Hey, Class 10 students! Welcome to your ultimate guide for Chapter 5: Periodic Classification of Elements from the NCERT Science syllabus. This chapter is like a treasure map for understanding how all the elements in the universe are organized. It’s super important for your exams and pretty cool to learn! We’ll answer key questions in a simple way and even give you an exercise to practice. Let’s explore the periodic table together!
What’s the Big Idea?
Imagine trying to organize 118+ elements—like sorting a giant toy box! Scientists did just that with the periodic table, grouping elements by their properties. Chapter 5 takes you through how this table evolved and how it works today. Let’s get to the questions!
Key Questions and Answers from Chapter 5
Here are the most important questions from Periodic Classification of Elements with clear, easy answers:
Q1: Why do we classify elements?
Answer:
We classify elements to:
- Make studying them easier by grouping similar ones.
- Predict properties of new elements.
- Understand patterns (e.g., metals vs. non-metals).
Example: Sodium and potassium act alike—they’re in the same group!
Q2: What was Dobereiner’s Triads?
Answer:
Dobereiner grouped elements in sets of three (triads) where the middle element’s atomic mass was the average of the other two.
Example: Lithium (7), Sodium (23), Potassium (39).
Average: (7 + 39) ÷ 2 = 23 (matches sodium!).
Limit: Only worked for a few elements.
Q3: What was Newlands’ Law of Octaves?
Answer:
Newlands said if you list elements by increasing atomic mass, every 8th element repeats properties—like musical notes.
Example: Li, Be, B, C, N, O, F, Na—sodium (8th) is like lithium.
Problem: Worked only up to calcium; later elements didn’t fit.
Q4: How did Mendeleev create his periodic table?
Answer:
Mendeleev arranged elements by:
- Increasing atomic mass.
- Similar properties in columns (groups).
He left gaps for undiscovered elements and even predicted their properties!
Example: He predicted “eka-silicon” (later germanium).
Cool Fact: His table was a hit because it worked!
Q5: What are the merits of Mendeleev’s periodic table?
Answer:
- Grouped elements with similar traits (e.g., alkali metals in Group 1).
- Left gaps for new elements (e.g., gallium, germanium).
- Corrected atomic masses (e.g., gold’s mass was adjusted).
Why It Rocked: It was a crystal ball for chemistry!
Q6: What was wrong with Mendeleev’s table?
Answer:
- Atomic Mass Issues: Some elements didn’t fit (e.g., potassium’s mass is less than argon’s, but it comes after).
- No Place for Isotopes: Same element, different masses confused things.
- Hydrogen’s Spot: Didn’t fit perfectly anywhere.
Fix: Modern table uses atomic number instead.
Q7: How is the modern periodic table different?
Answer:
The modern table, based on Moseley’s work, uses:
- Atomic Number (number of protons) instead of mass.
- 18 groups and 7 periods.
- Elements still grouped by properties.
Why Better?: Solves mass mix-ups (e.g., Ar = 18, K = 19 fits perfectly).
Q8: What are periods and groups in the periodic table?
Answer:
- Periods: Horizontal rows (1 to 7). Show number of electron shells.
- Period 1: 2 elements (H, He).
- Period 2: 8 elements (Li to Ne).
- Groups: Vertical columns (1 to 18). Show similar properties and valence electrons.
- Group 1: Alkali metals.
- Group 17: Halogens.
Tip: Periods = shells, Groups = behavior!
Q9: What trends do we see across a period?
Answer:
From left to right in a period:
- Atomic Size: Decreases (more protons pull electrons closer).
- Metallic Nature: Drops (left = metals, right = non-metals).
- Electronegativity: Increases (atoms grab electrons more).
Example: Period 2: Li (metal) → Ne (non-metal).
Q10: What trends do we see down a group?
Answer:
From top to bottom in a group:
- Atomic Size: Increases (more shells added).
- Metallic Nature: Increases (easier to lose electrons).
- Electronegativity: Decreases (less pull on electrons).
Example: Group 1: Li (small) → Cs (big and super reactive).
Q11: Where are metals, non-metals, and metalloids in the table?
Answer:
- Metals: Left side (e.g., Na, Fe)—shiny, conductive.
- Non-Metals: Right side (e.g., O, Cl)—dull, poor conductors.
- Metalloids: Zigzag line (e.g., Si, Ge)—mix of both traits.
Visual: Imagine a staircase splitting left (metals) from right (non-metals)!
Q12: What are valence electrons? How do they change?
Answer:
Valence electrons are in the outermost shell and decide how elements react.
- Across a period: Increases (1 in Li to 8 in Ne).
- Down a group: Stays same (Group 1 all have 1).
Example: Na (1 valence) loves to lose it; Cl (7 valence) grabs one!
Q13: Why was atomic number better than atomic mass?
Answer:
Atomic number (protons) is unique for each element and fixes order issues. Atomic mass varies with isotopes, causing confusion (e.g., Ar vs. K).
Moseley’s Win: X-ray studies showed protons rule!
Q14: What’s special about noble gases?
Answer:
Noble gases (Group 18: He, Ne, Ar, etc.) have full valence shells (2 for He, 8 for others), so they don’t react easily.
Use: Neon in lights, helium in balloons!
Q15: How do we predict an element’s properties?
Answer:
Look at its group (similar traits) and period (size trends).
Example: Element in Group 17, Period 3 (Cl) is a reactive non-metal gas like F above it.
Tips to Master Chapter 5
- Learn the Table: Sketch periods and groups—mark metals, non-metals.
- Memorize Trends: Size, metallic nature—practice with examples.
- Know History: Dobereiner, Newlands, Mendeleev—key names!
- Focus on Valence: It’s the key to reactions.
- Use Mnemonics: “LiBeB” for Period 2 elements.
Exercise for Chapter 5: Periodic Classification of Elements
Here’s a practice exercise with different question types. Answers are at the end!
A. Multiple Choice Questions (MCQs)
- Who proposed the Law of Octaves?
a) Mendeleev
b) Newlands
c) Dobereiner
d) Moseley - In the modern periodic table, elements are arranged by:
a) Atomic mass
b) Atomic number
c) Density
d) Valence electrons - Which group contains noble gases?
a) Group 1
b) Group 17
c) Group 18
d) Group 2
B. Short Answer Questions
- What is a triad? Give an example from Dobereiner’s work.
- Why did Mendeleev leave gaps in his table?
- How does atomic size change across a period? Why?
C. Long Answer Questions
- Compare Mendeleev’s and the modern periodic table. List two differences and one similarity.
- Explain trends in metallic character and electronegativity down a group with examples.
D. Fill in the Blanks
- The number of periods in the periodic table is _____.
- Elements in Group 1 are called _____ metals.
E. True or False
- Metalloids are found only on the left side of the periodic table.
- Valence electrons increase down a group.
Answers
- b) Newlands
- b) Atomic number
- c) Group 18
- A triad is a group of three elements where the middle one’s atomic mass is the average of the others. Example: Li (7), Na (23), K (39).
- Mendeleev left gaps for undiscovered elements and predicted their properties.
- Atomic size decreases across a period because more protons pull electrons closer.
- Differences: Mendeleev used atomic mass, modern uses atomic number; Mendeleev had gaps, modern is complete. Similarity: Both group by properties.
- Metallic character increases down a group (e.g., Li to Cs) as electrons are easier to lose; electronegativity decreases (e.g., F to I) as atoms pull less.
- 7
- Alkali
- False (they’re on the zigzag line)
- False (they stay the same)
Chapter 5: Periodic Classification of Elements is your key to unlocking the periodic table’s secrets. With these questions, answers, and exercises, you’re ready to shine in your Class 10 exams! Practice hard, stay curious, and you’ll ace it. Need help? Comment below—I’ve got you covered. Happy studying, periodic pros!
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