how many sigma bonds or pi bonds are used to join the 3 carbon atoms in this allene structure H2C=C=CH2

In the allene structure H2C=C=CH2, there are 5 sigma bonds and 2 pi bonds. Here's how:

1. Each single bond is a sigma bond. So, the bonds between the terminal carbons (C) and the hydrogens (H) are sigma bonds. There are 4 of these (2 on each end).

2. The bond between the central carbon and each of the terminal carbons is also a sigma bond. There are 2 of these.

3. Each double bond is composed of one sigma bond and one pi bond. In the allene structure, there are two double bonds, so there are 2 pi bonds.

So, in total, there are 5 sigma bonds (4 C-H and 1 C-C) and 2 pi bonds (in the C=C double bonds).