Function is_valid_isotope_numbers 

pub const fn is_valid_isotope_numbers(
    atomic_number: u8,
    mass_number: u16,
) -> bool

Returns true when the isotope numbers are structurally valid.

This only checks that atomic_number is between 1 and 118 inclusive and that mass_number >= atomic_number. It does not imply that the isotope is known, stable, naturally occurring, or abundant.