3.5 Atomic Weight

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.  After all, she had just learned that weight had something to do with the earth and it didn't seem to fit.  Thankfully, a quick Google search said the same thing.  It was a term people have used so long that scientists still use it, even though it really doesn't fit.

"What do you know about that?" she said to herself.  "I got it right!"

After reading and re-rereading the textbook, she decided that the atomic weight of an element was some kind of average of the different forms a single atom could take. She vaguely remembered something Spruce said about the same element being able to have different numbers of neutrons in its nucleus.

"So that means one form of an element might have a slightly different mass number than another."  Sure enough, there was a word Spruce had used, an "isotope."  "An isotope is a form of an element with a different number of neutrons from its other forms."

So apparently if you average together the different mass numbers depending on how frequently they appear in nature, you get the atomic weight.  So most carbon atoms have 12 particles in their nuclei (the plural of nucleus)--6 protons and 6 neutrons.  But a smaller number of carbon atoms have 14 particles--6 protons and 8 neutrons.  The average came out to about 12.01 as the atomic weight of carbon, the average of the mass numbers of its various forms.

"Wait a minute," Stef said.  "What was I reading about carbon 12?"  She couldn't quite remember what she had read but she suddenly understood what carbon 12 meant.  "That must be the way of referring to a particular isotope of carbon.  Carbon 12 is a form of carbon that has 12 particles in its nucleus.  So carbon 14 would be another isotope of carbon, with 14 in its nucleus."

She also noticed that the atomic weight was even written under each elements' symbol...