Looking for reliable SEBA Class 10 Science Chapter 5 (Periodic Classification of Elements) extra questions and answers for the 2026–27 academic year? Access important practice questions on Döbereiner’s Triads, Newlands’ Law of Octaves, Mendeleev’s Periodic Table, Modern Periodic Trends, valency, and atomic size to score top marks in your HSLC board exams.
Q1. What was the earliest classification of elements?
Ans: Elements were first classified into two basic categories: Metals and Non-metals.
- Metals are shiny, malleable, ductile, and good conductors of heat and electricity.
- Non-metals are dull, brittle, and poor conductors of heat and electricity.
Q2. What was Döbereiner’s Triad?
Ans: Johann Wolfgang Döbereiner grouped elements with similar chemical properties into sets of three, called triads. When arranged in increasing order of atomic mass, the atomic mass of the middle element was approximately equal to the average of the atomic masses of the other two elements.
Q3. Give one example of Döbereiner’s Triad.
Ans: Lithium (Li), Sodium (Na), and Potassium (K).
- Atomic mass of Lithium (Li) = 6.9
- Atomic mass of Potassium (K) = 39.0
- Average mass = (6.9 + 39.0) / 2 = 22.95 ≈ 23.0 (Atomic mass of Sodium)
Q4. What were the limitations of Döbereiner’s Triads?
Ans: The main limitations of Döbereiner’s Triads are:
- He could identify only three triads from the elements known at that time.
- The system failed to classify all known elements into triads, as many elements did not follow this rule.
Q5. State Newlands’ Law of Octaves.
Ans: Newlands’ Law of Octaves states that when elements are arranged in order of increasing atomic mass, every eighth element possesses properties similar to those of the first element, resembling the repetition of notes in an octave of music.
Q6. Why is it called the “Law of Octaves”?
Ans: It is called the “Law of Octaves” because, just like the eighth note in the musical scale (sa–re–ga–ma–pa–dha–ni–sa) repeats the sound of the first note, the chemical properties repeat at every eighth element.
Q7. Give one example of elements showing the Law of Octaves.
Ans: Lithium (Li) and Sodium (Na) exhibit similar physical and chemical properties and are positioned eight elements apart in the arrangement.
SEBA Class 10 Science Chapter 5 Extra Questions
Q8. Why did Newlands’ Law of Octaves fail?
Ans: Newlands’ Law of Octaves failed due to the following reasons:
- It was applicable only up to calcium (Ca); after calcium, every eighth element did not possess properties similar to the first.
- Newlands placed two elements in the same slot to fit them into his table (e.g., Cobalt and Nickel).
- Elements with dissimilar properties were grouped together in the same column.
- It assumed only 56 elements existed in nature and had no provision for newly discovered elements.
Q9. Who is known as the “Father of the Periodic Table”?
Ans: Dmitri Ivanovich Mendeleev, a Russian chemist, is known as the Father of the Periodic Table (published in 1869).
Q10. State Mendeleev’s Periodic Law.
Ans: Mendeleev’s Periodic Law states that:
“The physical and chemical properties of elements are a periodic function of their atomic masses.”
Q11. What were the rows and columns in Mendeleev’s Periodic Table called?
Ans:
- Horizontal rows are called Periods.
- Vertical columns are called Groups.
Q12. How did Mendeleev arrange the elements?
Ans: Mendeleev arranged the known 63 elements in increasing order of their atomic masses in such a way that elements with similar physical and chemical properties fell into the same vertical group.
Q13. Mention one achievement of Mendeleev’s Periodic Table.
Ans: Mendeleev left gaps in his periodic table for elements that were yet to be discovered and successfully predicted their atomic masses and chemical properties (e.g., Eka-Aluminium, which was later discovered as Gallium).
Q14. Name three elements predicted by Mendeleev.
Ans:
- Eka–Boron -> Scandium (Sc)
- Eka–Aluminium -> Gallium (Ga)
- Eka–Silicon -> Germanium (Ge)Ge)
Class 10 Science Chapter 5 Important Questions SEBA 2026-27
Q15. How did the discovery of noble gases support Mendeleev’s table?
Ans: When noble gases (such as Helium, Neon, and Argon) were discovered, they could be placed in a completely new group (Group 0 / Group 18) without disturbing the existing layout, demonstrating the flexibility and strength of Mendeleev’s periodic arrangement.
Q16. State one limitation of Mendeleev’s table related to hydrogen.
Ans: Hydrogen shares properties with both alkali metals (Group 1) and halogens (Group 17), creating confusion regarding its exact position in the periodic table.
Q17. Why did isotopes create a problem in Mendeleev’s classification?
Ans: Isotopes of an element have the same chemical properties but different atomic masses. Since Mendeleev arranged elements by increasing atomic mass, isotopes should have been given separate positions, which was not possible without breaking the periodic arrangement.
Q18. What anomaly did Mendeleev face with cobalt and nickel?
Ans: Cobalt (atomic mass 58.9) was placed before Nickel (atomic mass 58.7) so that elements with similar properties fell into the same group, violating the rule of strictly increasing atomic mass.
Q19. Who improved Mendeleev’s table using atomic number?
Ans: Henry Moseley in 1913 demonstrated that atomic number is a more fundamental property of an element than atomic mass.
Q20. State the Modern Periodic Law.
Ans: The Modern Periodic Law states that:
“The physical and chemical properties of elements are a periodic function of their atomic numbers.”
Periodic Classification of Elements Class 10 SEBA Extra Question Answer
Q21. How many groups and periods are there in the Modern Periodic Table?
Ans:
- 18 Groups (vertical columns)
- 7 Periods (horizontal rows)
Q22. What determines the position of an element in a group?
Ans: The position of an element in a group is determined by the number of valence electrons in its outermost shell. Elements in the same group have the same number of valence electrons.
Q23. What determines the position of an element in a period?
Ans: The position of an element in a period is determined by the total number of electron shells occupied in its atom.
Q24. What is the formula to calculate the maximum number of electrons in a shell?
Ans:he maximum number of electrons that can be accommodated in a shell is given by the formula 2n², where n represents the shell number (e.g., K=1, L=2, M=3, N=4).
Q25. How many elements are present in the first four periods?
Ans:
- 1st Period: 2 elements (Very short period)
- 2nd Period: 8 elements (Short period)
- 3rd Period: 8 elements (Short period)
- 4th Period: 18 elements (Long period)
SEBA Class 10 Science Chapter 5 Practice Notes
Q26. Why does hydrogen have a unique position in the periodic table?
Ans: Hydrogen exhibits dual behavior:
- Like Group 1 alkali metals, it forms compounds like H2O and HCl.
- Like Group 17 halogens, it exists as a diatomic molecule (H2) and forms covalent compounds.
- Hence, it does not fit perfectly into any single group.
Q27. Define atomic size (atomic radius).
Ans: Atomic size refers to the distance between the center of the nucleus and the outermost shell of an isolated atom.
Q28. How does atomic size change across a period and down a group?
Ans:
- Across a period (left to right): Atomic size decreases due to an increase in nuclear charge, which pulls electrons closer to the nucleus.
- Down a group (top to bottom): Atomic size increases because new electron shells are added, increasing the distance from the nucleus despite higher nuclear charge.
Q29. Define valency and state how it varies.
Ans: Valency is the combining capacity of an atom, determined by the number of valence electrons in its outermost shell.
- Across a period: Valency first increases from 1 to 4 and then decreases down to 0.
- Down a group: Valency remains the same for all elements in that group.
HSLC Science Periodic Table Important Questions
Q30. How does metallic and non-metallic character vary in the periodic table?
Ans:
- Metallic character (electropositive nature):
- Across a period: Decreases (due to increased nuclear pull making loss of electrons harder).
- Down a group: Increases (due to increased atomic size making loss of electrons easier).
- Non-metallic character (electronegative nature):
- Across a period: Increases (due to stronger pull for gaining electrons).
- Down a group: Decreases (due to larger atomic radius reducing effective pull on outer electrons).
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