The atomic number tells the number of positively charged protons in the nucleus of an atom. The atom is neutral because this is also the number of negatively charged electrons in the atom.
Below, I have defined each key term within the answer above:
Atom: An atom is the smallest unit of matter which retains properties unique to its element. The atom is composed of three subatomic particles- positively charged protons, neutrally charged neutrons, and negatively charged electrons.
Atomic number: The atomic number is the number of protons within the nucleus of an atom. Elements are defined by their atomic numbers, and the number of protons will not vary within atoms of a specific element.
Nucleus: The center of the atom that houses the protons and neutrons.
Electron: An electron is the negatively charged subatomic particle that revolves around the nucleus in an electron orbital.