Standard Form by Boxed Rockets

Are These Numbers in Standard Form?

5.98 \times 10^{6}

Yes

9.95 \times 10^{3}

Yes

0.604 \times 10^{-4}

No

0.178 \times 10^{4}

No

6.68 \times 10^{-3}

Yes

12 \times 10^{3}

No