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What are sigma, pi, and coordinate covalent bonds -- and which of the following exhibits them?

The molecules are H2O, N2, and NH3. The question is: What type of bonds are the molecules above? Sigma, pi or coordinate covalent? I have a rough idea, but I don't know how I should know which "overlap" and stuff. Help?

Public Comments

1. Single bonds are sigma bonds or coordinate covalent bonds. Sigma bonds are formed when two atoms share an electron each and the overlap is direct; coordinate covalent bonds are formed when a single atom shares two electrons. Pi bonds are the bonds occurring in multiple bonds, alongside a sigma bond. Whenever you have n bonds in a multiple bond, n-1 bonds will be pi bonds. H2O and NH3 have only sigma bonds; N2 has 1 sigma and 2 pi bonds.
2. H2O only has sigma bonds. It's molecular geometry is H-O-H (Not exactly like that, since it's bent.) N2 has a sigma bond, and two pi bonds. It's molecular geometry is N≡N NH3 has sigma bonds in it, and a coordinate covalent bond is created when another hydrogen is attached to make an Ammonium Ion. The covalent coordinate bond is created when it bonds the Hydrogen on top of the electron pair.
3. It's really pretty simple. Any single covalent bond between two elements is a sigma bond, which means that there is direct orbital overlap between the two elements. Any ADDITIONAL bonds between those two elements are pi bonds (sidewise overlap of orbitals). A coordinate covalent bond is where one element donates both electrons to the bond (usually each element in a covalent bond donates one electron). H2O - There are two single bonds between O and H. Single bonds are sigma bonds. N2 - There is a triple bond between the two N atoms. The first bond is sigma, and the two additional bonds are pi. NH3 - There are three single bonds between N and H, all sigma. None of these shows a coordinate covalent bond. As an example of one, take the ammonium ion, NH4+. In NH3, there are three single N-H bonds and an unbonded pair on the N. When an H+ ion (which has no electrons) comes along, N donates its unbonded pair to the H and forms the fourth N-H bond. Once the bond is formed, it is identical to the three other N-H bonds, i.e., it's not "different" because both electrons in the bond came from N.