Mercury Hg

This isn't as hard as the high ratio would suggest. The 20 bonus levels are each unlocked by collecting all bonus atoms from certain levels, and thereby unlocking the "anti-" version of that element's level.

In these bonus levels, you start with a small amount of Mercury and collect vials of it as you progress through the level. You must complete the level with 100% Mercury, meaning you MUST collect every vial and MUST NOT lose any Mercury along the way.

The vials are (I believe) in the same positions as the bonus "atoms" are in the regular levels.

Many of these levels are very easy. The biggest tip for the harder ones is to take your time. There is no time limit to worry about, so go slow and easy, be careful and accurate. Don't forget you can use the A button to bring your mercury together if bits become detached (Thanks, TheDarKnyht). Obviously one consolidated puddle is going to be far easier to manage, although only a bit of it needs to touch the finish as long as all 100% is still in the level.

Also consider the order in which you collect the vials. Some are in positions that are easier to get at if you have a smaller puddle of Mercury, so get those first, bypassing any others along the way until later.

Follow these tips, and persevere with those levels that are a bit tricky, and you should be able to complete all 20 levels in a couple of hours.

Good luck!