What is meant by the atom, did the Arabs know it in the time of the Prophet, what does the word mean in His (the Almighty's) saying: "smaller than the atom," and will deeds smaller than the atom not be accounted for?

The Holy Quran must be understood according to the Arabic language in which it was revealed, not according to modern terminology and concepts. For whoever does not know the language of the Companions and their customs of speech may err in understanding the intent of Allah. An example of this is the word "dharr" (ذرة) in the Quran; it should not be understood as the atom in its modern physical sense, but rather as the smallest thing that can be seen with the naked eye at the time of revelation, such as what the Arabs used to see. This is to warn that a person will be held accountable for everything, no matter how small, as in His (Allah's) saying: ﴿وَمَا يَعْزُبُ عَنْ رَبِّكَ مِنْ مِثْقَالِ ذَرَّةٍ فِي الْأَرْضِ وَلَا فِي السَّمَاءِ وَلَا أَصْغَرَ مِنْ ذَلِكَ وَلَا أَكْبَرَ إِلَّا فِي كِتَابٍ مُبِينٍ﴾ [Yunus: 61], which indicates the vastness of Allah's knowledge, and not to specify the names of small things.