The mass of an atom is concentrated in its center, specifically in the nucleus, due to the presence of protons and neutrons.
The nucleus is the central part of an atom and contains most of its mass. Protons, which have a positive charge, and neutrons, which have no charge, are located within the nucleus. These subatomic particles are much more massive than electrons, which orbit around the nucleus in electron shells.
The reason the mass is concentrated in the nucleus is primarily due to the strong nuclear force. The strong nuclear force is a fundamental force that acts between protons and neutrons within the nucleus. This force is responsible for binding the nucleus together despite the repulsive electromagnetic force between positively charged protons.
The strong nuclear force is extremely powerful but operates only at very short distances, typically confined to the size of an atomic nucleus. It overcomes the electromagnetic repulsion between protons and holds the nucleus together, creating a compact, densely packed structure.
In contrast, electrons are much lighter and occupy relatively large regions of space in electron shells surrounding the nucleus. Their mass is negligible compared to the mass of the nucleus, so the majority of the atom's mass is located in the tiny volume occupied by the nucleus.
Therefore, due to the concentration of protons and neutrons in the nucleus and the strong nuclear force that binds them together, the mass of an atom is concentrated in its center.