Everyone knows the surname 김, which is 金 in hanja. However, 金 is written as 금 everywhere except the surname, e.g. 금요일 Friday, 금성 Venus, 백금 platinum, 합금 alloy, 기금 fund. Why is the surname 김 written differently?
Like Chinese and Japanese, Korean also has two or more ways (음훈) to pronounce one Chinese character.
The Chinese character '金' is pronounced as '김' only when it is used as a family name and some city names such as 김해시(金海市), 김포시(金浦市). In other context, it is always pronounced as '금'. That's the way it is and the family name is not the only name that is pronounced as '김'.
The first person who used the family name '김' is 김수로왕 and it has been around since AD 42 when he was born. According to the myth, six golden eggs came down from the sky and 김수로왕' was born out of one of them.
Another interesting Chinese character that is pronounced in two ways is 不:
아닐 불,아닐 부: 불가피, 부동산