The periodic table is an example of
a model. It allows scientists to make
predictions by highlighting patterns in the properties of elements. The discovery of new elements
allowed scientists to fill gaps and correct mistakes in the original periodic
table. Which of the following words best
describes the model used to construct the original periodic table?
Arguably, the very first periodic
table came in 1863 from Alexandre-Émile Béguyer de Chancourtois, who put elements on
a spiral on a piece of paper. Elements were arranged on the
cylinder left to right by relative atomic mass and arranged vertically using the
spiral by chemical behavior. However, this version isn’t
generally considered a traditional table.
Traditionally, the original
periodic table is that of Dmitri Ivanovich Mendeleev in 1869, which arranged
elements bottom to top by relative atomic mass and left to right in periodic
chemical behavior. In 1871, Mendeleev produced a
revised version where periods went top to bottom and groups went left to right, more
like our modern periodic table. The key feature that distinguished
Mendeleev’s system from previous systems was that he left gaps using existing data
to predict the properties of unknown elements. This made Mendeleev’s table a very
good model because it allowed for accurate prediction. New elements likes scandium,
gallium, and germanium were discovered later and inserted naturally into the gaps,
fitting the predictions very closely.
Now, let’s have a look at the
question. We need to look at five words and
find the one that best describes the model used in the original periodic table. These three organizing principles
constitute the model used to make the table. Now, it would be perfectly accurate
to say that Mendeleev’s table was wrong. There were lots of things that have
since being changed.
But the question isn’t just asking
for any description. We’re looking for the best
description, one that does justice to the great work that it was. So the original table was wrong in
some respects. But it was also correct in many
respects. It would also be fair to say that
Mendeleev’s tables were fundamentally flawed because they use relative atomic mass
rather than atomic number as we use today.
However, based on the data of the
time, tellurium and iodine were the only pair of elements that seemed out of
sequence. Tellurium had a higher relative
atomic mass, but its chemical behavior meant it fit better if it was before iodine
rather than after. What would be unfair is to call
Mendeleev’s tables unscientific because they reflected insight into the data
available at the time.
The fact that Mendeleev left gaps
suggested by the data and the fact that he switched round tellurium and iodine
despite it not fitting the relative atomic mass principle suggests that he was
genuinely thinking about what he was doing. He didn’t want to just make the
data fit his theory.
The last word that we could apply
sensibly to Mendeleev’s tables is simply “incomplete.” It was made before we understood
atoms in any more detail and before we understood protons and their impact on
chemical behavior. Out of all the answers, this is the
fairest. While there were wrong and correct
and a flawed aspects to the table, it was a step in the right direction, a decisive
turn in our understanding of the elements.
As with many scientific models and
theories, development happens in stages. And we don’t necessarily need to
discard a model just because it’s not perfect. So of the five words we’ve been
given, the one that best describes the model used to construct the original periodic
table is “incomplete.”