Chem Tool #1 · DE Bio 101 Unit 2 · Campbell Ch. 2 and 3

Bond Budget

An atom’s outer shell wants eight electrons, and hydrogen wants two. How many e- it is short is how many bonds it makes, and that number never changes: carbon four, nitrogen three, oxygen two, hydrogen one. Build seven molecules by spending each atom’s budget down to zero.

Tap one atom, then another, to bond them. Tap a bond again to make it a double. Tap a finished bond to remove it. The number on each atom is how many bonds it still needs. Don’t forget to practice drawing lewis dot structures, Mr. Schwebach can help with that. Valence is a concept to understand, if you need to create and master a SAM on this topic.

Once a molecule is finished you can turn it. Drag on it with a finger or the mouse and the whole thing rotates, lone pairs and all, so you can see the shape it actually takes in space.

carbon, 4nitrogen, 3 oxygen, 2hydrogen, 1 lone pair

Reading the model. Atoms nearer to you are drawn larger and brighter, atoms behind are smaller and dimmer — that is the only depth cue, so turn the molecule when you are unsure which way a bond runs. Two dots beside an atom are a lone pair: electrons the atom keeps rather than shares. Lone pairs take up room and push the bonds, which is why water is bent and carbon dioxide is not. On paper the same shapes are drawn with plain lines, a solid wedge coming toward you and a dashed wedge going back.

When all seven are built, look back at the last three. Each one hangs a small group off a carbon skeleton, and each of those groups is what gives a macromolecule its behaviour in water. That is where Unit 3 begins.

Do this away from the screen

On paper, draw the Lewis structure for ethanol, then redraw it in its real shape: tetrahedral at both carbons, bent at the oxygen, with both lone pairs on the oxygen shown. Mark which end of that molecule water will stick to. Do it without looking anything up. If you stall, the atom you stalled on is the one to go back to.