Lesson 6, Topic 2
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Various Models of the Atoms

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1. Thompson’s Atomic Model

Thompson proposed that an atom consists of homologous sphere of positive charges which are embedded in negatively charged electrons

2. Rutherford Model of Atom

Rutherford proposed a planetary model of the atom with the following assumptions:

• Protons with positive charges are concentrated in a small nucleus
• The nucleus is centrally located in an atom
• The electrons are located at the peripheral of the nucleus and they render the atom electrically neutral
• Electrons revolve round the nucleus of an atom
• There is a wide gap between nucleus of an atom and its orbiting nucleus

Limitation of Rutherford Atomic Model

(i) It predicts that light of a continuous range of frequencies will emitted whereas experiment reveals line spectra instead of continuous spectra

(ii) He stated that atoms are unstable- electrons quickly spiral into the nucleus, but generally atoms are stable

3. Bohr’s Postulate of  Atomic Model

Bohr suggested model of atoms that

(i) Electrons occupy fixed orbit around the nucleus of an atom, when an electron is in  one of these orbits, it does not radiate energy

(ii) When an electron jumps from a higher energy level to a lower energy level, it emits an indefinite amount of radiation (quanta)

(iii) An electron absorbs radiation when it jumps from a lower energy level to a higher level

(iv) The energy emitted is given by hf = Eu =Ec where Eu = energy at upper level,   h = Planck’s constant, f = frequency of emitted light or radiation, EL = energy at lower level

Bohr’s model was a success in

(i) Calculating the radius of allowed orbit and total energy of the electron in each allowed orbit of hydrogen atom

(ii) Predicting the spectral line of glowing hydrogen atom

(iii) Explaining the quantized energy state of photon in photoelectric effect

Failure or Limitations of Bohr’s Atom

(i)It could not explain bright spectral lines of complex atoms like oxygen

(ii) It views electrons as particles moving round the nucleus at definite orbit

(iii) Experiments have shown that electrons can behave like waves or particles

4. Electron Cloud Model

An atom consists of positively charged nucleus surrounded by electrons in a cloud of different energy level. The location of electrons depends on how much energy electron possesses.

Electrons with the lowest energy are found in the energy level close to the nucleus while those with the highest energy level are found on the outermost energy levels, farther from the nucleus.

The electrons change from one energy level to another level depending whether they gain or lose energy

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