Avogadro's Number, the Mole, and Molar Mass- Bohr and Quantum Atomic Models

Below, pick one topic that you feel confident in your understanding of and one that you find the most challenging to grasp:
- Avogadro's Number, the Mole, and Molar Mass
- Bohr and Quantum Atomic Models
- Ionic and Covalent Bonds

For the topic you understand well, explain the topic. You can explain through text, illustration, using an example. Envision that you are giving an explanation to someone who is not in our class and has never heard of your chosen topic.
For the topic you are struggling with, pinpoint exactly what you do and do not understand about the concept.

Example
Let's pretend I'm taking a hypothetical class on how to play baseball. And for this "week" in my hypothetical class I was taught about pitching and catching and the relationship between the pitcher and the catcher.
The topics we learned are: left-handed vs right-handed pitching, the distance between the pitcher and the catcher and how that affects pitching speed and strategy, non-verbal communication between pitcher and catcher, and how pitchers and catchers engage in fielding when necessary.
I'm good with how the distance between mound and home plate is important. I'm less clear and how non-verbal communication in baseball works.

I understand how distance between mound (where the pitcher is) and home plate (where the catcher is) is important. I envision a baseball kind of like a runner. Runners typically train for a certain distance race. Also, even for the same distance run you might moderate your speed if its a recovery or a training run. Pitches are similar. The pitcher knows what range of speeds they can work with given the distance from mound to home plate. If they are going to walk a player (not even give them a chance to hit the ball) they need a different (lower) speed than if they're aiming for a strike.

I understand that there are a variety of pitches in baseball and that the catcher must be informed about what pitch is coming. I am less clear on how they have their unspoken communication from such a far distance. How does their signaling system change from game to game or season to season? What happens if there is a last minute change in plans? I've never had to communicate nonverbally in such a way before so I don't have a way to relate it to my life.