Compatible CPUs must:
-Have the same or lower TDP
-Have the same socket
Even if they have those two things, they still might not work if:
-They use a different lithography
-They use a different generation of core design
^ These factors depend on whether the laptop's BIOS has support for later generation CPUs, which OEMs like Dell don't typically bother to add. You could contact Dell for a complete list of supported CPUs.