Fill in the blanks:
$(1)$ In the crystal structure of ice, the oxygen atom is at a distance of $276 \ pm$ from four hydrogen atoms.
$(2)$ The relative proportion of tritium to protium is $1: 10^{18}$.

(N/A) $(1)$ The oxygen atom in the crystal structure of ice is surrounded by four hydrogen atoms at a distance of $276 \ pm$.
$(2)$ Tritium is the rarest isotope of hydrogen, and its relative abundance compared to protium is $1: 10^{18}$.