How do you calculate atomic number of an element?

1 Answer
Apr 3, 2016


Number of protons, number of electrons, mass number minus number of neutrons.


Some questions are considered as trick questions but they are not.

The element X has a mass number of 20 and it has 10 protons/electrons. What is the atomic number?

1 Atomic number is the number of protons so the answer is 10.

2 It is also the number of electrons because, elements have no overall charge as the positive protons cancel out the negative electrons. In order for this cancellation to happen, the number of protons and electrons must the same. Therefore, if the question only gives the number of electrons, you write the number of electrons as the answer.

The element Y has a mass number of 30 and has 20 neutrons. What is the atomic number?

When you find the number of neutrons of an element, you do mass number minus atomic number. You can use this to build an equation:

The atomic number is 10