The Prophet (saws) called this verse: The collective comprehensive verse, when he was asked about the obligatory charity of the donkeys and he replied:
(Nothing has been revealed to me regarding this except this comprehensive verse which includes everything: “So whoever does good equal to the weight of an atom, shall see it; and whoever, does evil equal to the weight of an atom, shall see it.”)
A weight of anything is its heaviness, which is the amount of the earth gravity to it. When the weight of an object is low and it is thrown from high, it falls slowly, as for the heavy thing, it falls quickly because, the power of gravity is stronger. So, when we read the word (weight) we understand it as the heaviness. Therefore, the measure of weight here is the atom.
What is the atom? Ibn Abbas (May Allah be pleased with him) said:
“If you put your hand on the ground and then raise it, every piece of dust that stick on it is an atom”
It is that is called “dust dispersed”, so the atom is a dust or the dust dispersed. Thus, Allah (swt) does not oppress by the weight of an atom. This is only an “example”, it can be bigger or smaller. This does not mean that an atom is the lowest weight object on earth, since the scientists, through their researches, could break the atom, which involves in the synthesis of the substance, into positive and negative neutrons. This does not contradict the Holy Qur’an; Allah (swt) says:
﴾ لَا يَعْزُبُ عَنْهُ مِثْقَالُ ذَرَّةٍ فِي السَّمَاوَاتِ وَلَا فِي الْأَرْضِ وَلَا أَصْغَرُ مِن ذَٰلِكَ وَلَا أَكْبَرُ إِلَّا فِي كِتَابٍ مُّبِينٍ ٰ﴿
(Not absent from Him is an atom’s weight within the heavens or within the earth or [what is] smaller than that or greater, except that it is in a clear register -.) (Saba: 3)
Therefore, there is a thing that is smaller than an atom.
خَيْرًا يَرَهُ (of good will see it): The good is the opposite of evil. The man can recognize the good because it is useful and well, and on the other hand, he can recognize the evil because it is harmful and ugly. However, the word “good” is sometimes used not as an opposite of evil. For example, it is said that this matter is better than the second one, although the second is also good. Similar to what was narrated by Abo Horayra that the Prophet (saws) said:
(A strong believer is better and dearer to Allah than a weak one, and both are good.)
The strong believer and the weak believer are good, but the goodness has degrees. However, in this verse, (good) does not mean such that, rather, it means the type of the deed itself which can be described as being good, likewise, the type of the deed itself which can be described as being evil.
This is the end, which the man is satisfied with.
﴾وَنَضَعُ الْمَوَازِينَ الْقِسْطَ لِيَوْمِ الْقِيَامَةِ فَلَا تُظْلَمُ نَفْسٌ شَيْئًا ۖ وَإِن كَانَ مِثْقَالَ حَبَّةٍ مِّنْ خَرْدَلٍ أَتَيْنَا بِهَا ۗ وَكَفَىٰ بِنَا حَاسِبِينَ ٰ﴿
(And We place the scales of justice for the Day of Resurrection, so no soul will be treated unjustly at all. And if there is [even] the weight of a mustard seed, We will bring it forth. And sufficient are We as accountant.) (Al-Anbiyaa: 47)
The justice that Allah (swt) has established:
ٰ﴾وَالسَّمَاءَ رَفَعَهَا وَوَضَعَ الْمِيزَانَ﴿
(And the heaven He raised and imposed the balance.) (Ar-Rahman: 7)
Hence, everything is in a balance.
 Sahih Al-Bukhari, Book of Tafsir, hadith No. 4678.
 Sahih Muslim, Book of Destiny, hadith No. 2664.