An isotope of an element, atomic number z, has mass number 2z + 4. How manyneutrons are in the nucleus of the element?A) z + 4 B) z + 2 C) z D) 4

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jeew-m | College Teacher | (Level 1) Educator Emeritus

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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)