How does atomic number, mass number, and number of electrons, relate to atomic structure?

1 Answer
Nov 29, 2015


You are almost there, but you'll need to refine some definitions.


  1. Protons are fundamental particles, that are conceived to (i) reside in the atomic nucleus, and (ii) have a positive charge. The number of protons determines the identity of the element: if 1 proton is present, then it's hydrogen, if 3, then lithium, if 26 then iron.

  2. The electron filling scheme is broadly correct, but this is not really stressed at O levels. I would not try to commit this to memory. In later years you will learn a much better system of classification.

  3. Protons are fundamental positively charged particles, and electrons are fundamental negatively charged particles. Now as I have said, the number of protons determines the identity of the element, so in no chemical reaction can an element lose a proton (why not?). But elements can and do lose or gain electrons to become positively or negatively charged species, called ions . Note that for neutral elements there are always an equal number of electrons (whizzing outside the nucleus) and protons (residing in the nucleus). Why? Because most matter is electrically neutral.

Given what you know now, can you tell how many electrons there are in an iron atom; a calcium atom; a chlorine atom? You will need a Periodic Table. Now tell me how many electrons there are in #Fe^(3+)#, #Ca^(2+)#, and #Cl^-#.

You are certainly free to ask follow up questions.