Periodic Table Questions and Answers
Page no-81
1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Some of Döbereiner’s triads can be found in Newlands’ Octaves. Example: Lithium (Li), Sodium (Na), and Potassium (K), which form a triad in Döbereiner’s classification, are also placed in the same column in Newlands’ Octaves. However, not all of Döbereiner’s triads fit perfectly into Newlands' arrangement.
2. What were the limitations of Döbereiner’s classification?
- Limited to only a few elements – Döbereiner could identify only three triads from known elements.
- Did not apply to all elements – Many elements did not fit into the triad pattern.
- No clear periodic trend – The classification did not provide a comprehensive way to arrange all elements systematically.
3. What were the limitations of Newlands’ Law of Octaves?
- Worked only up to Calcium (Ca) – After Ca, the pattern did not hold.
- Assumed only 56 elements existed – Later, new elements were discovered that did not fit the law.
- Misplaced elements – Cobalt (Co) and Nickel (Ni) were placed in the same column as Fluorine (F), Chlorine (Cl), and Bromine (Br), which have different properties. Iron (Fe), which is similar to Co and Ni, was placed far away from them.
- Noble gases were not discovered – The discovery of noble gases made the law irrelevant.
Page no-85
1. Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements:
- Potassium (K) → K₂O
- Carbon (C) → CO₂
- Aluminium (Al) → Al₂O₃
- Silicon (Si) → SiO₂
- Barium (Ba) → BaO
2. Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (Any two)
- Scandium (Sc) → Eka-boron
- Germanium (Ge) → Eka-silicon
3. What were the criteria used by Mendeleev in creating his Periodic Table?
- Atomic Mass: Elements were arranged in increasing atomic mass.
- Chemical Properties: Elements in the same group had similar chemical properties.
- Formulae of Oxides & Hydrides: Grouping was based on how elements combined with oxygen & hydrogen (e.g., R₂O, RO₂).
- Periodic Law: "The properties of elements are the periodic function of their atomic masses."
- Gaps for Undiscovered Elements: Mendeleev predicted missing elements and their properties.
4. Why do you think the noble gases are placed in a separate group?
- Highly Unreactive: Noble gases (He, Ne, Ar, etc.) are chemically inert due to their full valence electron shells.
- Late Discovery: They were discovered after Mendeleev’s table due to their low atmospheric concentrations.
- No Chemical Similarity with Other Groups: Unlike other elements, noble gases do not form many compounds, so they were placed in a new group (Group 0) without disturbing the table’s order.
Page no-90
1. How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?
- Atomic Number as Basis: Elements are arranged by increasing atomic number rather than atomic mass, resolving inconsistencies like the position of Co (Cobalt) and Ni (Nickel).
- Position of Isotopes: Isotopes (e.g., Cl-35 and Cl-37) have the same atomic number and are placed in the same group.
- Position of Hydrogen: Hydrogen's placement remains ambiguous, but it is often placed in Group 1 due to its single valence electron.
- Grouping of Elements: Elements with similar properties are in the same group based on electronic configuration.
2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Calcium (Ca) and Beryllium (Be). Reason: They belong to Group 2 (Alkaline Earth Metals) and have two valence electrons, similar to Magnesium (Mg).
(a) Three elements that have a single electron in their outermost shells:
Hydrogen (H), Lithium (Li), and Sodium (Na) (Group 1 elements).
(b) Two elements that have two electrons in their outermost shells:
Beryllium (Be) and Magnesium (Mg) (Group 2 elements).
(c) Three elements with filled outermost shells:
Helium (He), Neon (Ne), and Argon (Ar) (Noble gases with complete outermost shells).
(a) Lithium, sodium, and potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
Yes. All belong to Group 1 (Alkali Metals) and have one valence electron, making them highly reactive.
(b) Helium is an unreactive gas, and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Both have a completely filled outermost shell. Helium has 2 electrons (1st shell full, 1s²), while Neon has 8 electrons (2nd shell full, 2s²2p⁶), making them stable and chemically unreactive (Noble Gases).
In the Modern Periodic Table, which are the metals among the first ten elements?
Lithium (Li) and Beryllium (Be). They are found on the left side of the Periodic Table and are classified as metals.
By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic character?
Beryllium (Be). Reason: Metallic character increases down a group and decreases across a period from left to right. Among Ga (Gallium), Ge (Germanium), As (Arsenic), Se (Selenium), and Be (Beryllium), Be is in Group 2 and is more metallic compared to the other elements, which are farther right in the Periodic 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?
- (a) The elements become less metallic in nature. ✅ (Correct)
- (b) The number of valence electrons increases. ✅ (Correct)
- (c) The atoms lose their electrons more easily. ❌ (Incorrect)
- (d) The oxides become more acidic. ✅ (Correct)
Answer: (c) The atoms lose their electrons more easily.
As we move from left to right across a period, effective nuclear charge increases, so atoms hold onto their electrons more tightly, making it harder to lose them.
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:
- (a) Na (Sodium) – Group 1
- (b) Mg (Magnesium) – Group 2 ✅ (Correct Answer)
- (c) Al (Aluminum) – Group 13
- (d) Si (Silicon) – Group 14
Explanation: The formula XCl₂ suggests that X has a valency of +2, meaning it belongs to Group 2 (Alkaline Earth Metals), just like Magnesium (Mg). High melting point also indicates it is a metal, reinforcing its placement in Group 2.
3. Which element has:
- (a) Two shells, both of which are completely filled with electrons? Neon (Ne) – Atomic number 10 (Electronic configuration: 2, 8) ✅
- (b) The electronic configuration 2, 8, 2? Magnesium (Mg) – Atomic number 12 ✅
- (c) A total of three shells, with four electrons in its valence shell? Silicon (Si) – Atomic number 14 (Electronic configuration: 2, 8, 4) ✅
- (d) A total of two shells, with three electrons in its valence shell? Boron (B) – Atomic number 5 (Electronic configuration: 2, 3) ✅
- (e) Twice as many electrons in its second shell as in its first shell? Carbon (C) – Atomic number 6 (Electronic configuration: 2, 4 → 4 is twice of 2) ✅
4. (a) What property do all elements in the same column of the Periodic Table as boron have in common?
Same number of valence electrons (3 in Group 13) → They form trivalent (3+) ions.
(b) What property do all elements in the same column of the Periodic Table as fluorine have in common?
Same number of valence electrons (7 in Group 17) → They are highly electronegative and tend to gain 1 electron to form negative ions (anions).
5. An atom has an electronic configuration 2, 8, 7.
- (a) What is the atomic number of this element? Atomic number = 2 + 8 + 7 = 17 (Chlorine, Cl) ✅
- (b) To which of the following elements would it be chemically similar? Fluorine (F) – Atomic number 9 ✅
- Reason: Both belong to Group 17 (Halogens) and have 7 valence electrons, showing similar chemical properties.
6. The position of three elements A, B, and C in the Periodic Table are shown below:
Group 16 Group 17
- -
- A
- -
B C
- (a) State whether A is a metal or non-metal. A is a non-metal (since it belongs to Group 17, Halogens). ✅
- (b) State whether C is more reactive or less reactive than A. C is less reactive than A because reactivity in Group 17 decreases down the group. ✅
- (c) Will C be larger or smaller in size than B? C will be smaller than B because atomic size decreases across a period from left to right. ✅
- (d) Which type of ion (cation or anion) will be formed by element A? A will form an anion (negative ion) because it gains electrons to complete its octet. ✅
7. Nitrogen (atomic number 7) and Phosphorus (atomic number 15) belong to Group 15. Write their electronic configurations. Which is more electronegative?
- Nitrogen (N): 2, 5
- Phosphorus (P): 2, 8, 5
More electronegative: Nitrogen (N) because:
- Smaller atomic size → Stronger attraction for electrons.
- Higher effective nuclear charge than phosphorus. ✅
8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Period Number = Number of electron shells.
Group Number = Number of valence electrons (for Groups 1-2 & 13-18).
Example: Sodium (Na, Atomic number 11) → Electronic configuration: 2, 8, 1
Period = 3 (because it has 3 shells).
Group = 1 (because it has 1 valence electron). ✅
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?
Magnesium (12) and Strontium (38). ✅ Reason: They all belong to Group 2 (Alkaline Earth Metals) and have two valence electrons.
10. Compare and contrast Mendeléev’s Periodic Table and the Modern Periodic Table.
| Mendeléev’s Periodic Table | Modern Periodic Table |
|---|---|
| Elements arranged by atomic mass. | Elements arranged by atomic number. |
| 63 elements were known. | 118 elements are known today. |
| Groups were divided into subgroups (A and B). | Groups are numbered 1 to 18, no subgroups. |
| No place for noble gases (they were discovered later). | Noble gases are in Group 18. |
| Isotopes caused problems (they had different masses). | Isotopes have same atomic number, so no issue. |
| Position of hydrogen was uncertain. | Still ambiguous, but often placed in Group 1. |
| Transition elements were not placed separately. | Transition elements are in Groups 3-12. |
✅ The Modern Periodic Table is more accurate and resolves many inconsistencies of Mendeléev’s Table.
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