An Overview of Class 10 Science Mcq Chapter 5 Periodic Classification Of Elements
FAQs on Class 10 Science Mcq Chapter 5 Periodic Classification Of Elements
1. What types of important questions should I prepare from Chapter 5, Periodic Classification of Elements, for the CBSE Class 10 board exams 2025-26?
For the CBSE Class 10 Science exam, you should focus on a mix of question types from this chapter. Expect:
- 1-mark questions (MCQs): Based on the modern periodic law, identifying element positions from atomic numbers, and names of specific groups (e.g., halogens, noble gases).
- 3-mark questions: Explaining trends like atomic size, valency, and metallic character across periods and groups. Also, stating the achievements and limitations of Mendeleev's Periodic Table is a frequently asked question.
- 5-mark/Case-based questions: These often involve a table snippet and require you to identify elements, compare their properties (like reactivity, atomic radius), and write chemical formulas of the compounds they form.
2. How do atomic size and metallic character change as we move from left to right in a period and down a group in the Modern Periodic Table?
This is a very important concept for board exams. The trends are as follows:
- Across a Period (Left to Right): Atomic size decreases because the nuclear charge increases, pulling electrons closer to the nucleus. Consequently, the metallic character also decreases as the tendency to lose electrons reduces.
- Down a Group (Top to Bottom): Atomic size increases because a new electron shell is added at each step. This increases the distance of the valence electrons from the nucleus, making them easier to lose. Therefore, the metallic character increases down a group.
3. Why is understanding the limitations of Mendeleev's Periodic Table considered an important question for the Class 10 exam?
Understanding Mendeleev's limitations is crucial because it explains the need for the Modern Periodic Table. Examiners test this to see if students appreciate the evolution of scientific models. Key limitations that are often asked about include:
- Position of Isotopes: He could not assign a fixed position as they have different atomic masses but similar chemical properties.
- Anomalous Pairs: Some elements with higher atomic mass were placed before elements with lower atomic mass (e.g., Co before Ni) to maintain chemical similarity.
- Position of Hydrogen: Its position was not justified as it resembles both alkali metals and halogens.
4. State three significant achievements of Mendeleev's Periodic Table that made it a landmark in the classification of elements.
Mendeleev's Periodic Table was a major breakthrough. Its three most important achievements, frequently asked in exams, are:
- Systematic Arrangement: He was the first to systematically arrange all known elements into groups (vertical columns) and periods (horizontal rows) based on their properties.
- Prediction of New Elements: He left gaps for undiscovered elements and predicted their properties with remarkable accuracy. For example, eka-aluminium (Gallium) and eka-silicon (Germanium).
- Correction of Atomic Masses: The table helped in correcting the atomic masses of several elements, like Beryllium (from 13.5 to 9), by placing them in the correct group based on their properties.
5. An element 'X' has an atomic number of 17. How can you determine its position in the Modern Periodic Table and predict its nature?
This is a typical HOTS (Higher Order Thinking Skills) question. Here is the step-by-step method:
- Electronic Configuration: The configuration for atomic number 17 is 2, 8, 7.
- Period Number: The element has 3 electron shells, so it belongs to the 3rd Period.
- Group Number: It has 7 valence electrons. For elements with more than 2 valence electrons, the group number is 10 + (valence electrons). So, the group is 10 + 7 = 17.
- Nature: Since it has 7 valence electrons, it needs to gain 1 electron to achieve a stable octet. Elements that gain electrons are non-metals. Therefore, 'X' is a reactive non-metal (specifically, a halogen).
6. What is the fundamental difference between the basis of Mendeleev's Periodic Table and the Modern Periodic Table?
The fundamental difference, a key 1-mark question, lies in the property used for classification. Mendeleev's Periodic Law stated that the properties of elements are a periodic function of their atomic mass. In contrast, the Modern Periodic Law states that the properties of elements are a periodic function of their atomic number (Z).
7. What is a common trap or misconception students have in questions related to periodic trends in the Class 10 Science exam?
A very common trap is confusing the trends of atomic size and electronegativity/non-metallic character across a period. Students often forget the inverse relationship. Remember: as you move left to right in a period, the atomic size decreases, but the atom's ability to attract electrons (electronegativity) increases. This is because the increasing nuclear charge pulls the electron shells tighter, making the atom smaller and better at attracting electrons.
8. Compare Mendeleev's Periodic Table with the Modern Periodic Table on the basis of how they resolve the position of isotopes.
This is a critical comparison question.
- In Mendeleev's Periodic Table, isotopes posed a major problem. Since his table was based on atomic mass, isotopes of the same element (which have different masses) would require different positions, which contradicted their similar chemical properties.
- The Modern Periodic Table elegantly solves this. It is based on atomic number, and all isotopes of an element have the same atomic number. Therefore, they are all placed in the same slot in the table, which aligns perfectly with their identical chemical behaviour.
9. Why were early models like Dobereiner's Triads and Newlands' Law of Octaves rejected, and why is this an important topic?
Understanding why early models were rejected is important because it shows the development of scientific principles. These models had significant limitations:
- Dobereiner's Triads: This model was rejected because it was not universal. He could only identify a few sets of three elements (triads) that followed the rule. It was not applicable to all the elements known at that time.
- Newlands' Law of Octaves: This model was rejected because it worked well only for lighter elements up to calcium. Furthermore, the discovery of noble gases completely disrupted its pattern of every eighth element having similar properties.
