Question

An isotope of which element could have 19 protons and 21 neutrons?
calcium
zirconium
scandium
potassium

Answer 1

You can think of protons are the identity of any element, each element has its own number, which is proton. So element number 19 from the periodic table is potassium.

You don't need to care about neutrons here.

Periodic table:
http://www.ptable.com/

Answer 2

oo thanks for your help ;D

Answer 3

welcome :)