Comparing Old and New Atomic Theories

Early Ideas About Atoms

1. Democritus (460-370 BC):

   • He was the first to suggest that everything is made of tiny, indivisible particles called "atomos."
   • His ideas were mostly based on thoughts rather than real experiments.

2. Dalton's Atomic Theory (1803):

   • Dalton brought a more scientific approach to understanding atoms.
   • Key Points:
     • Everything is made of atoms.
     • Atoms of the same element are identical in weight and properties.
     • Compounds form when different atoms join together in specific ratios (for example, water, or H₂O, has 2 hydrogen atoms and 1 oxygen atom).
   • However, he couldn't explain the existence of isotopes or smaller particles inside atoms.

3. Thomson’s Model (1897):

   • Thomson discovered the electron and created the "plum pudding model."
   • He suggested that atoms consist of electrons scattered in a positively charged "soup."
   • This was the first time scientists had evidence of particles even smaller than atoms, leading to the understanding of the electron's charge and mass.

4. Rutherford’s Model (1911):

   • Rutherford used the gold foil experiment to show that atoms have a small, dense center called the nucleus.
   • He proposed that most of an atom's mass and positive charge is in the nucleus, with electrons orbiting around it.
   • The nucleus is very tiny, about 1/100,000th the size of the whole atom!

5. Bohr Model (1913):

   • This model explained how electrons travel around the nucleus.
   • Bohr introduced energy levels, where electrons are in specific paths with certain energies.
   • He had a formula to show these energy levels, but it was quite complicated.

Modern Atomic Theories

1. Quantum Mechanical Model (1926-present):

   • This theory is based on the rules of quantum mechanics.
   • Instead of fixed paths, electrons exist in areas of probability, called orbitals, shown by wave functions.
   • This model is much better at predicting how atoms behave in chemical reactions. It explains things like how electrons can spin and mix together.

2. Key Facts:

   • In today's view, the nucleus is made of protons and neutrons, which are called nucleons. The ratio of neutrons to protons is stable in many elements (for example, Carbon-12 has an equal amount of both).
   • With new tools like atomic force microscopy, scientists can even see individual atoms, helping us understand atomic structure better.

Summary

As we moved from ancient ideas about atoms to today's quantum mechanical model, our understanding has grown a lot. Now, we have strong evidence and advanced math to explain how atoms work, giving us knowledge that earlier thinkers couldn’t have imagined.