The weight of an atom, often referred to as its atomic mass, can be determined by adding up the masses of its individual particles—protons, neutrons, and electrons—taking into account their relative abundances.
The atomic mass unit (u) is commonly used to express atomic masses. It is defined as 1/12th the mass of a carbon-12 atom. The mass of a proton is approximately 1.007276 u, the mass of a neutron is roughly 1.008665 u, and the mass of an electron is extremely small and can be considered negligible compared to the masses of protons and neutrons.
To calculate the atomic mass of an atom, you need to know the number of protons and neutrons in its nucleus. The number of protons defines the atomic number and determines the element, while the sum of protons and neutrons gives the mass number.
Once you have the atomic number and mass number of the atom, you can determine the number of protons and neutrons it contains. The atomic mass is then calculated by multiplying the number of protons by the mass of a proton and the number of neutrons by the mass of a neutron, and finally summing these values.
For example, let's consider oxygen with an atomic number of 8 and a mass number of 16. This means it has 8 protons (since the atomic number is the number of protons) and 8 neutrons (since the mass number is the sum of protons and neutrons). The atomic mass of oxygen is calculated as follows:
Atomic mass = (Number of protons × mass of a proton) + (Number of neutrons × mass of a neutron) = (8 × 1.007276 u) + (8 × 1.008665 u) ≈ 15.9994 u
Therefore, the atomic mass of oxygen is approximately 15.9994 atomic mass units.