A hydrogen atom has a diameter of about 10 nm. Express this diameter in meters. Express this diameter in millimeters. Express this diameter in micrometers.

So, the first question is: how many meters are 10 nm?

1nm = 0.000000001 m.

So 10 nanometers are 0.00000001 m!

Now, how many milimeter are those?

let's start with meters, 1 meter are 1000 milimeters.

so

0.00000001*1000=0. 00001 m!

now, micrometers. 1 micrometer are 1000 nanometers.

so 10 nanometers are 0.01 micrometers! (1 nanometer is 0.001 micrometers)