Standard Form by Boxed Rockets

Are These Numbers in Standard Form?

4.33 \times 10^{-6}

Yes

7.76 \times 10^{4}

Yes

7.2 \times 10^{-6}

Yes

31 \times 10^{5}

No

0.3 \times 10^{-4}

No

5.09 \times 10^{-3}

Yes