How many neutrons does element x have if its atomic number is 30 and its mass number is 70?

The answer is 40.

We can solve this by finding out the number of protons, and neutrons. Atomic number of an element means the number of protons in that element. So, the atom has 30 protons if the atomic number is 30.

On the other hand, mass number is the total number of protons and neutrons, but not electrons, because they're too light comparing to the other 2. Therefore, we can simply solve the number of neutrons in the atom by subtracting the number of protons from the mass number. 70 - 30 = 40.

Therfore, the number of neutrons is 40.

Explanation:

For each of the isotopes in Exercise 21.1, determine the numbers of protons, neutrons, and electrons in a neutral atom of the isotope. 5. 6. Calculate the density of the 2412Mg nucleus in g/mL, assuming that it has the typical nuclear diameter of 1 × 10–13 cm and is spherical in shape.