NCERT Textbook Questions & Answers + Important Extra Questions
Section A: In-Text Questions
Did Döbereiner's triads also exist in the columns of Newlands' Octaves? Compare.
Answer
Yes, some of Döbereiner's triads were found within columns of Newlands' Octaves. For instance, the triad of lithium (Li), sodium (Na) and potassium (K) appeared in the same column of Newlands' arrangement. However, not all triads fitted neatly into the octave pattern, and Newlands' classification broke down after calcium.
What were the limitations of Döbereiner's classification?
Answer
Döbereiner could only identify three triads from all the elements known at that time. His method of grouping elements into sets of three based on the average of the first and third element's atomic mass equalling the middle element's mass could not be applied to the majority of elements. Many elements could not be arranged into triads at all.
What were the limitations of Newlands' Law of Octaves?
Answer
(i) The law was applicable only up to calcium — after calcium, every eighth element did not have properties similar to the first.
(ii) Newlands sometimes placed two elements in the same slot to make his pattern work (e.g., cobalt and nickel in one position).
(iii) He assumed that only 56 elements existed in nature and no more would be discovered — this was proven wrong with the discovery of noble gases and other elements.
(iv) Elements with very different properties were sometimes placed together (e.g., iron was grouped with non-metals like oxygen and sulphur).
Use Mendeleev's Periodic Table to predict the formulae for the oxides of the following elements: K, C, Al, Si, Ba.
Answer
Mendeleev arranged elements by increasing atomic masses and placed elements with similar properties in the same group. The formula of the oxide depends on the valency:
• K (Group I, valency 1) → K₂O
• C (Group IV, valency 4) → CO₂
• Al (Group III, valency 3) → Al₂O₃
• Si (Group IV, valency 4) → SiO₂
• Ba (Group II, valency 2) → BaO
Besides gallium, which other elements have properties that were predicted by Mendeleev? Which are these elements and what had Mendeleev originally named them?
Answer
(i) Scandium (Sc) — Mendeleev had named it Eka-boron. Its predicted properties closely matched the actual properties of scandium discovered later.
(ii) Germanium (Ge) — Mendeleev had named it Eka-silicon. Its density, atomic mass, and oxide formula matched Mendeleev's predictions remarkably well.
(Gallium was predicted as Eka-aluminium.)
What was the basis of Mendeleev's classification of elements?
Answer
Mendeleev used atomic masses of elements as the fundamental property for classification. He arranged all known elements in order of increasing atomic mass and observed that elements with similar physical and chemical properties recurred at regular intervals (periodically). He stated: "The properties of elements are a periodic function of their atomic masses."
Why do you think the noble gases are placed in a separate group in the periodic table?
Answer
Noble gases (helium, neon, argon, krypton, xenon, radon) have completely filled outermost electron shells and are extremely stable. They have a valency of zero — they do not normally form bonds with other elements. Since they do not fit into any existing group based on valency, they were placed in a separate group (Group 18 or Group 0) on the extreme right of the periodic table. Their discovery actually supported the periodic law rather than contradicting it.
How could the Modern Periodic Table remove various anomalies of Mendeleev's Periodic Table?
Answer
The Modern Periodic Table uses atomic number (not atomic mass) as the basis for classification, which resolved several problems:
(i) Position of isotopes: Isotopes have different atomic masses but the same atomic number — so they naturally belong in the same position.
(ii) Cobalt-Nickel anomaly: Mendeleev placed Co (58.9) before Ni (58.7) despite higher mass. By atomic number, Co (27) correctly comes before Ni (28).
(iii) Hydrogen's position: By electronic configuration (1 valence electron), hydrogen logically sits in Group 1.
(iv) No sub-groups needed: The 18-group structure separates elements cleanly without the confusion of sub-groups A and B.
Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Answer
Calcium (Ca) and Strontium (Sr) would show chemical reactions similar to magnesium. All three elements belong to Group 2 of the periodic table and have two electrons in their outermost shell (valency 2). Since chemical properties are determined by the number of valence electrons, elements in the same group exhibit similar chemical behaviour.
Q10. Name
(a) Three elements that have a single electron in their outermost shells.
(b) Two elements that have two electrons in their outermost shells.
(c) Three elements with filled outermost shells.
Ans.
(a) Lithium (Li), Sodium (Na), Potassium (K) — all in Group 1 with configuration ending in s¹.
(b) Beryllium (Be) and Magnesium (Mg) — both in Group 2 with configuration ending in s².
(c) Helium (He), Neon (Ne), Argon (Ar) — all noble gases in Group 18 with complete octets (or duplet for He).
(a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
Answer
Yes, all three elements have one electron in their outermost (valence) shell:
Li: 2,1 | Na: 2,8,1 | K: 2,8,8,1
This single valence electron is easily lost during chemical reactions, giving all of them a valency of 1 and similar reactive properties. They are all alkali metals in Group 1.
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Answer
Both helium and neon have completely filled outermost electron shells:
He: 2 (duplet complete) | Ne: 2,8 (octet complete)
Having stable, fully occupied valence shells means they have no tendency to gain, lose, or share electrons, making them chemically inert (or nearly so).
In the Modern Periodic Table, which are the metals among the first ten elements?
Answer
Among the first ten elements (H, He, Li, Be, B, C, N, O, F, Ne), only Lithium (Li) and Beryllium (Be) are metals. They are positioned on the left side of the periodic table, consistent with the pattern that metals occupy the left and centre of the table.
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?
Answer
Gallium (Ga) would have the maximum metallic characteristic. In the periodic table, metallic character increases down a group and decreases across a period from left to right. Among the given elements, Ga is furthest to the left and also has a larger atomic size compared to Be (which is in a higher period but far left). Ga is in Group 13, Period 4, giving it strong metallic character.
Section B: NCERT Exercise Questions
Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic table?
(a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their valence electrons more easily.
(d) The oxides become more acidic.
Answer
(c) is incorrect. As we move from left to right across a period, the nuclear charge increases while atomic size decreases. This makes it harder for atoms to lose valence electrons, not easier. Elements become less metallic and more electronegative across a period.
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:
Answer
(a) Na (b) Mg (c) Al (d) Si
Ans. (b) Magnesium (Mg). Since the formula is XCl₂, the valency of X is 2. Magnesium also has a valency of 2 and forms MgCl₂. The high melting point indicates ionic bonding, consistent with a Group 2 metal.
Q3. Which element has
(a) two shells, both of which are completely filled with electrons?
(b) the electronic configuration 2, 8, 2?
(c) a total of three shells, with four electrons in its valence shell?
(d) a total of two shells, with three electrons in its valence shell?
(e) twice as many electrons in its second shell as in its first shell?
Ans.
(a) Neon (Ne) — configuration 2, 8 (both shells full)
(b) Magnesium (Mg) — atomic number 12
(c) Silicon (Si) — configuration 2, 8, 4
(d) Boron (B) — configuration 2, 3
(e) Carbon (C) — configuration 2, 4 (second shell has 4, first has 2; 4 = 2 × 2)
(a) What property do all elements in the same column of the Periodic Table as boron have in common?
Answer
All elements in Group 13 (boron's column) have three valence electrons and a valency of 3. They form compounds with the general formula MX₃ (e.g., BCl₃, AlCl₃).
(b) What property do all elements in the same column as fluorine have in common?
Answer
All elements in Group 17 (halogens) have seven valence electrons and a valency of 1. They are highly reactive non-metals that readily gain one electron to form negative ions (X⁻).
An element has the electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
(b) To which of the following elements would it be chemically similar? N(7), F(9), P(15), Ar(18)
Answer
(a) Atomic number = 2 + 8 + 7 = 17 (this element is chlorine).
(b) It would be chemically similar to Fluorine (F) because fluorine also has 7 valence electrons (configuration 2, 7). Both are in Group 17 and have the same valency of 1.
The position of three elements A, B and C in the Periodic Table are shown below:
Answer
(A is in Group 16 Period 2; B is in Group 17 Period 2; C is in Group 16 Period 3)
Ans.
(a) A is a non-metal (it is in the upper right of the table, in Group 16 — this is oxygen).
(b) C is less reactive than A. Both are in Group 16, but C is in a lower period (larger atom). As we go down a group among non-metals, reactivity decreases.
(c) C has a larger atomic size than B. Even though B and C are in the same period, C is to the left (Group 16) while B is to the right (Group 17). Actually, comparing across the period is tricky — but since C is in Period 3 and B in Period 2, C is definitely larger. (If both were in Period 2, B would be smaller as it's further right.)
(d) A would form an anion with the formula A²⁻ because A has 6 valence electrons and needs to gain 2 to complete its octet.
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?
Answer
Nitrogen: 2, 5 | Phosphorus: 2, 8, 5
Nitrogen is more electronegative than phosphorus. Both have the same number of valence electrons (5), but nitrogen has a smaller atomic size (only 2 shells vs. 3 shells for P). The smaller atom means the nucleus has a stronger pull on incoming electrons, giving nitrogen a greater tendency to attract shared pairs of electrons toward itself.
How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Answer
The Modern Periodic Table is based on atomic numbers, and electronic configuration is directly determined by atomic number:
• The number of shells (energy levels) occupied by electrons determines the period. For example, Na (2, 8, 1) has 3 shells
⇒ Period 3.
• The number of valence electrons determines the group. For example, Na has 1 valence electron
⇒ Group 1.
Thus, knowing the electronic configuration immediately tells you where an element sits in the periodic table.
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?
Answer
Elements with atomic numbers 12 (Magnesium) and 38 (Strontium) have properties resembling calcium. All three are in Group 2 with 2 valence electrons. Mg is directly above Ca and Sr is directly below Ca in the periodic table. Elements 19 (Potassium, Group 1) and 21 (Scandium, Group 3) are in different groups and have different chemical properties.
Compare and contrast the arrangement of elements in Mendeleev's Periodic Table and the Modern Periodic Table.
Answer
Mendeleev's Table
• Based on atomic mass
• Had 8 groups (with sub-groups A and B)
• Could not explain position of isotopes
• Some anomalies (Co before Ni despite higher mass)
• Left gaps for undiscovered elements (strength)
Modern Periodic Table
• Based on atomic number (Moseley's contribution)
• Has 18 groups and 7 periods
• Isotope problem resolved (same atomic number = same position)
• No anomalies in element ordering
• Clear separation of metals, non-metals, and metalloids
• Position directly linked to electronic configuration
Section C: Important Extra Questions
State the Modern Periodic Law.
Answer
The Modern Periodic Law states: "The physical and chemical properties of elements are a periodic function of their atomic numbers." This means that when elements are arranged in order of increasing atomic number, elements with similar properties recur at regular intervals.
How do atomic size, metallic character, and electronegativity vary (a) across a period and (b) down a group?
Answer
Across a period (left to right)
• Atomic size decreases (increased nuclear charge pulls electrons closer)
• Metallic character decreases (harder to lose electrons)
• Electronegativity increases (greater tendency to attract electrons)
Down a group (top to bottom)
• Atomic size increases (more electron shells added)
• Metallic character increases (outermost electrons farther from nucleus, easier to lose)
• Electronegativity decreases (weaker pull on electrons due to larger size and shielding)
What are metalloids? Give two examples.
Answer
Metalloids are elements that show properties intermediate between metals and non-metals. They are found along the zig-zag line (staircase) in the periodic table that separates metals from non-metals. They can behave as both conductors and insulators depending on conditions, making them useful as semiconductors.
Examples: Silicon (Si) and Germanium (Ge) — both widely used in electronics and semiconductor technology.
An element has atomic number 16. Predict its position in the periodic table and state whether it is a metal or non-metal.
Answer
Atomic number 16
⇒ Electronic configuration: 2, 8, 6
• Number of shells = 3 → Period 3
• Number of valence electrons = 6
⇒ Group 16
• This element is Sulphur (S), a non-metal.
It has 6 valence electrons, so its valency is 2 (needs 2 more electrons to complete octet). It forms the oxide SO₂ (or SO₃) and the hydride H₂S.