# A nuclide has 20 electrons and a positive charge. There are 18 neutrons. What is the nuclide?

Well the mass number is $\text{39 amu}$, and we can even quote the element as scandium....
We gots an ion with 20 electrons, with a POSITIVE charge. There are thus 21 nucular protons, i.e. $Z = 21$ (because the neutral element would have 21 electrons). And if $Z = 21$ then clearly a glance at the Periodic Table tells us that we got the element scandium..... And since this nucleus has 18 neutrons we got ""^(39)Sc, which is an exceptionally rare scandium isotope, and is likely (momentarily) observed in nuclear reactions of calcium.