I reversed gnd and vin...

[BluePhoenix]

I've connected the gnd to 7.2v and the pin vin to the minus of the battery... as result the chip near the DC power connector blow up!
Ok... now I have two questions
1-Which is the name of the chip? There's written something like 113nf
2-Is that chip the only thing I broke?

I'm so afraid T_T

[BluePhoenix]

I'm using an arduino UNO r3 

[CrossRoads]

Post a picture. I find this works well: copy from the product page, paste into powerpoint, draw an arrow, save as .jpg.
Then clickk Additional Options, Browse to your file, and Attach it.

[BluePhoenix]

Post a picture.

Thanks for your reply, I hope to find a solution in order to not buy another arduino

There are 4 arrows in the left-bottom corner

[retrolefty]

You  likely damaged  other stuff on the board sensitive to having reverse polarity applied. It will take some time and expense to replace the obviously bad component(s) only to see that the board still doesn't work.

That is too bad because if you had applied the reverse polarity to the external power connector instead of directly to the shield Vin pin you would have suffered no damage because the arduino folks installed a reverse polarity protection diode between the connector and the Vin trace just for such newbie type of mistake.

[CrossRoads]

That part is the 5V regulator.
http://www.digikey.com/product-detail/en/NCP1117ST50T3G/NCP1117ST50T3GOSCT-ND/1967217
Remove it, then measure with an ohm meter between +5 and Gnd. Should see a pretty high resistance, like 10s of K-ohms. If not, remove the 328 chip & measure again. If still low, then something else surface mount is damaged - could be the 16U2 USB/Serial chip, the 3.3V regulator, the op-amp, the FET T1 that does the autoswitching.
http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf

[BluePhoenix]

Ok... i removed the burnt chip and now (via usb and via 5v pin) it works! In a few days I will buy some reg sot223 5v
I will notice you about what will happen

[CrossRoads]

Make sure the part you purchase has the same pinout as what's on the board.

[BluePhoenix]

The only reg I found here, in Italy is a sot-223 5v BUT the pin out is input-gnd-output  :~ :~ :~

[CrossRoads]

Keep looking then. Or, use a leaded part, and bend the pins to end up where needed.
Or get a 5V wallwart, and jumper over the regulator pads (and reverse polarity diode).
Or get a USB wallwart, and power the board thru the USB port.