How is the atomic number of an element defined?

1 Answer
Aug 29, 2017


By the number of positively charged, massive particles that are contained in the element's nucleus.


And thus atomic number, #Z#, represents the number of protons contained in the element's nucleus.

If #Z=1#, we gots hydrogen; if #Z=2# we got helium; if #Z=3# we got lithium; .......if #Z=46# we got palladium.... You need not learn these numbers, because for most tests in Chemistry and Fysics a copy of the Periodic Table will be made available to you. You still have to learn how to use it.

Now given that the atomic number #Z# represents the number of positive nuclear charges, it ALSO represents the number of EXTRA-NUCLEAR, negatively charged particles, i.e. electrons, IN THE NEUTRAL ELEMENT. Got it? Electrons are conceived to whizz about the nuclear core, and exchange and sharing of electrons between nuclei underlies all of chemistry.

We are not finished there, because the nucleus can also contain various numbers of neutrons; massive, neutrally charged nuclear particles. For example most hydrogen nuclei only contain one proton, i.e. #""^1H#; a few nuclei contain one neutron to give #""^2H#, deuterium. The differing number of neutrons, and the DIFFERENT resultant mass gives rise to the phenomenon of #"isotopes"# nuclei of the same element with different atomic mass. Interactions between neutrons and protons, at impossibly short nuclear distances, give rise to the strong nuclear force, that binds the nucleus together, and is strong enuff at these short nuclear ranges, to overcome electrostatic repulsion between protons. (And if you want more info, consult a particle physicist, this is the ignorant chemists' section!)

It seems that I have covered all of chemistry and fyziks in a few sentences. The concepts and ideas are (I think!) easy and straightforward, but you should read your text in order to consolidate and expand your understanding. You should also read old exam papers in order to get an idea of how far your understanding will be tested.