What is the nuclear charge of an element?

And then you have to have a copy of the Periodic Table. There should be one beside you now if you are doing your chemistry/physics homework. So let's say that we have the element nitrogen. And at the moment the element nitrogen, as #N_2# gas, surrounds you.

From the Periodic Table, this tells me that #Z=7# for nitrogen. What does this mean? It means that in the nitrogen nucleus (the nucleus of ALL nitrogen atoms), there are 7 massive, positively charged particles, 7 protons; this number defines the particle as nitrogen.

But if there are 7 positive charges associated with nitrogen, electrical neutrality would insist that there are 7 negative charges. And indeed there are 7 negative charges, 7 electrons, fundamental negative charges of negligible mass, that are conceived to whizz about the nitrogen nucleus.

But the mass of the nitrogen atom is usually #14# #"atomic mass units"#. The nucleus ALSO contains 7 neutrons, 7 massive, NEUTRALLY charged particles. (Interactions between protons and neutrons, at short nuclear ranges, are attractive, and this binds the nucleus together. But you don't learn about this in Chemistry.)

We would identify this nitrogen nucleus as the #""^14N# isotope. The fact that a nucleus can contain various numbers of neutrons gives rise to the existence of isotopes, which are chemically identical, because of course they have equal numbers of electrons and protons, but have slightly different atomic masses because of the different number of neutrons.

The atomic mass quoted on the Periodic Table is the weighted average of the individual isotopes. Most elements, even the simplest element hydrogen, have a number of accessible isotopes: cf, hydrogen, #""^1H#, #"the protium isotope"#, #""^2H#, #"the deuterium isotope"#, #""^3H#, #"the tritium isotope"#. You should be able to tell me how many neutrons each isotope contains.

And that's the basis of atomic physics in a couple of paragraphs. Remember to read your text to consolidate your understanding, and consult old exam questions to see what level of detail you have to know.