In the section "Introduction to the atom"
sub section "Isotopes and mass spectrometry"
there is this example for Chlorine
"The atomic mass of chlorine-35 is 34.97u, and the atomic mass of chlorine-37 is 36.97u. And yet, if you look on the periodic table, you'll find that the mass of chlorine is given as 35.45u. Where does this number come from?"
My misunderstanding is in the atomic mass of the isotope Cl-35. In that isotope, the number of protons, Z, =17 and the number of neutrons, N, = 18.
From earlier lessons, it shows that 1 proton= 1.007u and 1 neutron= 1.009u
so from there, I gather that the isotope Cl-35 should have a mass of 17(1.007u) + 18(1.009u) = 35.281u
Can someone please explain why the atomic mass of Cl-35 = 34.97u?