The question asks for the most serious anomaly or limitation of Mendeleev's periodic table. We need to evaluate each option based on the historical context and scientific understanding of Mendeleev's time and subsequent discoveries.
A) Placement of elements in wrong order of atomic masses — e.g., Cobalt (58.9) placed before Nickel (58.7) despite higher mass. This is considered the most serious anomaly because it directly contradicted Mendeleev's primary organizing principle (increasing atomic mass) and indicated that atomic mass was not the most fundamental property for periodicity. This issue was later resolved by the discovery of atomic number.
The question asks for the Modern Periodic Law, which was established after Mendeleev's work and based on Moseley's discoveries. We need to identify the statement that correctly defines this law.
Correct Option: D) The properties of elements are a periodic function of their atomic numbers
The question asks about the number of periods and groups in the Modern Periodic Table. This is a fundamental concept in chemistry, requiring knowledge of the table's structure.
Correct Option: C) 7 periods and 18 groups
The question asks for the number of elements in the first, second, and third periods of the Modern Periodic Table. To answer this, we need to recall the structure of the periodic table and the rules governing the filling of electron shells, which determine the length of each period.
Correct Option: B) 2, 8 and 8
The question asks why elements in the same group of the periodic table exhibit similar chemical properties. This is a fundamental concept in chemistry related to the arrangement of electrons in an atom.
Correct Option: C) The same number of valence electrons (same outer electronic configuration)
This is the correct answer because the number of valence electrons dictates an atom's chemical reactivity and bonding behavior. Elements with the same number of valence electrons will participate in chemical reactions in similar ways, leading to similar chemical properties.
The question asks about the significance of the period number of an element in the periodic table. We need to relate the period number to the electronic configuration of an atom.
B) The highest principal quantum number (n) of the outermost occupied shell. This statement accurately defines what the period number represents. It signifies the number of the outermost main energy level (shell) where electrons are present.