Looking for accurate SEBA Class 10 Science Chapter 5 (Periodic Classification of Elements) textual solutions for the 2026–27 academic year? Access step-by-step textbook exercise answers, periodic law explanations, Modern Periodic Table trends, valency, atomic size, and metallic properties to help you score top marks in your HSLC board exams.
Blue question
1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Ans.
Yes, some of Döbereiner’s triads were also present in Newlands’ Octaves. For example, lithium (Li), sodium (Na), and potassium (K) appeared together in both classifications. This shows that Newlands’ Law of Octaves partly supported Döbereiner’s triads.
2. What were the limitations of Döbereiner’s classification?
Ans.
Döbereiner could arrange only a few elements into triads. Many known elements could not be placed in any triad. As more elements were discovered, the classification became unsuitable for all elements.
3. What were the limitations of Newlands’ Law of Octaves?
Ans.
Newlands’ Law of Octaves was valid only up to calcium. Elements discovered later could not be fitted properly into the table. Also, some dissimilar elements were placed in the same group. Therefore, the law was not applicable to all elements.
4. Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, Al, Si, Ba.
Ans.
The formulae of the oxides are:
- Potassium (K) – K2O
- Carbon (C) – CO2
- Aluminium (Al) – Al2O3
- Silicon (Si) – SiO2
- Barium (Ba) – BaO
SEBA Class 10 Science Chapter 5 Solution
5. Besides gallium, which other elements have since been discovered that were left by Mendeléev in his Periodic Table? (Any two)
Ans.
Two elements that were discovered later and filled the gaps in Mendeléev’s Periodic Table are Scandium (Sc) and Germanium (Ge).
6. What were the criteria used by Mendeléev in creating his Periodic Table?
Ans.
Mendeléev arranged the elements according to their atomic masses. He placed elements with similar chemical properties in the same group. He also used the valency and the formulae of oxides and hydrides to determine the position of elements.
7. Why do you think the noble gases are placed in a separate group?
Ans.
Noble gases are placed in a separate group because they are chemically inactive. Their outermost shell is completely filled, so they do not react easily with other elements.
8. How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?
Ans.
The Modern Periodic Table is based on atomic number instead of atomic mass. This removed many defects of Mendeléev’s table. It also gave the correct position to isotopes and arranged elements in a proper order.
9. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Ans.
Calcium (Ca) and Beryllium (Be) show chemical reactions similar to magnesium. This is because all three elements belong to Group 2 and have two valence electrons. Therefore, they have similar chemical properties.
Class 10 Science Chapter 5 Question Answer SEBA 2026-27
10. Name the following:
(a) Three elements that have a single electron in their outermost shells.
Ans. Hydrogen (H), Sodium (Na), Potassium (K)
(b) Two elements that have two electrons in their outermost shells.
Ans. Beryllium (Be), Magnesium (Mg)
(c) Three elements with filled outermost shells.
Ans. Helium (He), Neon (Ne), Argon (Ar)
11.(a) Lithium, sodium and potassium react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
Ans.
Yes, all three elements have one electron in their outermost shell. They belong to Group 1 of the Periodic Table. Therefore, they show similar chemical properties.
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What do their atoms have in common?
Ans.
Both helium and neon have completely filled outermost shells. Therefore, they are chemically inactive and do not react easily.
12. In the Modern Periodic Table, which are the metals among the first ten elements?
Ans.
Among the first ten elements, Lithium (Li) and Beryllium (Be) are metals.
Periodic Classification of Elements Class 10 SEBA Textual Exercise Solutions
13. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic: Ga, Ge, As, Se, Be?
Ans.
Beryllium (Be) has the maximum metallic character among these elements because it is placed on the left side of the Periodic Table. Metals are generally found on the left side of the table.
Exercise
1. Which of the following statements is not a correct statement about the trends when going from left to right across the periods of the Periodic Table?
Ans.
(c) The atoms lose their electrons more easily.
As we move from left to right across a period, atoms hold their electrons more strongly due to increased nuclear charge. Therefore, they do not lose electrons easily.
2. Element X forms a chloride with the formula XCl2, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as—
Ans.
(b) Magnesium (Mg)
Since X forms XCl2, it has a valency of 2. Therefore, it belongs to Group 2, like magnesium.
3. Which element has—
(a) Two shells, both of which are completely filled with electrons?
Ans. Neon (Ne)
(b) The electronic configuration 2, 8, 2?
Ans. Magnesium (Mg)
(c) A total of three shells, with four electrons in its valence shell?
Ans. Silicon (Si)
(d) A total of two shells, with three electrons in its valence shell?
Ans. Boron (B)
(e) Twice as many electrons in its second shell as in its first shell?
Ans. Carbon (C)
4. (a) What property do all elements in the same column of the Periodic Table as boron have in common?
Ans.
All elements in the same group as boron have three valence electrons. Therefore, they show similar chemical properties.
(b) What property do all elements in the same column of the Periodic Table as fluorine have in common?
Ans.
All elements in the same group as fluorine have seven valence electrons. They are highly reactive non-metals and have similar chemical properties.
SEBA Class 10 Science Chapter 5 Textbook Answers
5. An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
Ans.
The atomic number is 17 because the total number of electrons is 2 + 8 + 7 = 17.
(b) To which of the following elements would it be chemically similar? N(7), F(9), P(15), Ar(18)
Ans.
It would be chemically similar to Fluorine (F) because both have seven valence electrons and belong to the same group.
6. The position of three elements A, B and C in the Periodic Table are shown below.
(a) State whether A is a metal or non-metal.
Ans. A is a non-metal.
(b) State whether C is more reactive or less reactive than A.
Ans. C is less reactive than A.
(c) Will C be larger or smaller in size than B?
Ans. C will be smaller in size than B.
(d) Which type of ion, cation or anion, will be formed by element A?
Ans. A will form an anion by gaining one electron.
7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to Group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?
Ans.
The electronic configuration of nitrogen is 2, 5 and that of phosphorus is 2, 8, 5.
Nitrogen is more electronegative than phosphorus because its atomic size is smaller and it attracts shared electrons more strongly.
8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Ans.
The number of electron shells in an atom shows its period number, while the number of valence electrons determines its group number.
For example, sodium (2, 8, 1) has 3 shells and 1 valence electron, so it is placed in Period 3 and Group 1.
9.In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?
Ans.
Elements with atomic numbers 12 (Magnesium) and 38 (Strontium) have properties similar to calcium because they belong to the same group (Group 2) of the Periodic Table.
HSLC Science Periodic Table Question Answer
10. Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the Modern Periodic Table.
Ans.
- Mendeléev’s Periodic Table was based on atomic mass, whereas the Modern Periodic Table is based on atomic number.
- In Mendeléev’s table, the position of isotopes could not be explained. In the Modern Periodic Table, isotopes occupy the same position because they share the same atomic number.
- The position of hydrogen was ambiguous in Mendeléev’s table, whereas in the Modern Periodic Table hydrogen is placed in Group 1.
- Thus, the Modern Periodic Table provides a more scientific framework and resolves the defects of Mendeléev’s table.
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