The number of protons present in the nucleus of an atom is known as the atomic number. It is denoted by Z.
Hydrogen has one proton in its nucleus. So, the atomic number of hydrogen is 1.
Sodium has 11 protons present in its nucleus. The atomic number of sodium is 11.
Lithium has 3 protons and oxygen has 8 protons in its nucleus respectively. So, the atomic number of lithium is 3 and oxygen is 8.