# Why is the teacher saying here that relative atomic mass unit is equal to total number of nucleons?

Why is the teacher saying that relative atomic mass is equal to the total number of nucleons here? Can't a nucleon be either a proton or neutron, but the mass of a proton or neutron is slightly different, so how can he say that mass of one atom of an element is equal to total number of nucleons. Sorry if this question is too simple or I understood something wrong.

• Consider the level of simplification and approximation. Commented Jun 8 at 17:07
• No, he just implicitly used simplifying approximation. Relative mass of a nucleus is roughly proportional to the nucleon number. Commented Jun 8 at 17:11
• Near any high school-like education of science is intentionally not as complicated as the known level of complexity of nature. The same here. The assumed audience is IMHO less advanced than you are. Commented Jun 8 at 17:55
• Let's take any example : Iron $\ce{Fe}$, It has $26$ protons, and a number of neutrons which can vary from $28$ to $32$. The most abundant isotope ($81.8$% of all isotopes) is by far the iron with $30$ neutrons. If you sum up the number of neutrons and of protons of this most abundant isotope, it gives $26 + 30 = 56$. The measured atomic mass of iron is $\ce{55.65}$. If you are not interested by precision, you can forget about decimals and admit that its atomic mass is $56$ g/mol. $56$ is also the sum of protons + neutrons of the most abundant isotope. It is a wrong af course, but well... Commented Jun 8 at 19:11
• @Maurice Iron-56 has isotopic mass 55.9349375(7) Da, very close to 56. Commented Jun 8 at 19:29