An isotope of an element, atomic number z, has mass number 2z + 4. How many

neutrons are in the nucleus of the element?

A) z + 4

B) z + 2

C) z

D) 4

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We know that mass number of an atom is the sum of protons and neutrons inside the atom. Usually this is expressed in unified atomic mass units or 1/12 of the mass of a single C-12 (carbon-12) atom at rest.

Isotope means an atom which has same number of electrons but different neutrons.

atomic mass number `= N_p+N_n`

`2z+4 = z+N_n`

`N_n = 2z+4-z = z+4`

*So there will be (z+4) neutrons in the isotope.*

*So the correct answer is A)*

**Sources:**

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