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Structure of atom

STRUCTURE OF ATOM

Thomson’s Model of the Atom

Rutherford’s Model

Bohr’s Model

 

Thomson’s Model of the Atom:

  • When J. J. Thomson proposed his model of the atom in 1903, then only electrons and protons were known to be present in the atom.
  • Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons, in a sphere of positive charge, were like dry fruits in a spherical Christmas pudding.

 

According to Thomson’s model of the atom:

  • An atom consists of a sphere (or ball) of positive charge with negatively charged electrons embedded in it.
  • The total negative charge of electrons is equal to the total positive charge of the sphere.
  • An atom is electrically neutral.

  • The shaded area in the sphere contains all the positive charge in the atom.
  • The negatively charged electrons are spread throughout the positive charge.

 

Rutherford’s Model of an Atom:

  • Rutherford’s alpha particle scattering experiment led to the discovery of a small positively charged nucleus in the atom containing all the protons and neutrons.
  • When fast moving alpha particles are allowed to strike a very thin gold foil in vacuum, it is found that:
  1. Most of the alpha particles passed straight through the gold foil without any deflection from their original path.
  2. A few alpha particles were deflected through small angles and a few were deflected through large angles.
  3. A very few alpha particles completely rebound on hitting the gold foil and turn back on their path (just as a ball rebounds on hitting a hard wall).

 

Rutherford’s explanation of observations:

  • Gold foil is made up of atoms.
  • If the atoms were solid throughout their volume, then every alpha particle striking them should have charged its path and got deflected.

  • Since most of the alpha particles passed straight through the gold foil without any deflection, it shows that there is a lot of empty space in the atom.
  • The observations that the alpha particle are deflected through small and large angles shows that there is a “centre of positive charge” in the atom which repels the positively charged alpha particles and deflects them from their original path.
  • A very few of the alpha particles are turned back on their path.
  • This fact cannot be explained only on the basis of repulsion due to positive charge of the nucleus.
  • It can, however, be explained by assuming that the nucleus is very dense and hard.
  • So, the observation that a very few alpha particles completely rebound on hitting the gold foil shows that the nucleus is very dense and hard which does not allow the alpha particles to pass through it.

 

Rutherford’s alpha-particle scattering experiment shows the presence of a nucleus in the atom.

  1. Nucleus of an atom is positively charged.
  2. Nucleus of an atom is very dense and hard.
  3. Nucleus of an atom is very small compared to the size of the atom as a whole.

 

Rutherford’s Model of the Atom:

  1. An atom consists of a positively charged, dense and very small nucleus containing all the protons and neutrons (protons have positive charge whereas neutrons have no charge). Almost the entire mass of an atom is concentrated in the nucleus.
  2. The nucleus is surrounded by negatively charged electrons. The electrons are revolving round the nucleus in circular paths at very high speeds. The circular paths of the electrons are called orbits.

  1. The electrostatic force of attraction between the positively charged nucleus and negatively charged electrons holds the atom together.
  2. An atom is electrically neutral. This is because the number of protons and electrons in an atom is equal.
  3. Most of the atom is empty space.

 

Drawback of Rutherford’s Model of the Atom:

  • A major drawback (or defect) of Rutherford’s model of the atom is that it does not explain the stability of the atom.
  • The negatively charged electrons are revolving around the positively charged nucleus is circular paths.
  • Any particle in circular orbit would undergo acceleration.
  • According to the electromagnetic theory of physics, if a charged particle undergoes accelerated motion, it will radiate energy (or lose energy) continuously.
  • Revolving electrons will lose their energy continuously in the form of radiation.
  • Thus, the energy of revolving electrons will decrease gradually and their speed will also go on decreasing.
  • The electrons will then be attracted more strongly by the oppositely charged nucleus due to which they will come more and more close to the nucleus.
  • And ultimately the electrons should fall the nucleus by taking a spiral path.
  • This should make the atom very unstable and hence the atom should collapse.
  • Electrons do not fall into the nucleus of an atom. Atoms are very stable and do not collapse on their own.
  • The Rutherford’s model, however, does not explain the stability of an atom.

 

Bohr’s Model of the Atom:

  1. An atom is made up of three particles: electrons, protons and neutrons. Electrons have negative charge, protons have positive charge whereas neutrons have no charge, and they are neutral. Due to the presence of equal number of negative electrons and positive protons, the atom on the whole is electrically neutral.
  2. The protons and neutrons are located in a small nucleus at the centre of the atom. Due to the presence of protons, nucleus is positively charged.
  3. The electrons revolve rapidly round the nucleus in fixed circular paths called energy levels or shells. The energy levels or shells are represented in two ways: either by the numbers 1, 2, 3, 4, 5 and 6 or by the letters K, L, M, N, O and P. The energy levels are counted from the centre to outwards.

  1. The electrons could revolve around the nucleus in only “certain orbits” (or “certain energy levels”), each orbit having a different radius. The electrons in each orbit have a characteristic amount of energy. The electrons which are in orbits close to the nucleus have low energy while those in orbits farther from the nucleus have higher energy.
  2. When an electron is revolving in a particular orbit or particular energy level around the nucleus, the electron does not radiate energy (does not lose energy), even though it has accelerated motion around the nucleus. And since the electrons do not lose energy while revolving in certain permitted orbits, they do not fall into the nucleus and hence the atom remains stable.
  3. The change in the energy of an electron takes place only when it jumps from a lower energy level to a higher energy level or when it comes down from a higher energy level to a lower energy level. When an electron gains energy, it jumps from a lower energy level to a higher energy level, and when an electron comes down from a higher energy level to a lower energy level, it loses energy in the form of radiation.

  1. There is a limit to the number of electrons which each energy level (or shell) can hold. For example, the first energy level (or K shell) can hold a maximum of 2 electrons; second energy level (or L shell) can hold a maximum of 8 electrons; third energy level (or M shell) can hold a maximum of 18 electrons and fourth energy level (or N shell) can hold a maximum of 32 electrons.