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

1 Answer
Oct 19, 2016

Answer:

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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