Which of the following does an
atomic nucleus become if it loses only neutrons when it undergoes radioactive
decay? (A) A different element from the
one it was before the decay. (B) A different isotope of the
element it was before the decay.
This question asks us whether an
atomic nucleus that loses only neutrons changes what element it is a nucleus of or
if it changes what isotope of an element it is a nucleus of. If an atomic nucleus only loses
neutrons, this means that the number of protons in the nucleus does not change.
To answer this question, we must
recall the difference between an element and an isotope of an element. Recall that the number of protons
in an atomic nucleus determines what element an atom is an atom of. For example, these two nuclei have
the same number of neutrons, three, but different numbers of protons. The nucleus on the left has three
protons. If you look at a periodic table,
you’ll see that the element with an atomic number of three is lithium. The nucleus on the right has four
protons, which makes it a beryllium nucleus. These are nuclei of different
On the other hand, atoms with
nuclei that have the same number of protons but different numbers of neutrons are
atoms of different isotopes of the same element. In this example, we see two lithium
nuclei with different numbers of neutrons. We refer to the nucleus on the
right, with four neutrons, as a lithium-7 nucleus. The nucleus on the left is instead
a lithium-6 nucleus. It has one fewer neutron than the
lithuim-7 nucleus. These nuclei are both lithium
nuclei, but with different numbers of neutrons.
Therefore, we say the answer to
this question is (B). If an atomic nucleus loses neutrons
only, it becomes a different isotope of the element it was before the decay.