What is the significance of valence electrons?
Valence electrons are the electrons in the outermost "shell" of an atom. These electrons determine many properties of the atom such as what other atoms they will bond with, what chemical structure they will form and how volatile they are in relation to other chemicals and atoms. Generally, atoms will attempt to form octet meaning will they will combine to with another atom to have a total of 8 shared electrons in the outermost cell.
How does the theory of evolution apply to the study of infectious diseases?
The premise behind the theory of evolution, on the most fundamental level, is that as D.N.A is replicating it will make mistakes which are called mutations. Most mutations occur randomly and have little benefit to the organism or microorganism involved. However, occasionally a mutation will give the organism a significant adaptive advantage. When this happens organisms that have this mutation out-survive and out-reproduce organisms without the advantage and thus this adaptive mutations is perpetuated and eventually dominates and evolves whatever organism this mutation took place in, this process is called natural selection. When applied to larger organisms this process of mutations and adaptations takes significantly longer. However, when analyzing bacteria and viruses this process happens at near real time. A good example to demonstrate this principle applied to infectious disease is the HIV virus. Finding a cure for this deadly virus that leads to AIDS is so illusive because HIV mutates at extremely high rates, new strands of HIV are naturally selected that are resistant to the natural antibodies that fight the infection and thus HIV continues to grow and survive even as the body continually fights new strands of the virus. A similar phenomena occurs with anti biotic medication which overuse is very dangerous. The more anti biotic medication that is used the greater chance of a mutation of anti biotic resistant bacteria evolving.
How has nuclear deterrence benefited U.S foreign policy post WW2?
Former President Theodore Roosevelt said "speak softly and carry a big stick". Nuclear weapons have proven to be the "big stick" in the post WW2 geopolitical environment. Even though nuclear weapons have increased the fear of a devastating conflict, this fear has paradoxically helped the U.S avoid such conflict. It can be argued that Korea, Vietnam, Iraq and Afghanistan were major wars but they cost way less U.S lives than more devastating conflicts like the civil war or WWII. The threat of nuclear war is what thwarted a major conflict between the USSR and the U.S during the cold war. Despite the threat of nuclear war being a major concern, the assumption that decision makers employ rationality will continue to keep governments from using them since there is the promise of mutually assured destruction.