The rest mass of an electron is 0.000 000 000 000 000 000 000 000 000 000 910 938 kg. What is this written in Scientific Notation?

The rest mass of an electron is given as $0.000\,000\,000\,000\,000\,000\,000\,000\,000\,910\,938 \text{ kg}$.
Let's count the number of zeros after the decimal point before the first non-zero digit (9). There are 27 zeros.
If we move the decimal point to the right until it is after the first non-zero digit (9), we move it $27 \text{ (zeros)} + 1 \text{ (for the digit 9)} = 28$ places.
Since we moved the decimal point to the right, the exponent of 10 would be negative.
So, strictly based on the number as written, the scientific notation would be $9.10938 \times 10^{-28} \text{ kg}$.

All the options have an exponent of $10^{-31}$. This indicates a discrepancy between the number written in the question and the options. The actual rest mass of an electron is a well-known physical constant, which is approximately $9.10938 \times 10^{-31} \text{ kg}$. It is highly probable that the long form of the number in the question contains a typo and was intended to represent this actual value.

Assuming the question intends to refer to the actual rest mass of an electron, which is $0.\underbrace{000...000}_{30 \text{ zeros}}910938 \text{ kg}$.
To write this in scientific notation, we move the decimal point to the right until it is after the first non-zero digit (9).
We count the number of places the decimal point moves: $30 \text{ (zeros)} + 1 \text{ (for the digit 9)} = 31$ places.
Since the decimal point is moved to the right, the exponent of 10 is negative.
Thus, the scientific notation is $9.10938 \times 10^{-31} \text{ kg}$.

• $910938 \times 10^{-31} \text{ kg}$ (The 'a' part, 910938, is not