Remember, scientific notacion is used to express numbers with a lot of digits, numbers that represents things like the diameter of the Earth, or the diameter of an atom.
To write scientific notations, we have to express a whole part wich most be more than zero but less than ten. Also, we have to define a power base of 10, and at last, we have to define an exponent for that power, where the exponent is going to tell us the number of digits the number has.
Therefore, the three parts are whole number, base 10, and exponent.