The mass of an atom is the total mass of its constituent particles, which include protons, neutrons, and electrons. However, the mass of an atom is not a whole number due to several factors:
Protons and neutrons: Protons and neutrons have mass and contribute significantly to the total mass of an atom. The mass of a proton is approximately 1.0073 atomic mass units (u), while the mass of a neutron is slightly higher at approximately 1.0087 u. Since the atomic masses of protons and neutrons are not whole numbers, their presence affects the overall mass of the atom.
Electrons: Electrons are much lighter than protons and neutrons, with a mass of approximately 0.00055 u. In comparison to the masses of protons and neutrons, the mass of electrons is relatively negligible. Therefore, the presence or absence of electrons does not significantly affect the overall mass of the atom.
Isotopes: Most elements have multiple isotopes, which are atoms of the same element with different numbers of neutrons. Isotopes have different atomic masses due to variations in the number of neutrons. Since isotopes are present in nature and can contribute to the overall composition of a substance, they can impact the atomic mass.
Average atomic mass: The atomic mass listed on the periodic table for each element is often an average value that takes into account the abundance of different isotopes. The average atomic mass considers the masses of different isotopes and their relative abundance in a naturally occurring sample of the element. As a result, the average atomic mass may not be a whole number.
Considering these factors, the total mass of an atom is typically expressed in atomic mass units (u) or in kilograms (kg) in scientific contexts. It is not a whole number due to the varying masses of protons, neutrons, and the existence of different isotopes of an element.