Classification Of Elements and periodicity In Properties most Important Questions For exam 2026

Most Repeated Questions from the Chapter: Periodic Classification of Elements

Introduction to Important Questions

• The discussion focuses on frequently asked questions from the chapter on periodic classification, emphasizing their importance as they have appeared multiple times in exams.

• These questions are derived from previous years' papers and include queries from NCERT, state boards, and CBSE.

First Question: Second Electron Gain Enthalpy of Oxygen

• A key question asks whether the second electron gain enthalpy of oxygen is positive, more negative, or less negative.

• The concept involves adding a second electron to an already negatively charged oxygen ion (O-) to form O2-, which leads to repulsion due to electron-electron interactions.

• It is concluded that the second electron gain enthalpy for oxygen is positive because energy must be supplied to overcome this repulsion when adding another electron.

Second Question: Arranging Ions by Ionic Size

• Another important question requires arranging ions in increasing order of ionic size, highlighting that anions are larger than cations.

• Anions like N3-, O2-, and F- are discussed as being larger due to additional electrons causing increased repulsion compared to cations like Na+ and Mg2+.

Third Question: Electron Gain Enthalpy Comparison between Fluorine and Chlorine

• A significant question addresses why fluorine has a less negative electron gain enthalpy than chlorine despite being more electronegative.

• The reasoning lies in fluorine's smaller atomic size leading to greater repulsion when an additional electron is added, making it less favorable energetically.

Fourth Question: Nitrogen vs. Oxygen Electron Gain Enthalpy

• This question explores why nitrogen has a positive electron gain enthalpy while oxygen has a negative one.

• Nitrogen's half-filled p-orbital configuration (2p3) makes it stable and resistant to gaining an extra electron, resulting in a positive value for its enthalpy.

Fifth Question: Ionization Energy Comparisons

• The discussion shifts towards comparing ionization energies of nitrogen and oxygen, noting that removing an electron from oxygen (P4 configuration) is easier due to its instability compared to nitrogen's stable P3 configuration.

Sixth Question: Cation vs. Anion Size Explanation

• A critical question examines why cations are smaller than their parent atoms while anions are larger.

• Cations lose electrons leading to stronger attraction between remaining protons and electrons; thus they shrink in size. Conversely, anions gain electrons which increases repulsion among them causing expansion.

Seventh Question: Sodium vs. Magnesium Ionization Energies

• This section discusses why sodium has lower first ionization energy compared to magnesium but higher second ionization energy when forming Na+ versus Mg+2.

Eighth Question: Difference Between Electronegativity and Electron Gain Enthalpy

• Finally, the distinction between electronegativity (the ability of an atom to attract shared electrons in a bond), and electron gain enthalpy (energy change when an isolated atom gains an electron), is clarified with examples.

Understanding Ionization Energy and Noble Gases

Misunderstanding in Exam Questions

• A student expresses frustration over receiving the wrong question paper, specifically mentioning confusion regarding accountancy questions instead of science-related ones.

Ionization Energy Concepts

• The discussion highlights that nitrogen has a higher ionization energy than oxygen due to its half-filled stable configuration (1s² 2s² 2p³), making it harder to remove an electron.

• It is explained that cations are always smaller than their neutral atoms because they have fewer electrons, leading to a stronger attraction from protons.

Noble Gases and Electron Gain

• Noble gases, such as helium and neon, possess the largest positive electron gain enthalpy due to their fully filled electron shells (e.g., neon: 1s² 2s² 2p⁶).

• Neon’s electronic configuration is described as very stable (2,8), indicating that adding extra electrons would be challenging.

Stability of Noble Gases

• An analogy involving a family member (uncle) illustrates the stability of noble gases; just like an uncle who is already full after eating cannot easily take more food, noble gases resist gaining additional electrons.

• The concept emphasizes that for noble gases to accept extra electrons, they must enter a new shell due to their complete outer shell configuration.

Preparation Tips for Exams

• Students are advised on exam preparation strategies: if time allows, complete all NCERT exercises; if time is short before exams, focus on key questions from the chapter.

• Learning naming conventions for elements is emphasized as important for exams; students should memorize names based on atomic numbers provided in NCERT materials.