To calculate the mass of an atom, you need to sum the masses of its protons, neutrons, and electrons. The mass of a proton and a neutron is approximately 1 atomic mass unit (amu), while the mass of an electron is much smaller and can be considered negligible compared to protons and neutrons.
Given that the atom has 18 protons and 20 neutrons, the total mass contributed by protons and neutrons is:
18 protons × 1 amu/proton + 20 neutrons × 1 amu/neutron = 18 amu + 20 amu = 38 amu
Since the mass of electrons is negligible for this calculation, we can ignore them in this context.
Therefore, the approximate mass of the atom is 38 atomic mass units (38 amu).