Question

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

Short answer

Nickel (Ni) and Cobalt (Co) violate Mendeleev's periodic law in the fourth period.

Step-by-step solution

Understanding Mendeleev's Periodic Law

Mendeleev's original periodic law stated that elements were arranged in order of increasing atomic mass. According to this rule, the properties of elements should repeat periodically.

Identifying the Fourth Period Elements

The fourth period of the periodic table includes the elements with atomic numbers 19 to 36: Potassium (K), Calcium (Ca), Scandium (Sc), Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), Gallium (Ga), Germanium (Ge), Arsenic (As), Selenium (Se), Bromine (Br), and Krypton (Kr).

Checking for Mendeleev's Violation

Examine the fourth period elements to see if any violations of Mendeleev's law occur. A violation would occur if an element with a higher atomic mass precedes an element with a lower atomic mass.

Finding the Violating Elements

Within the fourth period, you find that Nickel (Ni), with an atomic mass of approximately 58.7 amu, precedes Cobalt (Co), which has a lower atomic mass of about 58.9 amu. This is a notable case where they are out of order by atomic mass.

Confirming the Elements

Confirm that Nickel (Ni) and Cobalt (Co) are the two elements in the fourth period that violate Mendeleev's original periodic law due to the reversed order by atomic mass.

Key concepts

Mendeleev's Periodic Law

Mendeleev's Periodic Law was a groundbreaking principle in chemistry. Dmitri Mendeleev, a Russian chemist, proposed this law in the 19th century. He suggested that when elements are arranged in order of increasing atomic mass, their properties recur periodically. This means that similar chemical properties would appear at regular intervals. Mendeleev's arrangement allowed him to predict the existence and properties of elements that had not yet been discovered, showcasing the power of his periodic table.

However, the simplicity of using atomic mass as the organizing principle came with its own challenges. As more elements were discovered, exceptions to Mendeleev's periodic law began to appear. Elements that seemed to break the order suggested by increasing atomic mass were identified, showing the need for further refinement in understanding atomic structure.

Atomic Mass

Atomic mass is a crucial concept in understanding the arrangement of elements. It is the mass of an element's atom, typically measured in atomic mass units (amu). This measurement includes the total number of protons and neutrons in an atom's nucleus.

Mendeleev originally used atomic mass to order the elements because it was the best metric available at the time. Most of the elements fell predictably into place when listed by increasing atomic mass. However, as our understanding of atomic structure has advanced, it became clear that atomic number, which increases with the number of protons, is actually the more accurate organizing principle.

• Atomic Number: Number of protons in an atom.
• Atomic Mass: Sum of protons and neutrons.
• Atomic Unit: Atomic Mass Unit (amu).

This crucial shift helped explain why some elements appeared out of sequence based on atomic mass alone.

Element Arrangement

The arrangement of elements in the periodic table is not arbitrary. Mendeleev initially arranged elements primarily by atomic mass, but modern periodic tables use atomic number. Why the change? This shift better reflects chemical properties because it aligns with the actual physical structure of the atom.

• Initial Arrangement: By increasing atomic mass.
• Modern Arrangement: By increasing atomic number.

In the case of elements like Nickel and Cobalt, it was discovered that although Nickel comes before Cobalt by atomic mass, Cobalt should precede Nickel due to its lower atomic number. This understanding corrected the sequence in the modern periodic table, making it not only more logical but also consistent with their observable chemical properties.

Fourth Period Elements

The fourth period of the periodic table is a fascinating segment to study. It includes elements with atomic numbers ranging from 19 to 36.

• Elements: Potassium (K), Calcium (Ca), Scandium (Sc), Titanium (Ti), and others up to Krypton (Kr).
• Atomic Numbers: 19 to 36.

These elements exhibit a range of properties and serve various roles in both chemistry and industry. Interestingly, it's in this period where we observe the order anomaly of Nickel (Ni) and Cobalt (Co). Although Nickel has a higher atomic mass, it precedes Cobalt in Mendeleev's arrangement, highlighting a key example where the use of atomic mass as a sorting criterion led to inconsistencies.

This discrepancy was an essential lesson and a stepping stone towards more accurate scientific methodologies, ultimately leading us to today's understanding that the properties of elements are best organized by atomic number.