An atom of a certain element has 19 protons, 19 electrons, and a mass number of 39. This element is very reactive. How many neutrons are in this atom?

There would be neutrons in that atom.

Explanation:

The mass number of an atom is the sum of:

Electrons do not contribute to the mass number of an atom.

From the question:

.

In this atom:

.

Hence, the number of neutrons would be .

it’s previous models   because when i picked that son of a gun’s answer it was wrong look for previous models because they might make it a different letter