Atom Whose Neutron Count Varies From the Standard
Puzzle 4 of The Moon Hard Pack
Question is: Atom Whose Neutron Count Varies From the Standard
and the answer is: ISOTOPE
Word Meaning:
ISOTOPE — (nuclear physics) Any of two or more forms of an element where the atoms have the same number of protons, but a different number of neutrons within their nuclei. Thus, isotopes have the same atomic number but a different mass number.
Definitions from Wiktionary.