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This question already has an answer here:

I just want to know if this equation for the mass defect is correct

$$\Delta m = Z * m_p + Z * m_e + N* m_n - A $$

where

  • Z- Atomic number
  • A- Mass number
  • N- number of neutrons
  • $m_p$- mass of the proton
  • $m_n$- mass of the neutron
  • $m_e$- mass of the electron

I came up with another formula also $$ΔM=[(Z(m_p)+N(m_n))−(M)]$$ where $m$ is the observed mass

Which one is correct and what is the difference between both these equations?

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marked as duplicate by ZeroTheHero, Jon Custer, Buzz, Emilio Pisanty, peterh Jan 22 at 19:10

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Possible duplicate of correct formula for Mass Defect / Binding Energy? $\endgroup$ – Anubhav Srivastava- WASAYANSI Jan 14 at 11:41
  • $\begingroup$ @Peaceman both are correct. $\endgroup$ – KV18 Jan 14 at 12:09
  • $\begingroup$ @AnubhavSrivastava-WASAYANSI The answer to your question depends on how (M) is defined. Please tell us what (M) means. Observed mass of what? $\endgroup$ – Farcher Jan 14 at 12:34
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    $\begingroup$ The first equation doesn't make sense in terms of units. The $-A$ term is unitless. BTW, please make more of an effort to use standard mathematical notation. The *'s and excessive parens make this a chore for other people to decode. $\endgroup$ – Ben Crowell Jan 14 at 14:19
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I just want to know if this equation for the mass defect is correct.

$$\Delta m = Z * m_p + Z * m_e + N* m_n - A $$

$A = $ mass number

No.
A is the mass of an $^A_Z X$ atom.

I came up with another formula also $$ΔM=[(Z(m_p)+N(m_n))−(M)]$$ where m is the observed mass.

This is correct if $(M)$ is the mass of an $^A_Z X$ nucleus.

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