Which of the following is not an isotope of hydrogen?

Isotopes of a given element differ in the number of __________.

Isotopes differ in the number of __________.

Chromium has four naturally occurring isotopes. has a mass of and an abundance of . has a mass of and an abundance of . has a mass of and an abundance of . has a mass of and an abundance of .

Using this information, estimate the atomic mass of chromium, in atomic mass units.