And an atom is conceived to consist of a dense nuclear core of massive, positively charged particles, #"protons"#, together with numbers of massive, neutrally charged particles, #"neutrons"#. The number of #"protons"# give the atomic number, #Z#, which is characteristic for the 100 or so elements: #Z=1,"hydrogen";# #Z=2,"helium";# #Z=3,"lithium";# ..............#Z=92,"uranium".#
Because most matter is electrically neutral, around the nuclear core of an element, there are a number of electrons (equal to #Z#), particles of negligible mass but unit electronic charge (i.e. an electric charge OPPOSITE to the of the proton), that are conceived to whizz about the nuclear core. Sharing and transfer of these electrons between different elements can explain the whole of Chemistry (easy innit?).
And thus we have rationalized all of chemistry and the existence of the known elements, on the basis of protons and electrons. The nucleus of many atoms also contains a number of massive, neutrally charged particles, #"neutrons"#. Neutrons, and protons, AT IMPOSSIBLY SHORT nuclear distances, engage in the strong nuclear force, which at such short ranges is strong enuff to overcome electronic repulsion between like charges. Varying numbers of neutrons within a nucleus gives rise to the existence of isotopes. Most elements have a number of isotopes.
For example, #""^1H#, #""^2H#, #""^3H#, are ALL isotopes of hydrogen because each nuclide contains the ONE PROTON (#Z# is unequivocally specified by the #H# symbol). But in #""^2H#, the nucleus also contains a NEUTRON, and likewise for #""^3H#, the nucleus contains 2 neutrons.
How do you know the atomic number of each element? Well, for every exam in Chemistry and Physics, you should be supplied with a Periodic Table, which gives #Z# for all the elements, and also the average atomic mass for each element.
If there is an issue, or query, or inconsistency in something I have said, ask away, and someone will help you.
Also see here for more of the same.
