pat (someone or something) on (something or some place)
To tap someone or something on a particular spot gently with something flat, usually one's hand. He patted me on the shoulder when it was my turn to go. He patted his car on the hood with the book he was holding to show me how sturdy the old thing was, and the bumper ended up falling off.
pat someone or something on something
to tap someone or something on a particular place with the open hand. She patted the child on the bottom. I patted the car on its hood to show how proud I was of it.