An oxygen atom is composed of eight protons, 8 neutrons, and eight electrons. The number of electrons and protons of an element is equivalent to the number in its atomic structure 8 for oxygen. Number of neutrons could be calculated by subtracting amount of protons in the mass of the atom that is 16 times eight for oxygen.An an atom is made up of electrons, neutrons and protons. Protons and neutrons are housed in the nucleus, which is a tightly space in the middle in the structure of the. The electrons, which are the smallest mass, reside in orbits surrounding the nucleus. Each orbital can only hold an amount of electrons. After a lower orbital is full, electrons begin to fill the higher energy orbital.