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Tuesday, 29 September 2015

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 4

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 4



Discovery of Nucleus & Rutherford Atomic Model:

Discovery of Nucleus: Rutherford bombarded a thin foil with alpha particles, which were emitted from a radioactive element. He noticed that most of the rays went straight, through the foil. Some were deflected at various angles. But a few were deflected just backward. Thus he concluded that the region in which the rays were deflected back, must contain a heavy positive charge inside the atom. This part of the atom was named as “Nucleus". Rutherford performed this experiment several times and thus charge of atom is concentrated at its center which is called "Nucleus"

Rutherford Atomic Model:

On the basis of the above mentioned experiment, Rutherford presented a theory about the structure of atom, which is known as Rutherford atomic model.
The main points of this theory are.

  1. An atom consists of a nucleus containing positive charge and it has the whole of the mass of atom.The electrons present in the atom revolve around the nucleus continuously.
  2. The no. of protons of atom is empty because many of the no. of electrons in an atom.
  3. Most of the portion of atom is empty between many of the alpha particles passed without any deflection.
  4. The electrons are in constant motion around the nucleus (like that planets around the sun) in such way that the centripetal force, required to keep the electron moving in a circle is provided by the electrostatic force of attraction between the electrons and the nucleus.

Defects: 

Rutherford law which based on the gravitation and laws of motion has the following drawback or deflects.

  1. According to Maxwell theory, any electron revolving in an orbit must radiate energy continuously and as a result the electron must revolve in a spiral way around the nucleus and ultimately must fell into the nucleus. But this doesn’t happen.
  2. If the electron revolving around the nucleus emits energy in the form of light, then its spectrum should be "continuously spectrum" but spectrum of an atom is always a  line Spectrum"

Light and Characteristics of Waves:

Light


  1. Light is a wave motion.
  2. A wave has the following characteristics.
  3. Light is a form of energy. Thus we can say that light possesses energy. This energy is directly proportional to the frequency of the waves produced by this light.

Some important parameters related to wave motion of light are as follow.

1. Wavelength:
The distance between two successive crests or trough of a wave is known as wavelength.
Its units are m, cm, nm, etc.
2. Frequency:

The no. of waves passing through a point per second is known as frequency. Its units are cycle / sec or waves /sec or Hertz (Hz)
3. Wave Number:
The no. of waves per unit distance (in cm ), is called wave number. Its unit is per centimeter ( cm -1).

Plank's Quantum Theory:

To explain the emission or absorption of radiations, Max Plank presented a theory in 1900. This theory is known as Plank's quantum theory.
The main points of this theory are as follow.

  1. The emitted or absorbed energy is not continuous. It consists of tiny discrete particles which are known as “quanta” (singular ---- quantum). A quantum is also named as a wave packet. Each wave packet or quantum has a particular amount of energy. In case of “light “(energy), the quantum is often called as “Photon".
  2. The energy if a quantum is directly proportional to the frequency of radiation.
  3. The amount "hv" is a particular amount of energy which is called a quantum of energy or a photon of energy.

Bohr's Model of Atom:

Bohr made an extensive use of quantum theory of plank and proposed that the electron, in hydrogen atom, can only exist in certain permitted quantized energy levels. In 1913, he presented a model about the structure of atom. The main postulates of Bohr's atomic model are.

  1. Electron revolves around the nucleus in fixed circular paths known as shell. Each shell has a fixed energy and a quantum number is assigned to it.
  2. As far as electron revolves in its own shell, it neither emits nor absorbs energy. However when electron jumps from lower energy shell to a high energy level (shell), it absorbs energy and when comes back, then it emits energy.
  3. The energy lost or gained by electron, when it jumps from high to low or low to high energy level, is given by plank's equation.

i.e. E = E2 — E1= hv.

Here E1 is lower energy level (closer to nucleus) and E2 is higher energy level ( away from the nucleus)

The angular momentum of electron (mvr) is the integral multiple of " h/2 ?

i.e. mvr= nh/2(pie)

Here m = mass of electron

v = velocity of electron

r = radius

N= integer (no. of shell) 1, 2, 3…………

H= Plank's constant (6062x10(-34) Js)

(pie) = 3.14.

The electron is bound to remain in one of the shell and can never stay between the shells.

Derivation of Radius of Electron in nth Orbit:

By using the ideas mentioned in “Bohr’s Atomic Model," he derived the expression for the radius of the nth orbit in hydrogen atom. For a General atom, consider an electron of charge "e" and mass "n" revolving around the nucleus having charge “Ze". Here Z is the no. of protons and "e" is the charge on proton. Let the electron revolves around the nucleus at a distance. Then according to coulomb's law, the electrostatic force of attraction between electron and the nucleus will be given as: Here eq is the vacuum permittivity and its value is 8.84x10(-12) C (2)/JM

Electromagnetic Radiations:

The radiation which are neither affected by electric nor by magnetic fields, are known as electromagnetic radiations

Spectrum:

When white light is passed through a prism, it splits into radiations of different wavelength which appear in the form of a bond. This band is known as spectrum. As light is electromagnetic in nature (e.g sum light), thus the spectrum may also be named as electromagnetic spectrum.
Electromagnetic spectrum consists of different radiations with particular wavelength range. The various radiations present in electromagnetic spectrum are. Cosmic rays, Gamma rays , X—rays, ultra violet rays, visible rays, infrared rays, microwaves & Radio waves.
Electromagnetic spectrum can be shown diagrammatically as:

  1. Cosmic rays
  2. Gamma rays
  3. Ultra violet rays
  4. Visible Infrared rays
  5. Microwaves Radio—waves

Electromagnetic spectrum consists of two portions. i.e. visible and invisible. The region of electromagnetic spectrum, below 400nm and above 750nm is known as invisible spectrum because the radiations in these ranges have no color. While the region of E.M.Spectrum from 400nm to 750mn, is known as visible spectrum because this region consists of radiations which have seven different colors.

Types:

There are two types of spectrum i.e.

  1. Continuous spectrum
  2. Line or Atomic spectrum.


1. Continuous spectrum:
The spectrum in which there is no clear boundary between any two colors is known as continuous spectrum. In this type of spectrum, the colors get diffused into each other at the point of their separation, therefore the boundary between the colors cannot be marked. Rainbow is an example of continuous spectrum.

2. Line or Atomic spectrum:
The spectrum in which, there is a clear boundary between any two colors, is known as line spectrum. For example when an element or compound is volatilized on a flame, due to heat absorptions, the electrons of their level ( ground state ) to high energy level ( excited state ) and when these electrons come back to the lower energy level, they emit the absorbed energy in the form of light. When this light is passed through a prism, we get a spectrum in which is clear boundaries between the colors. Therefore, line spectrum is also known as atomic spectrum. A particular element produces a spectrum of particular color and wavelength. Thus we can say that spectrum is the characteristic of an atom.

Types of Line or Atomic Spectrum:

There are two types of lines or Atomic spectrum which are :
1. Atomic Absorption Spectrum:
The atomic spectrum which consists of bright spaces separated by are lines is known as atomic or line absorption spectrum. Such a spectrum is formed when white light is passed through the cool vapours of an element. The element absorbs particular wavelengths due to the absence of which the dark lines appear in the spectrum.
2. Atomic Emission spectrum:
The atomic spectrum which consists of dark spaces separated by bright lines is known as atomic emission spectrum. Such a spectrum is formed when a substance is heated in a discharge tube and the emitted light is passed through the prism (or spectroscope). The bright lines appear due to the emitted wavelengths.

Written by: Asad Hussain

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