Question #c36ae
1 Answer
The mass of the second (heavier) isotope is 15.090 amu. See the calculation below.
Explanation:
Most elements exist as a mixture of atoms that have the same number of protons, but slightly different numbers of neutrons. Since protons and neutrons cause the atom to have mass (electrons have very little mass), these atoms (called isotopes) will have slightly different masses.
In particular, nitrogen exists in two forms. The far more common form has 7 protons and 7 neutrons in its nucleus. Its mass is 14.003 amu, and it makes up 99.632% of all N atoms. (One amu or "atomic mass unit" is just a convenient unit for stating the mass of atoms, as both the proton and neutron are very close to one amu.)
The remaining 0.368% of N atoms are slightly heavier, which is why the average mass of all N atoms is a bit more than 14.003, at 14.007 amu.
With all that, here is the solution to your question:
Average mass = contribution due to lighter isotope + contribution due to heavier isotope
where
Note that each bracket consists of the portion of atoms of a particular isotope times the mass of that isotope.
Now, "doing the math":
The heavier isotope must have a mass of 15.090 amu. (This happens because its nucleus contains 7 protons and 8 neutrons.)
