Option 1 states that two atoms are isotopes if they have the same atomic number but different mass numbers.

To understand this option, it`s important to understand what atomic number and mass number mean. The atomic number of an atom represents the number of protons in its nucleus. The mass number, on the other hand, is the total number of protons and neutrons in the nucleus.

Isotopes are atoms of the same element that have different numbers of neutrons, but the same atomic number. This means that they have the same number of protons, which determines the element, but different mass numbers due to the difference in neutrons.

For example, let`s take carbon as an element. Carbon has an atomic number of 6, meaning it has 6 protons. Carbon-12 is the most abundant isotope of carbon, with a mass number of 12, which means it has 6 protons and 6 neutrons. Carbon-14 is another isotope of carbon, with a mass number of 14, meaning it has 6 protons and 8 neutrons.

Option 1 correctly states that two atoms are isotopes if they have the same atomic number but different mass numbers.