a) The atom with an atomic number of 4 is beryllium. b) An atom of oxygen has 8 protons. The atomic number of oxygen is 8, which indicates the number of protons in its nucleus. c) An atom of helium has 2 electrons. The number of electrons in an atom is equal to its atomic number, and helium has an atomic number of 2. d) The number of neutrons in an atom of sodium can vary because sodium has multiple isotopes. However, if we assume a typical isotope of sodium, which is sodium-23, it has 12 neutrons. The atomic number of sodium is 11, which represents the number of protons. By subtracting the atomic number from the atomic mass (23 - 11), we can find the number of neutrons.