Structure of Atom

• In 1897, J. J. Thomson discovered a negatively charged particle called electron which he assumed to be 2000 times lighter than a proton.
• In his atomic structure model, he considered atoms to have a cloud of negative charge and the positive charges.
• Along with Rutherford he was also the first to demonstrate the ionization of air by X-rays.
• Thomson’s model of an atom is similar to plum pudding model or a watermelon.
• He compared the red edible part of the watermelon to positively charged sphere whereas the seeds of watermelon to negatively charged particles.

Postulates of Thomson’s Atomic Model:

He proposed that

• An atom consists of a positively charged sphere or ball with negatively charged electrons embedded in it.
• The positive and negative charges in an atom are equal in magnitude.
• So that the atom as a whole is electrically neutral. It has no overall positive or negative charge.

Limitations of Thomson’s Atomic Model:

The following are the drawbacks of Thomson’s atomic model:

• The model of atom failed to explain how a positive charge holds the negatively charged electrons in an atom. So, it failed to explain the stability of an atom.
• This theory also failed to account for the position of the nucleus in an atom.
• Thomson’s model failed to explain the scattering of alpha particles.
• Even though Thomson’s model was not an accurate to account for the atomic structure, it proved to be the base for the development of other atomic structure models.