How is a compound different from the elements that composed it?

Aboxcar traveling at 12 m/s approaches a string of 5 identical boxcars sitting stationary on the track. the moving boxcar collides and links with the stationary cars and they all move off together along the track. what is the final speed of the cars immediately after the collision? (you may take the mass of each boxcar to be 18,537 kg.)

Which is not true about oxygen-17 and oxygen-18?

What structural formula represents 4 electrons shared between two atoms?