1 Answer

Dfcorbin · Answer 1 · 2021-03-31T03:59:25+0000

Answer:

$\boxed {\boxed {\sf 39}}$

Step-by-step explanation:

The mass number of an atom can be found by adding the number of protons and neutrons.

$mass \ number =protons + neutrons$

There are 19 protons and 20 neutrons.

$protons=19 \\neutrons=20$

$mass \ number= 19+20$

$mass \ number =39$

The mass number of this atom is 39

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