To calculate the atomic mass of an atom, you need to add the number of protons and neutrons in its nucleus. In this case, the atom has 8 protons and 11 neutrons, so its atomic mass is 8 + 11 = 19.

The atomic mass of an element is typically expressed in atomic mass units (amu). One amu is defined as 1/12 of the mass of a carbon-12 atom. Therefore, the atomic mass of the atom in this question is 19 amu.