Yin Yang in center of atom on his shoulder.
The Yin-Yang represents the ancient Chinese understanding of how things work. The outer circle represents "everything", while the black and white shapes within the circle represent the interaction of two energies, called "yin" (black) and "yang" (white), which cause everything to happen and they cannot exist without each other.
"According to a 2006 Pew survey, 40% of Americans between the ages of 26 and 40 have been tattooed".