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Chemistry Youtube

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.

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    $\begingroup$ Consider the level of simplification and approximation. $\endgroup$
    – Poutnik
    Commented Jun 8 at 17:07
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    $\begingroup$ No, he just implicitly used simplifying approximation. Relative mass of a nucleus is roughly proportional to the nucleon number. $\endgroup$
    – Poutnik
    Commented Jun 8 at 17:11
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    $\begingroup$ 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. $\endgroup$
    – Poutnik
    Commented Jun 8 at 17:55
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    $\begingroup$ 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... $\endgroup$
    – Maurice
    Commented Jun 8 at 19:11
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    $\begingroup$ @Maurice Iron-56 has isotopic mass 55.9349375(7) Da, very close to 56. $\endgroup$
    – Buck Thorn
    Commented Jun 8 at 19:29

2 Answers 2

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Yes, the mass of a proton and neutron differ by ~0.14%, about 2.5 times the mass of an electron. As a simplification, that difference is ignored. The mass of electrons, ~0.2% that of the nucleons, is also ignored.

There is a further simplification: due to nuclear binding forces, the mass of a "free" proton, or of a temporarily "free" neutron is not the same as the mass of the particle bound in a nucleus. (By temporarily free, I'm acknowledging that naked neutrons have a half-life of just 10 minutes or so.) This is called the "mass deficit", discovered in 1905 by Albert Einstein.

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What he says is absolutely true if one is simply cancelling out the words in a word equation. However if the word equation represents nothing real it is just a math exercise and relates not to reality. The atomic mass is the mass of the atom: the protons + neutrons [aka nucleus] plus the electrons. The physical atomic mass is that of each individual nuclide; the chemical atomic mass is the weighted average of the natural isotopic abundance. The Relative atomic mass is the Mass of N, Avogadro's number, of atoms, molecules etc., N is now a defined constant. The definition is very close to the mass of N carbon-12 atoms so the atomic mass unit is almost exactly equal to 1/12 the mass of N C-12 atoms. The above [teachers] equation states that the average mass of a nucleon is constant; that is false! The average mass, a rather useless variable anyway, is different for each nuclide. The sum of protons and neutrons is the Atomic Mass Number. This number is the name or identifier of a nuclide[isotope] but it is not its mass. It happens that the mass difference between nucleons differs only slightly over the periodic table so defining Carbon 12 as 12units makes the Atomic Mass Number close to the relative atomic mass of a nuclide. This was done to make it easy for students and chemists; let it be so!

It is imperative that mathematical variables exactly describe physical identities before being manipulated. If they do not the best that can happen is an approximation. The more common result is gibberish. The worst is a proof that that is a fallacy and passed off as truth. The teacher's treatment results in an approximation and also in a fallacy that he is trying to pass off as truth. [2 out of 3] The teacher is wrong. Were it correct there could not be different elements and nuclear reactions because there would be no energy differences. These slight differences in mass that the comments are calling insignificant are the reasons that we and our universe exists. E=mc^2. Study up and confront your teacher.

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