What is the difference between stable and unstable isotopes

The reaction q+r2=r2q is found to be first order in r2 and

What’s the scientific notation for the number 6,840,000,000

Two metal blocks that have slightly different temperatures are placed next to one another. after five minutes, they both have lower but equal temperatures. according to the law of conservation of energy, what  most likelyhappened? energy was created inside the blocks.energy was destroyed inside the blocks.energy was absorbed into the blocks from outside the system.energy was transferred from the warmer block to the cooler block.