4ESO Física i Química , ARTICLE 58 : Atomic and mass numbers.

                                                   

                                                          ATOMIC NUMBER AND MASS NUMBER.

                       (Information got from https://global.britannica.com/science/chemical-element)

- Atomic number (Z) , the number of a chemical element in the periodic system, whereby the elements are arranged in order of increasing number of protons in the nucleus. Accordingly, the number of protons, which is always equal to the number of electrons in the neutral atom, is also the atomic number. An atom of iron has 26 protons in its nucleus; therefore the atomic number of iron is 26.

In the symbol representing a particular nuclear or atomic species, the atomic number may be indicated as a left subscript. 

Mass number (A) , in nuclear physics, the sum of the numbers of protons and neutrons present in the nucleus of an atom. The mass number is commonly cited in distinguishing among the isotopes of an element, all of which have the same atomic number (number of protons) and are represented by the same literal symbol; for example, the two best known isotopes of uranium (those with mass numbers 235 and 238) are designated uranium-235 (symbolized ²³⁵U) and uranium-238 (²³⁸U).