Internet giant Google will buy US telecom maker Motorola Mobility for $12.5 billion in cash, giving a boost to its Android mobile operating system, the companies said Monday. The firms have entered into a definitive agreement under which Google will buy Motorola Mobility for $40.00 per share, a 63 percent premium over the closing price of Motorola Mobility shares on Friday, they said in a statement indicating their boards of directors had unanimously approved the deal.