The atomic mass of an element is greater than its proton number due to the presence of neutrons in the atomic nucleus.
An atom consists of a positively charged nucleus at its center, which contains protons and neutrons, surrounded by negatively charged electrons. The number of protons in the nucleus determines the atomic number of the element and defines its identity. For example, hydrogen has an atomic number of 1 because it has one proton, while helium has an atomic number of 2 because it has two protons.
On the other hand, the atomic mass of an element takes into account the total mass of its protons, neutrons, and electrons. While protons and neutrons have similar masses (approximately 1 atomic mass unit each), electrons have a much smaller mass (about 1/1836th of a proton or neutron).
Since the atomic mass of an element considers the combined mass of protons, neutrons, and electrons, and the mass of electrons is negligible compared to protons and neutrons, the atomic mass is usually greater than the proton number. The difference between the atomic mass and the proton number is primarily due to the presence of neutrons. Neutrons add to the mass of the nucleus without significantly affecting the atomic number or the element's chemical properties.
The number of neutrons in an atom can vary, resulting in different isotopes of an element with the same atomic number but different atomic masses. Isotopes have the same number of protons but different numbers of neutrons, leading to variations in atomic mass.