Question

Which two elements in the fifth period violate the original periodic law as stated by Mendeleev?

Short answer

Tellurium (Te) and Iodine (I) violate Mendeleev's periodic law in the fifth period.

Step-by-step solution

Understand Mendeleev's Periodic Law

Mendeleev's original periodic law organized elements by increasing atomic mass, where elements with similar properties recur at regular intervals.

Identify the Fifth Period

The fifth period of the periodic table contains the elements Rubidium (Rb) to Xenon (Xe).

Locate Elements Out of Order by Atomic Mass

Within the fifth period, look for any anomalies where atomic mass does not increase consistently. In this period, Iodine (I) and Tellurium (Te) are exceptions; Tellurium comes before Iodine despite having a higher atomic mass.

Determine the Violation

The elements Tellurium (Te) and Iodine (I) violate the periodic law based on atomic mass because Te, with an atomic mass of 127.60, precedes I, which has an atomic mass of 126.90.

Key concepts

Mendeleev's Periodic Law

Mendeleev's Periodic Law was a groundbreaking step in the understanding of chemical elements. First introduced by Dmitri Mendeleev in 1869, it organized elements based on increasing atomic mass. This approach revealed that elements with similar properties appeared in regular intervals. Mendeleev's table allowed for the prediction of undiscovered elements by leaving gaps where the elemental pattern suggested that some pieces were missing. These gaps not only supported future discoveries but also showcased the underlying order in atomic interactions and properties.

Despite its brilliance, Mendeleev's initial law had some limitations, as atomic mass was not the perfect property to organize all elements. Today, the modern periodic table has adapted Mendeleev’s vision by organizing elements based on atomic number - the count of protons in an atom's nucleus - rather than atomic mass. This modified table aligns better with observed properties of elements and corrects the few mishaps seen with Mendeleev's original design.

Anomalies in the Periodic Table

The periodic table, as imagined by Mendeleev, was not without its anomalies, particularly when organized by atomic mass. One of the most notable anomalies involved the elements Iodine (I) and Tellurium (Te). Under Mendeleev's law, elements were to be in order of increasing atomic mass. However, Iodine, with an atomic mass of 126.90, follows Tellurium, which has a slightly higher atomic mass of 127.60.

This apparent contradiction was perplexing until the later discovery that atomic number, rather than atomic mass, dictated the chemical properties of elements. The switch to organizing by atomic numbers elegantly resolved such anomalies, positioning Iodine and Tellurium in line with their chemical behavior rather than their atomic weights. This demonstrated the robustness of the periodic table and its ability to adapt to new scientific insights.

Fifth Period Elements

The fifth period of the periodic table is an interesting segment that stretches from Rubidium (Rb) at atomic number 37 to Xenon (Xe) at atomic number 54. This period includes elements that span different groups and exhibit diverse chemical properties.

Exploring the fifth period reveals several transitions in element types, including metals, metalloids, and non-metals. It features:

• Alkali metals like Rubidium, known for their high reactivity and softness.
• Transition metals, including elements like Yttrium and Zirconium, which are valued for their strength and corrosion resistance.
• Lanthanides, which are rare earth elements that exhibit unique magnetic and luminescent properties.
• Noble gases such as Xenon, which are colorless, odorless, and largely inert.

Understanding the behavior and properties of the fifth-period elements further highlights the periodic table's capacity to organize and predict chemical characteristics efficiently.