Which atoms have the same number of neutrons?

which is the correct answer and why?

1. H-1 and He-3

2. H-2 and He-4

3. H-3 and He - 3

4. H-3 and He-4

4. H-3 and He-4

Explanation:

H-3 means hydrogen with a mass number of 3. Mass number is the addition of protons and neutrons in the nucleus. Hydrogen always has 1 proton because it has atomic number 1, therefore the number of neutrons is found by subtracting 1 from the mass number (3).

3-1 = 2 neutrons

He-4 also has 2 neutrons. He-4 has mass number 4, which means that its protons and neutrons added together equals 4. Helium has 2 protons because it has atomic number 2.

Therefore 2 + neutrons = 4

neutrons = 4-2

neutrons = 2