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

Oct 19, 2016

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 $Z , \text{the atomic number}$. And $Z$ defines the identity of the element:

$Z = 1 , \text{hydrogen"; Z=2, "helium"; Z=3, "lithium"; ......Z=6, "carbon"...........Z=92, "uranium}$

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 $\text{S}$. Every phyicist, chemist,and engineer regularly uses a Periodic Table.

But of course there is another layer: the phenomenon of $\text{isotopes}$.

Let's take the simplest atom, hydrogen. Since $Z = 1$, there is one nuclear proton, and around the nucleus an $\text{electron}$, 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 $\text{deuterium}$, an isotope of hydrogen, and represent is as ""^2H.

A smaller percentage of hydrogen nuclei contain 2 neutrons, to give the $\text{tritium}$ 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.

Confused yet? State your objections, and someone will try to help. Cheers and good luck with your studies.