Why does platinum metal make a good catalytic surface for reactant molecules? It causes reactant molecules to be less reactive.
It strengthens bonds between reactant molecules.
It prevents products from breaking away from the surface.
It holds reactant molecules in a good position for them to react.

it holds reactant molecules in a good position for then to react

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.