1) A solid is a state of matter where the atoms are ordered in a fixed lattice and packed closely together.
2) Only the charge of the two points is given, no extra properties are known. When we assume these represent the proton and electron, this scheme represents the hydrogen atom.
3) Two hydrogen atoms, seperated by a wall.
4) This forms a hydrogen molecule (H_2)
5) ~= -2
6) d=0