Final answer:
B) Atoms of element X and Y, being in the same group on the Periodic Table, have the same number of valence electrons but different numbers of protons, which identifies them as distinct elements. Isotopes have the same atomic number but differ in their mass number due to a different number of neutrons.
Step-by-step explanation:
Atoms of different elements are distinguished by the number of protons in their nuclei, which is known as the atomic number (Z). Elements that are placed in the same group of the Periodic Table have the same number of valence electrons, which influences their chemical properties, but they have a different number of protons and are thus different elements. The following points clarify the properties of atoms:
Atoms of element X and Y have a different number of protons, indicating they are different elements.
Isotopes of an element have the same number of protons and electrons but differ in the number of neutrons. This changes the atomic mass, but not the chemical properties.
The number of electrons in an atom changes during the formation of an ion, but this does not affect the number of protons or neutrons (nucleons).
The nucleon number or mass number (A) of an atom is the total number of protons and neutrons, not the total number of protons and electrons.
Therefore, the correct statement about atoms of element X and Y is that they have a different number of protons.