Because of their atom's composition. some are naturally packed closer together and stronger and more durable, while others tend to be more loose and breakable and/or weaker/less durable. also with how their atoms are composed is how the atoms interact with each other/ how they share or give or take electrons.
Answer:
I think that the statement is relative to speed because it is saying km per second.
6,484,649 every three numbers from the right
