Final answer:
Neon 21 has an atomic number of 10, meaning it has 10 protons, and it has 11 neutrons. The correct statement about Neon 21 is that it has 11 neutrons.
Step-by-step explanation:
The isotope Neon 21 is denoted by Ne-21, and for this isotope, the correct option from the choices provided is that it has 11 neutrons. Both Neon 20 (Ne-20) and Neon 21 (Ne-21) have the same atomic number, 10, which tells us how many protons are in the nucleus of the atom (and also how many electrons in a neutral atom). Since Neon 21 has one more neutron than Neon 20, we can infer that Neon 21 has 10 protons and 11 neutrons since the atomic mass is roughly the count of protons and neutrons. Therefore, the atomic mass of Neon 21 is 21 (10 protons + 11 neutrons). The answers about atomic mass being 42, the atomic number being 21, and it having 11 protons are incorrect for Neon 21.