1s22s22p63s 3p645230104p5 Which element is this?

fluorine (F)
bromine (Br)
chlorine (Cl)
krypton (Kr)

Draw the skeletal structures of two different molecules that are each made of 5 carbon atoms and 12 hydrogen atoms.

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.

What is the percentage of hydrogen in c2h4