The number of protons present in the nucleus of an atom of the element is called atomic number of the element. Since all atoms of a particular element have same number of protons in their nuclei, the atomic number of an element is characteristic of the element. For example, all the atoms of uranium have 92 protons in their nuclei. Hence, atomic number of uranium is 92.
In a neutral atom, the number of protons is equal to number of electrons. Therefore, atomic number of an element can also be taken as equal to the number of electrons in an atom of the element. The atomic number is denoted by the letter Z.
Atomic Number (Z) = Number of protons
= Number of electrons