* Mass Number: The mass number represents the total number of protons and neutrons in an atom's nucleus.

* Valence Electrons: These are the electrons in the outermost energy level of an atom, which are involved in chemical bonding.

Finding the Element:

1. Identify the number of protons: Since the mass number is 40, and we know that the number of protons defines the element, we need to find an element with 20 protons (to account for the remaining 20 neutrons).

2. Locate the element on the periodic table: The element with 20 protons is Calcium (Ca).

3. Check the valence electrons: Calcium is in Group 2 of the periodic table, which means it has 2 valence electrons.

Therefore, the element with a mass number of 40 and only 2 valence electrons is Calcium (Ca).