Take into consider isotopes where there are more or less than the ideal amount of neurtrons. These isotopes are radioactive and unstable to the point hey are used as atomic bombs. So with the proper amount of nuetrons it helps the atom maintain structure and not degrade
Sound waves traveling through air are indeed longitudinal waves with compressions and rarefactions. As sound passes through air (or any fluid medium), the particles of air do not vibrate in a transverse manner.
Negative tropism is the growth or movement of an organism away from a stimulus. A common example that can be used to explain negative tropism is the shoot of the plants that grow against gravity