If an element has an atomic mass of 19 and a proton number of 9, how many neutrons does it have and how many electrons does it have?

The atomic mass is made up of all the protons and neutrons (which together form the nucleus) which each have a mass of approx. 1g. Therefore if the mass of the element is 19g and it has 9 protons (proton number is 9), it must have 10 neutrons as 19-9=10. As electrons have negligible mass they are not relevant here. As the element has no charge, the number of protons which each give +1 charge must be be balanced by the number of electrons which each give -1 charge. Therefore there must be an equal amount of each. As the proton number is 9, the number of electrons must also be 9.