Define atomic number and isotope, and explain why 49 extX and 40 extY are not isotopes.

1) a) i) Define the following terms:

• Atomic number: The number of protons found in the nucleus of an atom. It uniquely identifies a chemical element.
• Isotopy: The phenomenon where atoms of the same element have the same number of protons but different numbers of neutrons, leading to different mass numbers.

1) a) ii) Explain why $^{49}\text{X}$ and $^{40}\text{Y}$ are not isotopes.
Isotopes are atoms of the same element, meaning they must have the same atomic number (number of protons). Since $\text{X}$ and $\text{Y}$ represent different chemical symbols, they correspond