Before the 1930s, most scientists assumed that atoms were simply made of protons and electrons. So a hydrogen and a helium atom placed side by side would look something like this:


But scientists measured the mass of a hydrogen atom to be 1.00 AMU and measured the mass of a helium atom to be 4.00 AMU. It didn’t make sense! How could the double the particle have quadruple the mass? It was almost as if something were missing…

The particles that were absent from earlier models are now called “neutrons.” It is a neutral particle with no magnetic charge and it has roughly the same weight as a proton. To figure out the number of neutrons in any normal atom, you simply take the atom’s atomic mass, round it off, and subtract the number of protons. Using this method it is easy to figure out that helium has two neutrons, but hydrogen doesn’t have any.


Today in class, students practiced making these calculations on Page 210. When they were done, they read about the weird, wild world of isotopes (atoms who don’t have the normal number of neutrons) and their applications.

October 14 – Neutrons (pg210)

To complete tonight’s homework, please click here. Then select: 20 questions; protons, neutrons, & electrons; and select “No rounding.” After answering the 20 questions, print out your score report. You will hand it in tomorrow as your HW grade.

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