An unknown element has 19 electrons and 20 neutrons, what is the mass number of this element?

The mass number of an element is calculated by adding the number of protons and neutrons in an atom. In a neutral atom, the number of electrons is equal to the number of protons. So, this unknown element has 19 protons (since it has 19 electrons in a neutral state) and 20 neutrons.

Step 1: Identify the number of protons = 19 (equal to the number of electrons in a neutral atom) Step 2: Identify the number of neutrons = 20 Step 3: Add the number of protons and neutrons to find the mass number

So, the mass number of the element = number of protons + number of neutrons = 19 protons + 20 neutrons = 39.

Therefore, the mass number of this unknown element is 39.