Question

If the atomic mass of an atom is `23*"amu"`, and there are 12 neutrons, how many protons are present?

Answer 1

If the atomic mass is `23`, and there are `12` neutrons, there MUST be `11` protons........

Explanation:

The mass of an atom depends on the number of nuclear particles: neutrons, massive particles with zero charge; and protons, massive particles with positive charge.

The number of protons gives `Z,"the atomic number"`. And `Z` defines the identity of the element:

`Z=1, "hydrogen"; Z=2, "helium"; Z=3, "lithium"; ......Z=6, "carbon"...........Z=92, "uranium"`

You don't have to be remember these numbers, because you should always have access to a Periodic Table (yes, even in exams). And thus you should be able to tell me for what element `Z=11`; it starts with an `"S"`. Every phyicist, chemist,and engineer regularly uses a Periodic Table.

But of course there is another layer: the phenomenon of `"isotopes"`.

Let's take the simplest atom, hydrogen. Since `Z=1`, there is one nuclear proton, and around the nucleus an `"electron"`, a non-massive particle of near zero mass, and an opposite electronic charge to the protons is conceived to whizz about it. Some few hydrogen nuclei contain a neutron in their nucleus. The nucleus is still hydrogen, because that's how we define hydrogen, `Z=1`. We call this species `"deuterium"`, an isotope of hydrogen, and represent is as `""^2H`.

A smaller percentage of hydrogen nuclei contain 2 neutrons, to give the `"tritium"` isotope, `""^3H`. All of these are hydrogen isotopes. Most elements have a number of isotopes, and their mass is reported as the weighted average of the individual isotopic masses.

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