Al-Zalzalah (The Earthquake)
Chapter 99: Verses 7-8

Atom's Weight of Deeds
"Then shall anyone who has done an atom's weight of good, see it! And anyone who has done an atom's weight of evil, shall see it."

An atom's weight

The early commentators on the Qur'an explain this phrase as "a mosquito" or "a particle of dust" which could be seen only when exposed to the light of the sun. These were the smallest things they could think of, and which may be referred to as an "atom". But now we know that the word "atom" refers to a definite thing which is much smaller than that "particle of dust" seen in the sunlight. For the particle of dust can be seen by the human eye while it is impossible to see the atom, even with the help of the most powerful microscopes in laboratories. It is only "conceived" by the scientists. None of them has seen it either with his eyes or with his microscope. All that they have seen is its effects.

The Precious Deeds

This atom, or what is similar to it in weight, whether good or bad, will be brought forth and shown to its doer, who will then receive its reward or punishment. At that time man does not undervalue any of his actions and deeds, whether good or bad. He does not say, "Oh, this is a trivial thing which has no weight or consideration". On the contrary, his conscience will be as sensitive to everything he has done as an accurate scale registering even the weight of an atom favourably or unfavourably.

The Believing Heart

There is nothing parallel or similar to this measure in this world, except the believing heart. For the believing heart is sensitive to even an atom's weight of either good or evil. But there are some hearts in this world which are unmoved even by mountains of sins and crimes. They remain unaffected while suppressing fountains of good which are far firmer than the mountains. These hearts are conceited in this earth but on the Day of Judgement they are crushed under their own burdens.

Source:
"In The Shade of the Quran" - Syed Qutb, Juz 30