Scientific notation allows us to write very large numbers in a clear and concise way. For example if you look at the number 1000000000000, it is not clear how many 0's there are! So a better way is to write it as 1012.
This means there are 12 zeros. The expression 1. x 10nmeans 1 multiplied by 10, n times. So:
1 x 101=10
1 x 102=100
1 x 103=1000
and so on.
That's fine if you want to write a large number like 10000000000. But suppose you want to write 16500000 ? The answer is you replace the 1 in the expression 1. x 107 by 1.65, so in this case the number would be 1.65 x 107
Notice that now the number of zeros is five, yet we have multiplied by 10 seven times. This is because the number n represents how many digits there are to the right of the first digit (the 1 in this case), whether they are zeros or not.