Two solids are depicted at the particulate level.
• Solid X: identical nickel atoms arranged in a close-packed metallic lattice.
• Solid Y: the same nickel lattice with smaller boron atoms occupying many tetrahedral voids.
Which prediction about hardness is most consistent with the representations?
a) Solid X is harder because the introduction of boron disrupts metallic bonding and weakens the lattice.
b) Solid Y is softer because the boron atoms increase the distance between nickel atoms, decreasing metallic bond strength.
c) Solid Y is harder because the boron atoms impede the ability of nickel layers to slide past one another.
d) Solid X is harder because pure nickel contains a greater concentration of mobile electrons that reinforce metallic bonding.