Solids are packed together closely, liquids are farther apart but still relatively close, and gases are very far apart with lots of space.
Explanation:
Lithium diisopropylamide (LDA) is used in many organic synthesis and is a strong base. It is prepared by the acid base reaction of N,N-diisopropylamine ( [(CH₃)₂CH]₂NH ) and butyllithium ( Li⁺⁻CH₂CH₂CH₂CH₃ ).
The equation is show below as:
[(CH₃)₂CH]₂NH + Li⁺⁻CH₂CH₂CH₂CH₃ ⇒ [(CH₃)₂CH]₂N⁻Li⁺ + CH₃CH₂CH₂CH₃
N,N-diisopropylamine ( [(CH₃)₂CH]₂NH ) is a weaker acid and hence, LDA ( [(CH₃)₂CH]₂N⁻Li⁺ ) is stronger base. (Weaker acid has stronger conjugate base)
Butyllithium ( Li⁺⁻CH₂CH₂CH₂CH₃ ) is a very strong base and hence, butane ( CH₃CH₂CH₂CH₃ ) is a very weak acid. (Strong base has weaker conjugate acid)
Since we're talking about a chain reaction, I think this would be a) self-sustaining. Chain reactions are not usually described as "weak" and can be highly explosive depending upon the compounds (think Uranium!).
The first (artificial) self-sustaining nuclear reaction is attributed to Enrico Fermi in 1942. Here's a bit of history:
http://www.atomicarchive.com/History/firstpile/firstpile_01.shtml
The alkali metals, which occupy group 1 of the periodic table. This is because the valence shells of these elements have only 1 electron, so easily form an ionic bond with a non-metal compound by donating this. A cation is formed by this donation, since there is one fewer electron orbiting the nucleus than there is in the atomic form - conversely an anion is formed when an atom gains an extra electron to become negatively charged.
