromagnetic radiation.

Rydberg Formula

The rydberg formula explain the different energies of transitions that occur between energy level. when an electron move from a higher energy level to our lower one a photon is emitted . the hydrogen atom can I made different wavelength of light depending on the initial and final energy level of transition it emits of photon with energy equal to the difference of square to final(np) and initial (ni) energy level . 

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1. Bohr's atom model failed to explain the atomic spectrum of atoms other than hydrogen atom.

2. Bohr's model could not give better explanation of hydrogen spectrum because it not explain the spectrum because  two spectral lines are they very close to each other.   

3.  It fail to explain  zeman effect that is splitting of spectral line under the influence of magnetic field.

4.  It could not explain the ability of atoms to form molecule by chemical

bonds. 

5.   Bohr's model of an atom could not explain the line spectra of atom containing more than one electron called multi electron atoms.

Particle nature of electromagnetic radiation :-  Planck's Quantum theory

When a black body is heated, it emits thermal radiations of different wavelengths or frequency. To explain these radiations, Max Planck put forward a theory known as Planck’s quantum theory. The main points of quantum theory are 

Planck’s quantum theory explains the emission and absorption of radiation. 

Black body radiation :-

A black body is a body or object which absorbs all the radiation falling on it. An ideal black body absorbs and emits radiations of all frequencies. The emission of electromagnetic radiation from a black body depends on temperature. The frequency variation of emitted radiation from a black body can be described by Planck’s law. This type of radiation is also known as Planck’s radiation which is a type of thermal radiation. Varying with temperature, the higher the temperature of the body, the more is the emission of radiation of all wavelengths. 

Postulates of Planck's Quantum theory are as follows  :–

1. Matter radiates energy or absorbs energy in discrete quantities, discontinuously in the form of small packets or bundles. 

2.   The smallest bundle or packet of energy is known as quantum. In the case of light, a quantum of light is known as a photon. 

3.   The energy of the quantum     absorbed or emitted is directly   proportional to the frequency of the    radiation. So, the energy of the     radiation is expressed in terms of   frequency as follows -

 A body or matter can radiate energy   

or absorb energy in whole-number      multiples of a quantum as n=hv.       Where n is a positive integer. So,     energy can be absorbed or radiated   as hv, 2hv, 3hv, 4hv……etc. not in the   form of 1.5hv, 2.5hv…etc.  

   Photoelectric effect :-

In 1887,H. Hertz preformed a very interesting experiment.

When light shines on a metal, electrons can be ejected from 

  the  surface of the metal in a         phenomenon known as the  photoelectric effect. 

This process is also often referred to as photoemission, and the electrons that are ejected from the metal are called photoelectrons. In terms of their behavior and their properties, photoelectrons are no different from other electrons. The prefix, photo-, simply tells us that the electrons have been ejected from a metal surface by incident light.

  Wave nature of electromagnetic radiation  -

The electromagnetic radiation James Maxwell was the first scientist to explain the interaction of charges in the attandance of magnetic and electric fields in the the 1870 .

The electromagnetic radiation is a flow of energy in which electrical and magnetic fields vary simulataneously

 Radiation waves , microwave, infrared light ,visible light , ultraviolet light, x-rays and gamma rays are all electromagnetic radiation.

• when a charge put in a magnetic or a electric field , it encounters a force acting on it , if multiple charges are placed .they experience interaction due to one another. 

• the  electrically charged particles performs an accelerating motion alternating electrical and magnetic fields are produced and transmitted.

These fields transverse in the forms of waves known as electromagnetic radiation .

Properties of EMR :-

1.  The oscillating charged particles generate oscillating magnetic and electric fields. 

2.   Electromagnetic radiation travel in space and vaccum through oscillating electrical and magnetic fields generated by their particles .

3.   Electromagnetic radiation interferes with electrical and magnetic field travel at light speeds   it  does not contains weight of charge but travels through a series of dynamic energy packet called photons  or quanta . 

4. The fields are perpendicular to one one another and also perpendicular to wave's direction of propogation .

5.  There are various types of electromagnetic radiation that differ in wavelength or  frequency. 

6.   The electromagnetic spectrum is made up of all of this electromagnetic radiation . 

7.   Various properties electro magnetic radiation, such as wavelength, frequency and time period are used to classify it. 

formula :-  

Dual behaviour of matter :- 

The French physicist de broglie in 1924 proposed that matter like radiation should also exhibit dual behaviour that is both particles and wave like properties.

He presented the concept of dual nature of matter which was based on the following observation.

this means that just as a photo has momentum as well as wavelength. electrons should also have momentum as well as wavelength i.e. probably from this analogy gave the following relation between wavelength (   )and momentum (p) of material particle.

He presented the concept of dual nature of matter which was based on the following observations.

De broglie's problems predictions was confirmed experimentally when it was found that an electron beam undergoes diffraction,  phenomena characteristics of waves .

this fact has been use to  in making and electron microscope.  Which is based on the wave like behaviour of electrons just as an ordinary microscope utilise the wave nature of light.  and electron microscope is a powerful tool in modern scientific research because it achieves a magnification of about 15 million times.

He presented the concept of dual nature of matter which was based on the following observations.

• The universe is composed of electromagnetic radiation and matter. Both of them are forms of energy and they can be transformed into each other.
• The matter has symmetry. Just like the dual nature of radiation, the matter should also have dual nature.
• Through the work of Niels Bohr, Albert Einstein, Arthur Compton, Max Planck, and Louis de Broglie and many other scientists, the current scientific theory is holding the fact that all of the particles exhibit the wave nature and the vice versa. The properties of light and matter show that both the light and matter have a dual nature of wave-particle. While one thing should be kept in mind that the properties of wave and particle are elementary .