God's Wrath Roused and Restrained

Why does God cast all of his wrath upon the Egyptians in the ten plaques and allow his own people who have provoked him and tempted him and spoke against him to escape his wrath? What accounts for God's own people receiving mercy in response to their sin and the Egyptians receiving wrath in response to theirs? And what is it exactly that make the children of Israel "his own people"?