* Protons (p): The number of protons defines the element. In this case, 19 protons indicate the element is Potassium (K).

* Neutrons (n): Neutrons contribute to the atomic mass.

* Atomic Mass: Atomic mass is approximately equal to the sum of protons and neutrons.

Calculation:

Atomic mass ≈ protons + neutrons

Atomic mass ≈ 19 + 20

Atomic mass ≈ 39

Therefore, the atomic mass of this atom is approximately 39.