Why can the atomic number be used to identify an element but the neutron number cannot? Can the mass number be used to identify an element?

Question

 Why can the atomic number be used to identify  an element but the neutron number cannot? Can the mass number be used to identify an element?

Answer 1

The atomic number of an element is fixed for that element.  However, the mass number of an element may vary, as different isotopes of the same element have different numbers of neutrons for the same atomic number.  Think of the example of ³He and ⁴He.  Both are isotopes of helium, and both have atomic number Z=2.  However, ³He has 2 protons and 1 neutron, hence mass number A=3, while ⁴He has 2 protons and 2 neutrons, hence mass number A=4.  They are both the same element, i.e. He, but with different mass numbers.  So the mass number alone cannot uniquely identify an element.  But the number of protons in the nucleus identifies it uniquely.  In addition, if the atom is neutral, then the atomic number also equals the number of electrons in its orbitals.  The chemical reactive possibilities of the element are determined by the configuration of those electrons.

 

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