The uploaded PDF contains the complete solution set for the worksheet based on Chapter 3, Classification of Elements and Periodicity in Properties, for Class 11 Chemistry. This solutions document not only provides correct answers to multiple-choice questions but also includes clear explanations for reasoning, short answers, and long descriptive questions. It systematically covers Mendeleev’s periodic table, modern periodic law, Dobereiner’s triads, IUPAC naming system, periodic trends, and advantages of the long form of the periodic table.
I am writing about this solutions PDF because Chapter 3 is theory-heavy and students often get confused between Mendeleev’s periodic law and the modern periodic law. Having a structured solution guide helps in clearing doubts and understanding why a particular answer is correct. From my experience, when students analyse solutions carefully instead of just checking answers, their conceptual clarity improves significantly.
As seen in the uploaded solutions PDF Ch-3 Classification of elements…, the answers are arranged section-wise and follow the exact structure of the worksheet. This makes cross-checking easy and systematic.
Solutions Based on Mendeleev’s Periodic Table
The PDF clearly explains that Mendeleev arranged elements in increasing order of atomic weight and placed elements with similar properties in the same vertical column or group. It highlights that:
• Similar elements were grouped together
• He left gaps for undiscovered elements
• Eka-aluminium and eka-silicon were later discovered as gallium and germanium
• He mainly considered oxides and hydrides for classification
The solution also clarifies that Mendeleev’s periodic law was based on atomic masses, not atomic numbers.
Modern Periodic Law and Its Explanation
The solutions explain that the modern periodic law states that the physical and chemical properties of elements are periodic functions of their atomic numbers.
Important clarifications include:
• Period number indicates the principal quantum number
• Groups are numbered from 1 to 18 according to IUPAC
• Horizontal rows are called periods and vertical columns are called groups
• The periodicity is due to repetition of similar valence shell electronic configuration
This part strengthens conceptual understanding.
Dobereiner’s Triads Explained
The PDF provides detailed explanation of Dobereiner’s triads.
Key points include:
• Elements arranged in groups of three
• Atomic mass of the middle element approximately equal to the average of the other two
• Examples like Li, Na, K and Cl, Br, I
This shows early attempts at classification before Mendeleev.
Download this Ch-3 Classification of elements and periodicity in properties PDF File: Click Here
Periodic Trends and Conceptual Answers
The solutions cover trends such as:
• Atomic radius increases down the group
• Electropositive character increases with atomic size
• Bond dissociation energy trends
• Valency trends
For example, the correct order of atomic radii among halogens is F < Cl < Br < I, which is clearly explained in the document.
IUPAC Nomenclature and New Elements
The PDF explains naming of elements with atomic number greater than 100 using roots like:
nil, un, bi, tri, quad, pent, hex, sept, oct, enn
It provides examples such as:
• Element with atomic number 110 – Ununnilium (temporary name), official name Darmstadtium
• Element with atomic number 120 – Unbinilium (symbol Ubn)
These explanations help students decode systematic names.
Features and Advantages of Long Form Periodic Table
The solutions clearly compare Mendeleev’s periodic table with the long form of periodic table.
Advantages of long form include:
• Based on atomic number
• Removes anomalies of atomic mass inversions
• Provides clear position to isotopes
• Separates transition and inner transition elements
• Links position of element with electronic configuration
This section is important for long-answer questions.
Drawbacks of Mendeleev’s Periodic Table
The PDF lists major limitations such as:
• Uncertain position of hydrogen
• No explanation for isotopes
• Anomalies in increasing atomic mass order
• No explanation for cause of periodicity
These points are frequently asked in board exams.
Numerical and Application-Based Answers
The document also explains:
• Calculation of atomic mass in triads
• Determining period and group from atomic number
• Why sixth period contains 32 elements
• Why element with atomic number 126 cannot fit in present setup
These answers strengthen analytical skills.
Why This Solutions PDF Is Useful
From my perspective, this solutions document is highly beneficial because:
• It provides reasoning behind every correct option
• It covers both objective and descriptive questions
• It explains historical development clearly
• It strengthens exam preparation
Students who carefully study the explanations will find it easier to attempt board-style long answers.
How Students Should Use This PDF
• First solve the worksheet independently
• Then compare with the solutions
• Identify conceptual gaps
• Revise periodic trends and laws
• Practise long-answer writing using the explained points
