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an electrically neutral atom of gallium has 31 electrons and 39 neutrons. what is the mass number for an atom of gallium

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Mass number = protons + neutrons.

An electrically neutral atom has not formed an ion yet, so as of now, the number of electrons = number of protons.

So, number of protons in gallium is = 31

Thus, mass number= 31 + 39
= 70.
User Evgeny Karpov
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