Chapter 5: Periodic Classification of Elements

Answer: Yes, some triads of Dobereiner were found in Newlands' octaves. For example, the triad Li, Na, K had atomic masses 7, 23, 39 respectively. In Newlands' classification, Li (1st element), Na (8th element), K (15th element) fell into the same column (every 8th element), showing similar properties.

Answer: (i) It could only classify a few elements into triads. (ii) All known elements could not be grouped into triads. (iii) In some triads, the atomic mass of the middle element was not exactly the arithmetic mean of the other two.

Answer: (i) It was applicable only up to calcium. After calcium, every 8th element did not have similar properties. (ii) Newlands placed two elements in the same slot (e.g., Co and Ni). (iii) It did not leave any space for new elements. (iv) It placed iron with elements of different properties.

Answer: (a) Groups – vertical columns containing elements with similar properties. (b) 8 groups (I to VIII) and 7 periods (horizontal rows). Some groups were subdivided into A and B sub-groups. (c) Mendeleev's classification was based on atomic mass and similarity in chemical properties.

Answer: Hydrogen has properties similar to both alkali metals (Group I) and halogens (Group VII). It can lose an electron to form H⁺ like metals, or gain an electron to form H⁻ like halogens. This dual behaviour made its position uncertain.

Answer: The period number corresponds to the number of electron shells (principal quantum number). The group number for main group elements equals the number of valence electrons. Thus, electronic configuration determines both group and period.

Answer: Among the first ten elements (H to Ne), the metals are: Lithium (Li), Beryllium (Be). Boron (B) is a metalloid. Sodium (Na) is 11th, not in first ten.

Answer: Be (beryllium). Metallic character decreases along a period (from left to right) and increases down a group. Be is in group 2, period 2. Among given, Ga (group 13) is less metallic than Be. So Be has the maximum metallic character.

Answer: (c) The atoms lose their electrons more easily – this is false; across a period, ionization energy increases, so losing electrons becomes harder.

Answer: (b) Mg – since XCl₂ indicates X has valency 2 (group 2 metals like Mg, Ca).

Answer: (a) Element with atomic number 7 (Nitrogen) – non-metals form acidic oxides. Atomic number 3 (Li) and 12 (Mg) and 19 (K) are metals, form basic oxides.

Answer: (a) Electronic configuration: 2, 8, 2 (1s²2s²2p⁶3s²). (b) Period – 3; Group – 2 (alkaline earth metal). (c) Metal. (d) Valency = 2. (e) Chloride: MgCl₂, Oxide: MgO.

Answer: (a) X (Cl, atomic number 17) – Group 17, Period 3; Y (Ca, atomic number 20) – Group 2, Period 4. (b) Compound: CaCl₂ (calcium chloride). (c) Ionic compound (salt).

Answer: Across a period: atomic radius decreases, metallic character decreases, ionization energy increases, electronegativity increases. Down a group: atomic radius increases, metallic character increases, ionization energy decreases, electronegativity decreases (for main group elements).

Answer: (i) Na₂SO₄ (ii) AlCl₃ (iii) Ca(OH)₂ (iv) Mg(HCO₃)₂ (v) K₃PO₄.

Answer: Valency is the number of electrons lost, gained, or shared to achieve stable octet (or duplet). For Mg (2,8,2), it can lose 2 electrons to attain octet, so valency = 2.

Answer: Mendeleev's table arranged elements by increasing atomic mass, with groups for similar properties. Modern table arranges by increasing atomic number. Mendeleev's had gaps for undiscovered elements; modern has no gaps. Modern has 18 groups (vs 8 groups in Mendeleev's). Modern table explains periodicity through electronic configuration.

Answer: Modern periodic law states: "The physical and chemical properties of elements are periodic functions of their atomic numbers." It was proposed by Henry Moseley (1913).

Answer: Mendeleev left gaps for elements that were not yet discovered, predicting their properties. Later, elements like gallium (eka-aluminium), scandium (eka-boron), and germanium (eka-silicon) were discovered and filled those gaps.

Answer: (i) Based on atomic number, not atomic mass, so no anomalies (like K and Ar). (ii) Separates metals and non-metals clearly. (iii) Explains periodicity via electronic configuration. (iv) Positions of isotopes are not an issue.