Answer :
Label A - nucleus.
Label B- electron cloud.
Explanation :
An atom is the smallest unit of a matter that consist of three subatomic particles which are electrons, protons and neutrons.
The protons and the neutrons are located inside the nucleus or the center of the nucleus where the mass of the an atom is concentrated.
The electrons are located around the nucleus.
The protons are positively charged, the electrons are negatively charged and the neutrons are neutral that means it has no charge.
In the picture, label A shows the nucleus.
Electrons has the ability to revolve around the nucleus in a fixed circular orbit. Electron cloud represents the spreading of the electrons energy.
In the picture, label B shows the electron cloud.
Hence, the label A - nucleus and label B- electron cloud.
