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The mass number of a nucleus is

  1. always less than its atomic number
  2. always more than its atomic number
  3. equal to its atomic number
  4. sometimes more than and sometimes equal to its atomic number

Answer: 4

I know that mass number is equal to the sum of number of protons and neutrons in the nucleus. And also number of protons are actually the atomic number of the elements. So I know that, why mass number is greater than atomic number. But I don't understand How can atomic number be equal to it's mass number. This means number of protons is equal to mass number i.e. number of neutrons is zero. And if there are no neutrons then how can it be called as nucleus? Is the given answer is wrong?

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Hydrogen atom $^1_1H$ i.e. an atom with one proton (and an electron) and no neutrons is the most abundant isotope in the universe. It has atomic number $Z=1$, and mass number $A=1$. Only in this case $A=Z$ since, as you said, a nucleus with more protons cannot be stable without neutrons.

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  • $\begingroup$ So if hydrogen don't have neutrons, does that mean there is no nuclear forces operating? $\endgroup$
    – Avi
    Commented Mar 11, 2017 at 7:09
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    $\begingroup$ Indeed there are nuclear forces: Strong force between the quark content of the proton is still operating, decribed by QCD. If the proton were unstable (and it could decay in other products) we would see weak interaction as well. However, since there are not other nucleons, there are not "binding forces" in this nucleus, no interaction between nucleons. $\endgroup$
    – Alejandro Menaya
    Commented Mar 11, 2017 at 7:13

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