They are physically notched different, so that one cannot be firmly seated or inserted into the other.
The Slot layout.
the different notch positions keep someone from installing a DDR DIMM or DDR2 DIMM in the wrong memory slot.
A DDR2 memory stick cannot be installed correctly in a DDR slot.
The two communicate in different ways, so they are incompatible software-wise. They are also physically incompatible due to a different pin arrangement, different notch locations, and different voltages.
No
DDR3 uses 1.5V; DDR2 uses either 1.8V or 2.5V
No DDR2 and DDR are not compatible and so you can not add DDR2 RAM module in a DDR Slot. Electrical specifications like voltage, signal pinout and even mechanical size and shape of modules are different.
ddr3
240
2.66" 200-pin SO-DIMM contains DDR2 SDRAM. One notch is near the side of the module.
DDR3 uses lower voltage
Memory Ram