Classification of Elements and Periodicity in Properties Notes: Class 11 Chemistry Chapter 3
Class 11 Chemistry Chapter 3 Classification of Elements and Periodicity in Properties covers the development of the periodic table, the modern periodic law, and periodic trends in atomic properties.
Overview of Topics & Subtopics (Click to Jump)
1. Development of the Periodic Table
Short Note:
• Mendeleev's Periodic Law: properties of elements are a periodic function of their atomic masses (had limitations, e.g., placement of isotopes).
• Modern Periodic Law: properties of elements are a periodic function of their atomic numbers, which resolved the anomalies of Mendeleev's table.
2. Nomenclature of Elements with Atomic Number > 100
Short Note:
• IUPAC has recommended a systematic nomenclature using numerical roots for elements with atomic number greater than 100, until their names are officially confirmed.
3. Periodic Trends in Properties
Short Note:
• Atomic Radius: decreases across a period (left to right) due to increasing nuclear charge, and increases down a group due to addition of new shells.
• Ionization Enthalpy: generally increases across a period and decreases down a group.
• Electron Gain Enthalpy: generally becomes more negative across a period, and less negative down a group.
• Electronegativity: generally increases across a period and decreases down a group.
4. Periodic Trends in Chemical Reactivity
Short Note:
• Metallic character decreases across a period and increases down a group, while non-metallic character shows the opposite trend.
NCERT Textbook Exercise Questions
What is the basic theme of organisation in the periodic table?
Answer: The basic theme is to classify elements in such a manner that similar elements are grouped together, based on the periodicity of their properties as a function of their atomic number (electronic configuration).
Arrange the following in order of increasing atomic radii: Na, Mg, Al, Si.
Answer: Since atomic radius decreases across a period from left to right, the increasing order is: Si < Al < Mg < Na.
Explain why the ionization enthalpy of nitrogen is higher than that of oxygen, though oxygen has a higher atomic number.
Answer: Nitrogen has a half-filled 2p³ configuration, which provides extra stability, making it harder to remove an electron. Oxygen has a 2p⁴ configuration where electron-electron repulsion in a paired orbital makes it slightly easier to remove an electron, so O has a lower ionization enthalpy than N despite being heavier.
Among the elements B, Al, C and Si, which has the highest first ionization enthalpy?
Answer: Carbon (C) has the highest first ionization enthalpy among these, since ionization enthalpy generally increases across a period and C is further right/up (smaller size, higher effective nuclear charge) compared to B, Al and Si.
Why are noble gases said to have zero electron gain enthalpy in the general trend discussion?
Answer: Noble gases have a completely filled stable electronic configuration, so they have no tendency to accept an additional electron, making their electron gain enthalpy close to zero (or slightly positive).
