Question

Why did Mendeleev not include radium in his periodic table of \(1871 ?\)

Short answer

Radium was not in Mendeleev's 1871 periodic table because it wasn't discovered until 1898.

Step-by-step solution

Understand Mendeleev's Periodic Table

Dmitri Mendeleev created the periodic table in 1869 and published an improved version in 1871. The table was organized based on increasing atomic weights and the periodic recurrence of elements' chemical properties.

Consider the Element Discovery Timeline

Radium was discovered by Marie Curie and Pierre Curie in 1898, which is 27 years after Mendeleev's table was published. In 1871, radium had not yet been discovered.

Recognize the Limitation of Available Information

Mendeleev could only include elements that were known at his time. Discoveries of new elements often depended on advances in technology and scientific methods, which were not available in 1871.

Conclusion on Radium's Exclusion

The table could not include radium simply because it had not been isolated or identified until much later. Hence, any table or classification system by Mendeleev would not account for unknown elements like radium.

Key concepts

Atomic Weights and Mendeleev's Periodic Table

Dmitri Mendeleev's ingenious approach to the periodic table relied heavily on the concept of atomic weights. In the late 19th century, chemists used atomic weights, an early iteration of today's atomic masses, to order elements. At that time, the atomic weight of an element was the best measure they had of its properties.
Mendeleev noticed that by arranging elements in the order of increasing atomic weight, certain patterns began to emerge. These repeating patterns or "periodic" characteristics allowed Mendeleev to group elements with similar properties together, thus forming the basis of his periodic table.
This organization revealed spaces left for elements that had not yet been discovered but were predicted based on the trends in atomic weights and properties. Mendeleev's ability to predict these gaps demonstrates the significance of atomic weights in classifying elements at that time.

Chemical Properties and Element Classification

Chemical properties are what Mendeleev used to group elements in his periodic table. These properties include reactivity, the type of compounds elements form, and how elements behave under certain conditions. Mendeleev observed that elements with similar chemical properties occurred at regular intervals when ordered by atomic weight.
This allowed him to align elements into columns or "groups" bearing similar chemical behaviors, something crucial for chemists when predicting how elements would react with one another. By prioritizing similar chemical properties over strict numerical order of atomic weights when needed, Mendeleev sometimes placed elements out of order by atomic weight to preserve these groups.
This approach emphasized the importance of chemical properties and showed how they could be used in conjunction with atomic weights to form an effective system for categorizing elements.

Element Discovery Timeline and its Impact on Mendeleev's Periodic Table

The timeline of element discovery played a critical role in the composition of Mendeleev’s periodic table. As of 1871, many elements, including radium, had not yet been discovered due to limitations in scientific techniques and understanding at the time.
Advances in technology over the succeeding decades, such as improved spectroscopy and radioactive decay experimentation, allowed scientists like Marie Curie to discover elements like radium in 1898.
Mendeleev's table was thus only as complete as the era's scientific knowledge allowed. Each discovery after 1871 filled a gap or confirmed the predictions Mendeleev initially made based on atomic weights and chemical properties. Told through the lens of historical advancements, the element discovery timeline underscores why certain elements like radium were absent from early versions of the periodic table.