3.4 Mass Number

That was pretty much it for chemistry at supper.  After supper, Spruce dropped Stef off at her house and headed back to Indy.

Back in the room, it occurred to her that she already pretty much knew everything Spruce talked about at supper.  But they hadn't talked about what she didn't understand, like atomic mass.  She didn't have anything else to do, so she decided to give it one more shot.

The textbook confused her, "The way scientists calculate the mass of atoms is in terms of the atomic mass unit, which is defined as one twelfth the mass of a Carbon 12 atom."

"What the heck is a Carbon 12 atom?" she asked herself.  She decided it to google it instead.  The blog of some 45 year old janitor at a community college came up.  "Basically, a proton weighs one atomic mass unit or 'amu.'"

"So a proton is one atomic mass unit," she said to herself and kept reading.

"It's not exact," the blog continued.  "A proton weighs just a little more than 1 amu, and a neutron weighs just a tad more."

"Oh, great."  She decided she didn't care.  She was going to think of a proton or neutron weighing one atomic mass unit.

She went back to the textbook.  "The mass number of an atom is the total number of protons and neutrons it has."  Mass sounded familiar.  "Isn't that the same as weight?" she asked herself.

She googled it again: "Is mass and weight the same?"  This time the blog of someone who worked at a Starbucks came up.  "Mass and weight are not the same.  Weight has to do with gravity and depends on how high or low you are on the earth.  In space you have no weight."

"That's confusing," Stef said to herself again, but she kept reading.

"Mass is, like, how much stuff is there, which is the same anywhere, even in outer space."

That made a little sense to her.  I need to put this in the notebook, he suddenly thought, before it all evaporated out of her mind.  She wrote down everything she'd learned so far that night.

electron: very small particle with negative charge, surrounds the nucleus in fields

proton: particle in nucleus with a positive charge, has a mass of about one atomic mass unit

neutron: particle in nucleus with a neutral charge, just a little bigger than a proton with a mass of about one atomic mass unit

atomic number: the number of protons in an atom's nucleus, tells you what element it is

mass number: the number of protons and neutrons in an atom's nucleus

atomic mass unit: the way they calculate how much an atom "weighs" (she couldn't think of a better way to put it), one "amu" is about the mass of a proton

She felt pretty good about the list because she pretty much understood everything on it.  There was just one more key term she couldn't figure out, atomic weight...

3.2 Protons, Electrons, Neutrons

"I even remember what's in an atom," Stef continued.

"Protons, electrons, and neutrons, of course," Spruce threw in before she could continue.  She curled her lips in a way that said she was contemplating throwing something at him.  "Ah, but do you remember which particles are in the nucleus and which ones are swirling around the space above like planets around the sun."

"HA!" She fired back.  "The electrons outside the nucleus don't really swirl.  They're more like force fields and you never know exactly where they're at.  You only can say where they are probably at, so there."

"And which particle is the biggest and which is the smallest?" he asked with a smirky grin she wanted to smack off his face.

"The electron is way smaller than the other two and has a negative charge," she answered confidently.

"Yeah, so which particle in the nucleus is bigger?" he persisted.

"Ah," she hesitated.  "The proton has a positive charge and the neutron doesn't have a charge.  It's neutral."

"That's not what I asked," he came at her again.  After she continued to play with her pasta, he finally answered it himself: "It's the neutron.  In fact, it's kind of like a proton and electron put together."

She completely ignored him, not willing to give him the satisfaction of knowing something she didn't.

"So the number of which particle tells you what element it is?" he continued.  She was not at all in the mood by now.  She began to think about her yoga class and some of the ridiculous stretches she was working on for college credit.

Meanwhile, Spruce continued to talk to himself...