The unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier is known as the what?

The unstable arrangement of atoms that forms at the peak of the activation energy is called the transition state.  If I remember correctly It is the point in which all the atoms or molecules (involved in the reaction) collide in the correct orientation to brake the bonds of the reactants and form the bonds in the products.

I hope this helps.  Let me know if anything is unclear.