# Question Video: Describing the Periodic Table as a Developing Scientific Model Chemistry • 7th Grade

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? [A] Wrong [B] Correct [C] Flawed [D] Unscientific [E] Incomplete

04:03

### Video Transcript

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.”