Grey's Anatomy: Physical Attraction... Chemical Reaction

George and Izzie worry that they don't have any romantic chemistry. Derek asks for more than a physical connection from Meredith. Much to his own surprise, Mark develops a crush on a Erica Hahn. Erica refuses to let Cristina in on her surgeries. Callie's role as Chief Resident is placed in jeopardy after she delegates much of her job to Bailey. Richard walks in on Derek and Meredith having sex. Lexi continues to try and get closer to Meredith, despite Meredith pushing her away.