9.05.3 Development of the Periodic Table

Syllabus

Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights.
The early Periodic Tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.
Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered and in some places changed the order based on atomic weights.
Elements with properties predicted by Mendeleev were discovered and filled the gaps.
Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.

Students should be able to describe these steps in the development of the Periodic Table.

What does this mean?

Early Scientists tried to find patterns in the elements they knew about.

One idea was to arrange them in order of the mass of their atoms, or according to their properties (what they reacted with)

But as the number of elements known increased people began to notice other patterns.

Triads

The word Triad suggests a group of 3.

In this case, Triads was the name given to groups of three elements with similar properties.

Mysteriously, the mass of the middle element in the Triad was almost exactly the average of the other two.

This was thought to be interesting but there weren't many triads, so no one really knew what to make of them.

Octaves

An English chemist noticed that if he arranged the known elements in mass order and chopped the list into columns of seven elements the Triads lined up

But this only really worked for light elements and ended up with Oxygen lining up with Gold and Iron, which it is not like at all.

So, he was ignored and humiliated so much by colleagues that he lost interest in Chemistry all together. Imagine!

Gaps

Dimitri Mendeleev arranged the elements in mass order but left gaps for new elements that hadn't yet been discovered.

He described what he expected these elements to be like when they were found and was proved to be right.

He also realised that mass order wasn't quite right, so he reversed the order of a few elements so that they fitted his table better.

He didn't know why they needed to move because no one knew anything about sub-atomic particles at that time.

Now we put the elements in order of Atomic Number.

This means that they are arranged in order of how many protons each element has.

Anomalies

Cobalt comes before Nickel because it has fewer protons, but it weighs a little more.

So, if the table was still arranged by mass the elements would be in the wrong order

But how is it possible for an element with fewer protons to weigh more than one with more protons?

Cobalt and Nickel both have several isotopes.

The most abundant isotopes of Cobalt are the the heavy ones, but Nickel has several stable light isotopes.

So, the average mass of a Cobalt atom is a touch more than the average mass of a Nickel atom.

Videos

Past Paper Questions

Answers

(gallium) fitted in a gap (Mendeleev had left)
(gallium’s) properties were predicted correctly (by Mendeleev)

allow (Gallium’s) properties matched the rest of the group 1

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