What are the rules to observe to follow the writing numbers express in correct the scientific notation?

1) Move the decimal until your number appears to be between 1 and 10 count the spaces that you move the decimal.

2) use the number of spaces as the exponent of 10 (the base)

3) if the original number was greater than 10, the exponent is positive, if the original number was less than 1 then the exponent is negative.

EX: 250,000 becomes 2.5 x 105

but 0.0025 becomes 2.5 x 10-3