Answer the following question given this paragraph: 

At least by our current understanding, a given atom can be defined by its atomic number and its mass number. The atomic number is the number of protons in the nucleus. All atoms with a given atomic number are the same element, because the chemical properties of an atom are primarily determined by the number of positive charges in its nucleus (and therefore the number of negatively charged electrons needed to make it neutral). The mass number of an atom is equal to the number of protons plus the number of neutrons. Since these particles both have a mass of approximately 1 amu and electrons are much smaller, the mass number is approximately equal to the mass of the atom in units of amu.


Q: What is the number of protons in the nucleus?


A:
atomic number