Chapter 9 · Electrons in Atoms and the Periodic Table
9.1Blimps, Balloons, and Models of the Atom
7 min · two checks
Predict
Why did a planetary model of the electron fail, even though it looked like a solar system?
The idea
- Explain why models of the atom changed.
- Separate a useful picture from a literal one.
Chemists keep a model when it predicts and retire it when it cannot. Dalton’s indestructible atom explained mass laws and could not explain ions or spectra. The nuclear atom explained the gold-foil experiment and said nothing about why electrons do not fall inward. Bohr’s orbits explained hydrogen’s colors and failed for larger atoms. The quantum-mechanical model — orbitals as regions of probability — is the one this chapter uses because it organizes the periodic table.
You will still draw Bohr-like pictures sometimes, the way a map draws a city as a dot. The drawing is a tool. The claim of this chapter is narrower and stronger: electron arrangement, not atomic mass, is what makes the columns of the table behave alike.
Keep these
- Models are judged by what they predict.
- Classical orbits cannot explain stable atoms or multi-electron spectra.
- The periodic table is an electron-structure table.