Robert Millikan was an American physicist who made significant contributions to the field of atomic theory. He is best known for his famous oil-drop experiment, which helped to determine the charge of an electron and led to a more accurate understanding of atomic structure. In this article, we will examine Millikan’s contribution to the atomic theory in detail.

The Oil-Drop Experiment

In 1909, Millikan conducted a series of experiments that involved suspending tiny oil droplets in an electric field. By measuring the rate at which these droplets moved, he was able to determine the charge on each individual droplet. This allowed him to calculate the charge of an electron accurately.

The oil-drop experiment was significant because it provided a way to measure electrical charges at the atomic level accurately. It also confirmed that electrical charges are quantized – meaning that they come in discrete units rather than being continuous.

The Charge of an Electron

Millikan’s experiment provided a more precise value for the charge of an electron than had previously been known. Prior to his work, scientists had estimated the charge based on measurements taken during electrolysis experiments. However, these values were not very accurate and varied widely.

Millikan’s experiment gave a value for the charge of an electron that was within 1% of what is currently accepted as correct – 1.602 x 10^-19 coulombs. This allowed scientists to make more accurate predictions about how atoms interact with each other and helped to lay the groundwork for modern atomic theory.

Contributions to Atomic Theory

Millikan’s work on atomic theory didn’t stop with determining the charge of an electron. He also made significant contributions to our understanding of how atoms are structured.

One important concept that Millikan helped develop was the idea that electrons exist in discrete energy levels within an atom. These energy levels determine how electrons interact with each other and with other atoms.

Millikan’s work also helped to solidify the idea that atoms are made up of a nucleus containing protons and neutrons, surrounded by electrons in various energy levels. This model of atomic structure is still the basis for our understanding of the atom today.


In conclusion, Robert Millikan’s oil-drop experiment was a groundbreaking achievement in the field of atomic theory. By accurately determining the charge on an electron, he helped to pave the way for modern atomic theory and provided a more precise picture of how atoms are structured. His work serves as a reminder of how scientific advancements can change our understanding of the world around us.