Chapter 2 - Atoms and the Atomic Theory - Exercises - Atomic Number, Mass Number, and Isotopes - Page 62: 35

Ca

Mass number A= Number of protons+ Number of neutrons. Let the number of protons be $x$. Then, $A=44=(x+4)+(x)= 2x+4$ $\implies x=\frac{44-4}{2}=20$ Number of protons= 20= Atomic number Z. The element with $Z=20$ is Calcium (Ca).