MCQ Detailed View

An atom consists of a central nucleus surrounded by electrons. The nucleus contains two types of subatomic particles: protons and neutrons. Both of these particles are heavy compared to electrons. The mass of a proton is approximately 1.67×10−271.67 \times 10^{-27}1.67×10−27 kg, and the mass of a neutron is nearly the same. Electrons are much lighter, with a mass about 1/1836 that of a proton. Because electrons have negligible mass, they contribute very little to the overall atomic mass.

The nucleus is extremely small compared to the size of the entire atom but contains nearly all of its mass. In fact, over 99.9% of an atom's total mass is concentrated in the nucleus. The nucleus has a high density due to the tightly packed protons and neutrons bound together by the strong nuclear force.

Atomic shells, orbits, and energy levels describe the regions where electrons move around the nucleus. These regions define the size and chemical properties of an atom but not its mass. Since electrons are extremely light, their contribution to atomic mass is almost negligible compared to the nucleus.

The understanding of mass distribution in an atom was first established by Ernest Rutherford's gold foil experiment. It showed that atoms have a dense central nucleus containing almost all the mass, surrounded by mostly empty space where electrons orbit.

The nucleus is responsible for the atomic number (due to protons) and atomic mass (due to both protons and neutrons). Any change in the number of neutrons or protons alters the mass significantly, while adding or removing electrons barely changes the total atomic mass.

Therefore, the nucleus is the region where nearly the entire mass of an atom is concentrated, while shells, energy levels, and orbits only determine its volume and electron arrangement.