What is the mass number of an ion with 105 electrons, 159 neutrons and a +1 charge?
express your answer as an integer.

The sum of protons and neutrons in the nucleus of an atom is mass number of an isotope and the number of protons and electrons are equal in a neutral atom.

Here the ion has 105 electrons and 159 neutrons and +1 charge. This means the original atom has lost one electron, originally there are 106 electrons in the neutral atom which shows that there are also 106 protons because in neutral atom protons and electrons are same.

So the mass number of an ion = number of protons + number of neutrons = 106 + 159 = 265

265 is the mass number.