Explain why astronomers prefer that cities use sodium street lights instead of mercury street lights.

This is because red tinted lights are easier on your eye. If you want to see the stars your eyes first need to get used to the darkness. Since the sodium in a lamp gives an orange-ish tint this is better for your eyes.
Another reason could be that sodium lights are less bright and don't pollute the sky with as much light which also makes it better for seeing stars.

You can look up pictures of the streets lights to see the color difference.

Ithink chemical bonding