Final answer:
The atomic number for an element with a mass number of 115 and containing 65 neutrons per atom is 50. You find this by subtracting the number of neutrons from the mass number.
Step-by-step explanation:
The atomic number of an element is the number of protons in its nucleus. To find the atomic number of an element with a mass number of 115 and 65 neutrons, we need to subtract the number of neutrons from the mass number. The formula to find the number of neutrons in an atom is: number of neutrons = mass number - atomic number. Applying this formula, we get number of neutrons = 115 - atomic number.
Since we know the number of neutrons is 65, we can set up the equation as 65 = 115 - atomic number. Solving for the atomic number, atomic number = 115 - 65, which equals 50. So, the atomic number for this element is 50.