Classification of elements | 20 Important questions | Class 11 Chemistry | 2026-27

Important Questions for Class 11 Chemistry: Classification of Elements

Ionization Enthalpy

• Definition: Ionization enthalpy is the energy required to remove an electron from an isolated gaseous atom in its ground state.

• Process: When removing an electron from a gaseous atom (e.g., X), it transforms into a cation (X⁺) and releases energy, termed ionization enthalpy.

• Multiple Ionizations: The first ionization enthalpy refers to removing the first electron, while subsequent removals are referred to as second and third ionization enthalpies, with increasing values due to increased positive charge.

• Always Positive: Ionization enthalpies are always positive because energy must be supplied to remove electrons, making the process endothermic.

Comparison of Elements

• Beryllium vs. Boron: Beryllium has a higher ionization enthalpy than boron due to its electronic configuration and stronger nuclear attraction on its outer electrons compared to boron's p-orbital shielding effect.

• Shielding Effect: In boron, the 2p electrons experience repulsion from inner 2s electrons, making them easier to remove compared to beryllium's more tightly held s-electrons.

Isoelectronic Species

• Commonality in Ionic Species: N³⁻, O²⁻, F⁻, Na⁺, and Al³⁺ all have ten electrons each; thus they are isoelectronic species.

• Ionic Radii Order: The order of increasing ionic radii is Al³⁺ < Na⁺ < F⁻ < O²⁻ < N³⁻ due to varying nuclear charges attracting the same number of electrons.

Electron Gain Enthalpy

• Definition: Electron gain enthalpy is defined as the energy change when an electron is added to a neutral gaseous atom.

• Group 17 Halogens: Halogens have high negative electron gain enthalpies because they release significant energy upon gaining one electron for stable noble gas configuration.

Justification for Element Count in Periodic Table

• Fifth Period Elements: The fifth period contains 18 elements due to available subshell configurations (5s, 4d, and 5p), allowing for maximum filling of orbitals according to Aufbau principle.

Noble Gases' Electron Gain Enthalpy

• High Positive Values: Noble gases exhibit very high positive electron gain enthalpies because adding an electron requires entering a higher principal quantum level which destabilizes their configuration.

Chemical Behavior Similarities

• Lithium and Magnesium Resemblance: Lithium and magnesium show similar chemical behaviors due to diagonal relationships arising from comparable size and charge density despite being in different groups.

IUPAC Naming Convention

• Element with Atomic Number 120: For naming elements above atomic number 100 using IUPAC conventions involves combining prefixes based on digit values leading up to "Unbinilium" (Ubn).

Trends in Electron Gain Enthalpy

• Oxygen vs. Fluorine Comparison: Oxygen has less negative electron gain enthalpy than fluorine because smaller atomic sizes lead to greater repulsion among added electrons within the same shell.

Size Comparisons Among Copper Ions

• Copper Size Analysis: Among Cu, Cu⁺, and Cu²⁺ ions; copper has the largest size since losing electrons increases effective nuclear charge pulling remaining ones closer together.

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Understanding Electron Configuration and Ionization Energies

Electron Filling Order

• The sequence of electron filling includes 7s, 5f, 6d, and 7p orbitals, with a total of 28 electrons to be filled.

• Maximum occupancy for the 5f orbital is 14 electrons; thus, after adding these to the other orbitals, only two electrons will fill the 7p orbital.

Period and Group Determination

• The highest principal quantum number (n) indicates the period; here it is seven, representing the seventh period.

• To determine the group number, sum the number of s and p electrons: 2 (from : s) + 10 (from : d) + 2 (from : p), resulting in group 14.

Ionization Energies Comparison

• Question regarding why sodium's first ionization enthalpy is lower than magnesium's; removing one electron from sodium requires less energy compared to magnesium.

• However, sodium’s second ionization enthalpy is higher than that of magnesium due to stability achieved after losing one electron.

Atomic Size and Nuclear Charge Effects

• Sodium has a larger atomic size than magnesium which results in its outermost electron being less strongly attracted to the nucleus.

• This makes it easier to remove an electron from sodium compared to magnesium since sodium achieves noble gas configuration upon losing one electron.

Stability After Electron Removal

• Once sodium loses its first electron achieving stable configuration (2p6), removing a second would disrupt this stability requiring more energy.

• In contrast, when removing a second electron from magnesium+, it still retains noble gas configuration making it easier to lose another electron.

Trends in Atomic Radius Across Periodic Table

Increasing Atomic Radius Order

• For elements like sodium, aluminum, and magnesium within the same period: as you move left to right across a period nuclear charge increases leading to decreased atomic size.

Anomalous Properties of Second Period Elements

• Elements in the second period exhibit distinct properties compared to their respective group members; lithium shows significant differences from sodium or potassium.

Reasons for Anomalous Properties

• Smaller atomic size leads to larger charge radius ratio.

• Higher electronegativity compared with other group elements.

• Absence of d-orbitals contributes significantly.

Examples and Relationships in Electronegativity

Metalloid Examples

• Common metalloids include boron, silicon, germanium, and arsenic which possess both metallic and non-metallic properties.

Relationship Between Electronegativity and Non-Metallic Character

• Electronegativity refers to an atom's ability to attract shared electrons in covalent bonds while non-metallic character describes how easily an atom can gain electrons.

Trends Across Groups

• As effective nuclear charge increases across periods both electronegativity and non-metallic character increase. Conversely down groups both decrease due to increased atomic size leading to shielding effects.

Summary Insights on Key Concepts

Final Thoughts on Chapter Questions

• These notes summarize critical concepts related to electronic configurations and trends observed within periodic table elements. Further practice questions are available through linked playlists for deeper understanding.