This is an extension of my post Difference between תולדות אדם and תולדות שם. When looking at תולדות אדם we see that it starts with אדם and ends with Noach and the birth of his three sons. On the other hand, the build up to אברהם starts at Shem and ends with Terach and the birth of אברם נחור והרן as well as the birth of their children. Similarly, when נח dies, he is spoken of in the exact way that the rest of the people listed in תולדות האדם. Avraham on the other hand, begins the complete beginning of the narrative and is never linked to the previous generations. In fact, the first we hear of him is to be told how he had to leave and separate himself from all that went before. The Torah shows this by the names of the Parshios as well as the name of the people involved. נח means rest, staying the same and is the end of the progression that takes the world past the mabul. לך לך means movement, separation and going to the future. Similarly, אברהם is given the nam אב המון גוים which means that he is the beginning of the future and the start of the next phase of history rather than the end of the previous stage.