Answer:
[tex]\boxed {\boxed {\sf 39}}[/tex]
Explanation:
The mass number of an atom can be found by adding the number of protons and neutrons.
[tex]mass \ number =protons + neutrons[/tex]
There are 19 protons and 20 neutrons.
[tex]protons=19 \\neutrons=20[/tex]
[tex]mass \ number= 19+20[/tex]
[tex]mass \ number =39[/tex]
The mass number of this atom is 39
