[maccha]

Why do diatomic molecules separate in reactions? For example

I + Cs2 --> CsI

why doesn't Cs2 stay as Cs2? how does this separation happen?

[csrscience.com]

Don't you mean I2 and Cs?

Metals don't from diatomic bonds.. they form metallic bonds - which to my understanding is where the electrons just freely flow around in the metal item, not really attached to any specific atom.

[maccha]

Sorry I meant I2 and Cs but I still don't understand.

[cth]

I₂ + Cs_(s)  CsI_(s)

If you look at the elements electronegativities http://en.wikipedia.org/wiki/Electronegativity, iodine is much more electronegative than cesium. This means that iodine is eager to attract one electron in order to become the I^- which has a more stable electronic configuration. On the other side of the periodic table, cesium is willing to give away one electron in order to become Cs⁺, which is more stable.

So, when iodine comes in contact with cesium, both agree to exchange one electron to form the salt CsI.

Remark, CsI is not a molecule as such, but it is a salt constituted of cations Cs⁺ and anions I^-. And it is not diatomic, instead it contains many anions and cations arranged in a 3-dimensional structure http://en.wikipedia.org/wiki/CsI.