Q.Why is helium used in diving apparatus?

Group 18 elements (He, Ne, Ar, Kr, Xe, Rn) have completely filled valence shells, $ns^2np^6$ (just $1s^2$ for helium), giving them exceptional chemical stability and extremely high ionisation enthalpies; until 1962 they were believed to be totally unreactive, which is the origin of the name "noble gases" — reflecting their general reluctance to combine with other elements, much as a noble metal resists corrosion. All are monatomic gases held together only by weak, instantaneous-dipole/induced-dipole London dispersion forces, so they have very low boiling and melting points that rise down the group as the atoms become larger and more polarisable, strengthening these dispersion forces; helium in particular stays gaseous almost down to absolute zero and does not solidify at ordinary pressure. Down the group, atomic size increases substantially, and because noble gas atomic radii are measured as van der Waals radii — the closest distance of non-bonded contact between two touching atoms — rather than the shorter covalent radii used for bonded atoms elsewhere in the periodic table, noble gas atoms appear comparatively large for their period. Helium is used, mixed with oxygen, in diving apparatus because it is far less soluble in blood than …