Final answer:
The unknown element with an atomic mass of 50 u and 40 electrons has 10 neutrons, which can be calculated by subtracting the number of protons (40) from the atomic mass (50).
Step-by-step explanation:
To determine how many neutrons an unknown element with an atomic mass of 50 u (standing for unified atomic mass units) and 40 electrons have, we need to know its number of protons. Since the number of electrons is equal to the number of protons in a neutral atom, this element has 40 protons. The atomic mass or mass number approximately equals the sum of protons and neutrons. By using the formula:
number of neutrons = atomic mass - number of protons
We can calculate the number of neutrons:
number of neutrons = 50 - 40 = 10
Therefore, the element has 10 neutrons.