Building a Timeline
Democritus proposed that matter is made up of atoms and empty space around 400 BCE. John
Dalton began the development of modern atomic theory in the early 1800s. That’s roughly 2200
years during which chemists made no progress in understanding the structure of matter. In the 200
or so years since Dalton, many scientists have made contributions to improve our understanding.
In grade 9 you learned a simplified version of the history, culminating with Bohr’s model. Today
we consider Bohr’s model to be a first-order approximation, meaning that it is accurate enough to
explain some observations, but not all, and that it is in part built around assumptions. For certain
parts Bohr knew what the model should be, but not why it should be that way. Even at the time
people understood that while the Bohr model was useful and sometimes effective, it was not the
complete picture. It explained the structure of and predicted the behaviour of hydrogen, but failed
when applied to any other element.
To improve the effectiveness of the atomic model, scientists soon realized that the conventional
ways of visualizing objects and their movement weren’t going to work. Through the first half of the
20th century, researchers worked at developing a new and deeper understanding of what particles of
matter actually are and how they behave at the atomic and even subatomic scales.
In class, we reviewed the familiar scientists from grade 9 and their contributions to atomic theory.
We went into more depth for each one, describing not only what they discovered, but how they did
it. From there, we continued the story by introducing what we today call quantum mechanics, the
set of physical laws that predict and explain the behaviour of the very small. Having completed our
discussion, you should now understand and be able to explain what scientists identify as the
quantum mechanical model of the atom – Bohr’s proposed electron energy levels have become
energy shells, divided into subshells, each with a number of possible atomic orbitals. In this activity,
you will build a detailed timeline of atomic theory development, identifying the key contributors and
describing how each one moved our understanding forward.
PROCEDURE
Use the information you’ve learned in class and from your workbook or researched online to
construct a timeline. What to include and to what depth is up to you, but your design should include:
 The name of the key people involved in the development of atomic theory
o Provided in relative order – who followed who rather than by specific dates
 A description of each person’s discovery or contribution
o Include enough detail to show you understand it, rather than simply stating it
 A mix of writing and relevant illustrations
o Some things need a description, while others need an illustration
 Visual appeal and depth
o Decorate your piece and add detail. If you don’t like formal written tests,
embrace this activity as a different opportunity to show off what you’ve learned
Windsor Chemistry
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