Q. Determine mass of an electron?

Ans. Three years later after J.J Thomson discovered the electron, the American scientist Robert Millikan used an oil drop experiment to accurately measure the charge on an electron. Oil drops were sprayed into a chamber that was exposed to an electric field. As the oil drops entered the chamber, they randomly developed charges by either gaining or loosing electrons.

The electric field in the chamber was calibrated so that an drop that was negatively charged would move slowly upward in front of the telescope. Since he knew the rate at which this negatively-charged drops were rising, the strength of the electrical field, and also the weight of the drops, Millikan had enough information to calculate the charge of these drops. Looking at his data, Millikan realized that the charges of the oil drops were always a whole-number multiple of a smaller charge having a value of 1.60 x 10-19 coulombs. Millikan discovered that this charge was the charge of a single electron.

Combining this charge value for an electron with the mass-to-charge ratio of the electron discovered by Thomson, Millikan was able to calculate the mass of a single electron:

mass of an electron = charge of an electron x mass of an electron/charge of an electron

1.60 x 10-19 coulombs x 5.69 x 10^-9 grams/ coulomb

= 9.11 x 10-28 grams

This mass is about 1/2000 of the mass of the lightest atom--the hydrogen atom.

Every atom has a characteristic number of electrons. For example, all hydrogen atoms have one electron, and all oxygen atoms have eight electrons. As you will see throughout your study of chemistry, the chemical behavior of all atoms, ions, compounds, and molecules can be traced back to their electron composition.