What does the atomic number of an element represent?
Number of protons
The atomic number of an element is defined as the number of protons present in the nucleus of one atom of that element. It is the identity tag of an element in the periodic table; every element has its own unique atomic number.
In a neutral atom the number of electrons equals the number of protons, so the atomic number also equals the electron count, but the definition itself refers to protons. The number of neutrons or the total of electrons and neutrons is a different quantity (mass number and neutron number), and isotopes count is not what the atomic number measures.
Hence, the answer is Number of protons.
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