Apple overtook Exxon Mobil as the world’s most valuable company for the first time Tuesday, as the computer company’s stock continued to defy a slowing market. The market value of the consumer electronics giant was $50 billion away from overtaking that of the oil titan when it reported yet another quarter of record-breaking earnings on July 19. The changing of the guard is due in part to a market slowdown that has depressed oil prices and taken Exxon Mobil stock down significantly.