Well, if the atom is neutral (and I must assume it is!) there are 20 electrons.
How do I know? I know that for the element calcium, Z=20, where Z is the "atomic number", the number of positively charged, massive particles, that live in the calcium nucleus. If there are 20 positive charges, the "neutral atom" must contain 20 negative charges, 20 electrons, conceived to whizz about the calcium nucleus.
But why do I write ""^40Ca? Well, this isotope has a mass of "40 atomic mass units" (note that the number of protons, i.e. the "atomic number", has alreafy been specified by the Ca symbol, how?) We have have accounted for 20 of these massive particles in the protons. An additional 20 neutrons live in the nucleus and are responsible for the isotopic mass; neutrons are massive particles of neutral charge. If you have further questions, or want clarification, ax.
