The atomic number of helium-4 is half its mass number. How many neutrons are there in an atom of helium-4?
The atomic number represents the number of protons in the nucleus of all atoms, isotopes, and ions of an element. When looking at a cell on the periodic table, the atomic number is frequently written above the chemical symbol. The mass number is the sum of the number of protons and neutrons in a nucleus. In hyphen notation, the mass number is written after the hyphen. Returning to the question, this means that the mass number of helium-4 is four.
If the atomic number of helium-4 is half of its mass number, then the atomic number of helium-4 must be two. We want to determine the number of neutrons in an atom of helium-4. We know that the mass number is the sum of the number of protons and the number of neutrons. We also know that the atomic number is the number of protons in the nucleus. We can substitute the mass number and number of protons into the equation and determine that the number of neutrons in an atom of helium-4 is two.