The number of protons in the nucleus of an atom determines the species of the atom, i. e., the element to which the atom belongs. an atom has the same number of protons and neutrons. but the electron number cannot be used instead because

There are 3 well-known subatomic particles. These are: (1) electrons, (2) protons, and (3) neutrons. Both protons and neutrons are found inside of the nucleus of the atom while the electron lies outside of it. The number of protons in the nucleus is used to classify the atoms because that is specific property of the atom called the ATOMIC NUMBER.

The number of electrons cannot be used instead because there are times when the atom takes in electrons from other sources making it negatively charge. Similarly, the atom can also loss some of its electrons making it positively charge. The charged atoms are called IONS. 

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