P₄O₁₀ has ______ bridging O atoms

The correct option is 6.

To understand why phosphorus decaoxide ($P_4{O}_{10}$) has 6 bridging oxygen atoms, let us look at its chemical structure and bonding.

Phosphorus decaoxide has a cage-like structure based on a tetrahedron of phosphorus ($P$) atoms:
1. There are 4 phosphorus atoms situated at the corners of a tetrahedron.
2. Each of the 6 edges of this tetrahedron contains a bridging oxygen atom that connects two phosphorus atoms. These are the P-O-P ether-like linkages. Thus, there are exactly 6 bridging oxygen atoms.
3. Additionally, each of the 4 phosphorus atoms is bonded to a terminal oxygen atom via a double bond (P=O).
4. This gives a total of 6 (bridging) + 4 (terminal) = 10 oxygen atoms, matching the formula $P_4{O}_{10}$.

Therefore, the number of bridging oxygen atoms in $P_4{O}_{10}$ is 6.