# How many electrons in the neutral ""^40Ca isotope?

You want the number of electrons in ""^40Ca?
How do I know? I know that for the element calcium, $Z = 20$, where $Z$ is the $\text{atomic number}$, the number of positively charged, massive particles, that live in the calcium nucleus. If there are 20 positive charges, the $\text{neutral atom}$ 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 $\text{40 atomic mass units}$ (note that the number of protons, i.e. the $\text{atomic number}$, has alreafy been specified by the $C a$ 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.