When elements combine, they form molecules. Simply put, a molecule is a group of two or more atoms chemically bonded together. ... Certain molecules are also compounds. A compound is a group of two or more different atoms chemically bonded together.
Coal and oil.... Fossil fuel is formed from decaying plants/animals who were buried and eventually converted to crude oil, gas by the exposure to heat and the pressure of the earths crust. This takes about 1 million years to happen.