# Class 11 Chemistry Unit 3 Notes | SwaVid NCERT Class 11 Chemistry notes: key ideas, formulas, common mistakes and 3 quick checks with worked answers, written by SwaVid from the NCERT textbook. Canonical: https://www.swavid.com/cbse/class-11/chemistry/chapter/classification-of-elements-and-periodicity-in-properties Source: https://www.swavid.com/cbse/class-11/chemistry/chapter/classification-of-elements-and-periodicity-in-properties # Classification of Elements and Periodicity in Properties ## Formulas and definitions ## Key ideas ## Common mistakes ## Quick checks ## Keep going ## Questions parents ask ## Class 11 Chemistry on SwaVid ### Historical development of periodic table ### Modern periodic law and table ### Division into blocks ### Atomic and ionic radii ### Ionization enthalpy and electron gain enthalpy ### Electronegativity and periodic trends ### Next chapter ### Related chapter ### Related chapter ### All chapters SwaVid › CBSE › Class 11 › Chemistry › Classification of Elements and Periodicity in Properties Notes written by SwaVid from the NCERT textbook. Class 11 Chemistry, Unit 3. Key ideas, formulas and common mistakes. CBSE prints 6 marks for Unit 3, not for this chapter alone. Short answer. This chapter traces the historical development of the periodic table and describes the modern arrangement based on atomic numbers. It explains how electronic configurations govern periodic trends in physical and chemical properties of elements across periods and groups. Source: NCERT Chemistry Part I (Reprint 2026-27) , Unit 3, Classification of Elements and Periodicity in Properties Dobereiner arranged elements into triads where the middle element had an atomic mass roughly equal to the average of the other two. Newlands proposed the law of octaves based on increasing atomic masses. Mendeleev organised elements by atomic mass and chemical properties, predicting elements like gallium and germanium. Moseley showed that atomic number is more fundamental than atomic mass. The modern periodic law states that the physical and chemical properties of elements are periodic functions of their atomic numbers. The long form table has 7 horizontal periods and 18 vertical groups. Elements are classified into s, p, d, and f-blocks based on the subshell in which the differentiating electron enters. The s-block and p-block elements are main group elements, d-block contains transition elements, and f-block contains inner transition elements. Atomic radius decreases across a period from left to right due to increased effective nuclear charge. It increases down a group because new electron shells are added. Cations are smaller than their parent atoms, whereas anions are larger. Ionization enthalpy increases across a period and decreases down a group, with deviations seen at completely filled and half-filled subshells. Electron gain enthalpy generally becomes more negative across a period and less negative down a group, though second-period elements like fluorine and oxygen have less negative values than chlorine and sulfur. Electronegativity is the ability of an atom in a molecule to attract shared electrons to itself, increasing across a period and decreasing down a group. Second-period elements show anomalous properties and diagonal relationships with elements of the third period due to their small size, high charge-to-radius ratio, and lack of d-orbitals. Try each question first. Then tap it to see the answer. Write the valence electron configurations: Be is 2s² and B is 2s² 2p¹. Beryllium has a completely filled 2s subshell with higher stability and penetration power. Removing a 2p electron from boron requires less energy than removing a 2s electron from beryllium. Beryllium All four species have 10 electrons: Al³⁺ (Z = 13), Mg²⁺ (Z = 12), Na⁺ (Z = 11), F⁻ (Z = 9). For isoelectronic species, higher nuclear charge Z pulls electrons closer, making the radius smaller. Order of nuclear charge: 13 > 12 > 11 > 9, so radius increases as Z decreases: Al³⁺ < Mg²⁺ < Na⁺ < F⁻. Al³⁺ < Mg²⁺ < Na⁺ < F⁻ Identify roots for digits: 1 = un, 2 = bi, 0 = nil. Combine roots: un + bi + nil + ium = unbinilium. Symbol uses the first letter of each root: U for 1, b for 2, n for 0. Ubn, Unbinilium Chemical Bonding and Molecular Structure . Structure of Atom . Redox Reactions . The 9 chapters of Class 11 Chemistry . CBSE does not give marks to one chapter. It prints 6 marks for Unit 3, Classification of Elements and Periodicity in Properties, which includes Classification of Elements and Periodicity in Properties. No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed. Chemical Bonding and Molecular Structure. SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works. - Unit 3 NCERT book - 6 marks, Unit 3 - 3 quick checks - Assuming fluorine has a more negative electron gain enthalpy than chlorine . Chlorine has a more negative electron gain enthalpy than fluorine because adding an electron to the small, compact 2p-subshell of fluorine causes strong interelectronic repulsions. - Thinking nitrogen has a lower first ionization enthalpy than oxygen . Nitrogen has a higher first ionization enthalpy than oxygen because nitrogen has an extra-stable half-filled 2p³ configuration. - Confusing electronegativity with electron gain enthalpy . Electronegativity refers to a bonded atom attracting a shared pair of electrons in a molecule, whereas electron gain enthalpy is an energetic quantity for an isolated gaseous atom gaining an electron. - Assuming cations are larger than their parent atoms . A cation is always smaller than its parent neutral atom because the removal of electrons decreases electron-electron repulsion and increases effective nuclear charge. ## Key Links - [CBSE](https://www.swavid.com/cbse) - [Class 11](https://www.swavid.com/cbse/class-11) - [Chemistry](https://www.swavid.com/cbse/class-11/chemistry/chapters) - [Start free →](https://www.swavid.com/signup) - [All 9 chapters](https://www.swavid.com/cbse/class-11/chemistry/chapters) - [NCERT Chemistry Part I (Reprint 2026-27)](https://ncert.nic.in/textbook.php) - [Open notes](https://www.swavid.com/cbse/class-11/chemistry/chapter/chemical-bonding-and-molecular-structure) - [Open notes](https://www.swavid.com/cbse/class-11/chemistry/chapter/structure-of-atom) - [Open notes](https://www.swavid.com/cbse/class-11/chemistry/chapter/redox-reactions) - [Class 11 Chemistry](https://www.swavid.com/cbse/class-11/chemistry/chapters) - [Start free →](https://www.swavid.com/signup)