The number of neutrons in an atom does affect the size of the nucleus, but it does not directly influence the distance between atoms in a solid material. Isotopes of the same element differ primarily in their mass, as you mentioned.
The nucleus of an atom consists of protons and neutrons tightly bound together. Since neutrons do not carry a charge, they do not directly contribute to the electrostatic forces that hold the nucleus together. However, neutrons do add mass to the nucleus, and the mass of an atom does have a small effect on its overall size.
Isotopes of the same element have the same number of protons and electrons, which determines their chemical properties. However, they differ in the number of neutrons they possess. This variance in neutron count leads to differences in mass between isotopes. For example, hydrogen-1 (1H) and deuterium (2H) are isotopes of hydrogen, with hydrogen-1 having one proton and no neutrons, while deuterium has one proton and one neutron.
The variation in mass due to different isotopes does not significantly influence the distance between atoms in a solid material. The distance between atoms primarily depends on the bonding and arrangement of atoms in the solid, which is determined by the electron configuration and chemical interactions. Isotopes of an element share the same electron configuration, so their atomic sizes and the distances between their atoms in solid materials remain relatively constant.
To summarize, the number of neutrons in an atom affects the mass of the nucleus and leads to different isotopes of an element. However, the variation in mass between isotopes does not directly impact the distance between atoms in solid materials, which is primarily determined by the electron configuration and chemical bonding of the atoms.