molecular orbital theory

1. In molecular orbital theory, a covalent bond is formed whenever two atoms overlap all of their orbitals, regardless of whether they are valence orbitals or not, to create bonding and antibonding orbitals. 

2. Yes, this is found in p subshells when forming homonuclear molecules with some atoms. For atoms like nitrogen, the lack of electron repulsion from their unpaired electrons in the 2p subshells allows the pi orbital to be lower energy than the sigma one. So, in molecular orbital theory, a pi bond can actually be formed in the p subshells only for such atoms. This is different from valence bond theory, where a sigma bond must be made before a pi bond in any situation.

3. Bonds in molecular orbital theory are simply the creation of new orbitals, and if the total number of electrons in the molecule is odd, then it is definitely possible to have only one electron in a bonding or antibonding orbital.  

@Pauline

Yep, you can do it right here on Khan Academy! Here's the site home: https://www.khanacademy.org/science/chemistry

However, if you just want to brush up and want some quick review, I would recommend the Bozeman Science Chemistry Videos, as they provide a rapid but still accessible overview of main topics. http://www.bozemanscience.com/chemistry

In terms of good textbooks, I would recommend the Zumdahl & Zumdahl textbook, which is excellent, (I think the publisher is National Geographic), but I don't know of any available pdfs. 

I would love to learn inorganic chemistry on khan acaddemy with practices and quizes.

Anyone know and free online textbooks or pdfs where I can start learning chemistry from scratch I would love to brush up all I know what an atom and nuclease is a c, f like single lettered atomic elemetnts names.

if in molecular orbital theory bonds are formed simply creation of new orbitals then why we calculate bonde order after a schematic representation of a molecule?

The bond order is needed to calculate the strength of the bond, and it relies on the concept of bonding and antibonding orbitals. We need to create the schematic representation with its bonding and antibonding orbitals to find the bond order.