In this lesson, we will learn:
• The contributions of key scientists which led to the modern Periodic Table.
• An example of how a scientific theory relies on measurement and a standardized method.
• The key features of a scientific theory and its emphasis on empirical observation and prediction.
- Like the development of atomic theory, developing the Periodic Table has taken time and contributions by many scientists, each with their own theories and experiments, to lead to its current state today.
- The early work built on John Dalton's work, which tried to identify elements by their unique mass because this was the most obvious property scientists could measure. Because quality of equipment and analytical methods were poor and there was no standardized, 'proper' way to measure atomic mass, there were inconsistencies in different scientists' measuring the mass of elements. This held back progress; to organize, scientists measuring events or objects need consistency to spot any patterns emerging.
- The first move toward anything resembling the current Periodic Table was by Johan Dobereiner. He showed that the appearance and the reactions of certain known elements were quite similar. Because these certain elements with similarities came in threes, he called these groups triads. Some of his triads (A triad of Li, Na, K and a triad of Cl, Br and I) survived and now form groups in the current periodic table!
- In the 1860s, John Newlands showed by ordering the elements by mass, with hydrogen first, every eighth element had similarities in its properties. He called this the law of octaves. This was seen as a breakthrough in the arrangement of the elements – it was the beginning of the term 'periodic' being used to describe the elements, meaning a repeating pattern. However, his ordering had inconsistencies, such as placing metals in an octave with non-metals. This led to the suspected existence of undiscovered elements.
- In 1869, Dmitri Mendeleev published his work where he organized the elements according to properties and their masses. It received very little attention to begin with, but it was noticed after being republished. Mendeleev also noticed that when elements were ordered by mass, there was a periodic (repeating) pattern of chemical properties. The genius in Mendeleev's work was in doing the following:
- He organized the elements by row (called a period) and by column (called a group), where the groups showed the elements that had common properties.
- He chose to move some elements around the table, prioritizing grouping elements by their common properties, not ordering by their mass.
- He deliberately left gaps in his table where he supposed the existence of undiscovered elements. He even suggested the properties of these elements, using the properties of the groups in his table. When discovered, they matched.
- Mendeleev's table forms the basis of the current Periodic Table. The only major exception to his work – the existence of the noble gases – slotted in at the end of the table (group 0) when discovered. This didn't affect the pattern in his table.
- Mendeleev's Periodic Table is a classic example of a successful scientific theory. For any theory to be scientific, it needs to:
- Be able to explain the current empirical evidence. Does it explain why we see what we see in experiments and in the observable world?
- Be testable by future experiments. If a theory and its supporters want to be proved right, we need to be able to plan and do experiments that prove it at all!
- Be able to explain new evidence when it appears. Scientific theories need to account for all evidence; they can't 'pick and choose' when to work.
- If a theory can't explain new evidence, it needs to be either revised or replaced by another theory that is also testable and does explains the evidence so far!