Chapter – 3
Classification of Elements and Periodicity in Properties
1. Explain why the electron gain enthalpy of fluorine is less negative than that of chlorine.
Answer.
This is due to the small size of the fluorine atom. As a result of the strong inter electronic repulsions in fluorine’s relatively small 2 p orbitals, the incoming electron does not experience much attraction.
2. All transition elements are d-block elements, but all d-block elements are not transitionelements. Explain.
Answer.
All transition metals are d-block elements, but not all d-block elements are transition elements because all d-block elements that do not have completely filled d- orbitals are not counted as transition elements, making such elements exceptional. Zn, Cd, and Hg are a few examples.
3. Identify the group and valency of the element having atomic number 119. Also, predict theoutermost electronic configuration and write the general formula of its oxide.
Answer.
For element having atomic number 119 : Group – 1, Valency – 1, Outermost electronic configuration – 8 s 1 and the general formula of the oxide will be M 2 O.
4. Ionisation enthalpies of elements of second period are given below :Ionisation enthalpy/ kcal mol-1 : 520, 899, 801, 1086, 1402, 1314, 1681, 2080.Match the correct enthalpy with the elements and complete the graph given in Fig. 3.1.Also, write symbols of elements with their atomic number.
Answer.
The order of ionisation enthalpy of second period elements is: Li < B < Be < C < O < N < F < NeThe correct enthalpy with the elements and the complete graph is given as follows-
5. Among the elements B, Al, C and Si,
(i) which element has the highest first ionisation enthalpy?
(ii) which element has the most metallic character?Justify your answer in each case.
Answer.
Among the elements, B, Al, C and Si
(i) The element that has the highest first ionisation enthalpy is C.
(ii) The element that has the most metallic character is Al.
6. Write four characteristic properties of p-block elements.
Answer.
The four most important properties of p-block elements are as follows:
(a) Both metals and nonmetals are present in p-block elements, but the number of nonmetals is much greater than that of metals. Furthermore, within a group, the metallic character increases from top to bottom, while the non-metallic character increases from left to right along a period in this block.
(b) Their ionisation enthalpies are higher than those of s-block elements.
(c) They mostly combine to form covalent compounds.
(d) Some of them have compounds with multiple (variable) oxidation states. In a period, their oxidising character increases from left to right, while their reducing character increases from top to bottom.
7. Choose the correct order of atomic radii of fluorine and neon (in pm) out of the options given below and justify your answer.
(i) 72, 160
(ii) 160, 160
(iii) 72, 72
(iv) 160, 72
Answer.
The atomic radius of F is usually expressed in terms of covalent radius, whereas the atomic radius of neon is usually expressed in terms of van der Waals radius. An element’s van der Waals radius is always greater than its covalent radius. As a result, the atomic radius of F is less than the atomic radius of Ne (F = 72 pm, Ne = 160 pm).
8. Illustrate by taking examples of transition elements and non-transition elements that oxidation states of elements are largely based on electronic configuration.
Answer.
An element’s oxidation state is determined by its electronic configuration.S-block elements : Group 1 elements- General electronic configuration of the valence shell ns 1. Oxidation states =+1.Group 2 elements- General electronic configuration of the valence shell is ns 2. Oxidation state =+2 P-block elements.Group 13 elements: General electronic configuration of a valence shell is ns 2, np 1. The oxidation states are +3 and +1.Group 14 elements: General electronic configuration of the valence shell is ns 2, np 2. Oxidation states are +4 and +2.Group 15 elements: General valence shell electronic configuration ns 2, np 3. Oxidation states are -3,+3, and +5. Nitrogen exhibits +1,+2,+4 oxidation states as well.Group 16 elements General electronic configuration of the valence shell ns 2, np 4. Oxidation states are -2,+2,+4 and +6.Group 17: General electronic configuration of the valence shell is ns 2, np 5. Oxidation state is -1. Cl, Br, and I also have +1,+3,+5, and +7 oxidation states.Group 18: General electronic configuration of the valence shell is ns 2 np 6 . In bonding, the oxidation state = zero.A transition metal’s various oxidation states are caused by the presence of (n-1)d and outer ns electrons. (n-1)d1-10, ns 1-2 is a general electronic configuration. The most common oxidation states for these elements are +2 and +3.
9. Nitrogen has positive electron gain enthalpy whereas oxygen has negative. However,oxygen has lower ionisation enthalpy than nitrogen. Explain.
Answer.
The outermost electronic configuration of nitrogen is 2 s2 2 px 1, 2 py 1, 2 pz 1 whereas that of oxygen is 2 s 2 2 px 2, 2 py 1, 2pz1.Since oxygen acquires a stable configuration, i.e., 2 p 3, by removing one electron from the 2 p-orbital, it has a lower ionisation enthalpy than nitrogen. In the case of nitrogen, however, due to its stable configuration, it is difficult to remove one of the three 2 p -electrons.
10. First member of each group of representative elements (i.e., s and p-block elements)shows anomalous behaviour. Illustrate with two examples.
Answer.
The first member of each group of representative elements (i.e., the s- and p- block elements) exhibits anomalous behaviour due to:
(i) small size
(ii) higher ionisation enthalpy
(iii) higher electronegativity
(iv) the absence of d- orbitals.For example, in s – block elements, lithium behaves differently than the other alkali metals.
(a) Lithium compounds have a high covalent character. Alkali metal compounds are predominantly ionic.
(b) Lithium nitride is formed when lithium reacts with nitrogen, whereas other alkali metals do not form nitrides.In p- block elements, the first member of each group has four orbitals in their valence shell, one 2 s orbital and three 2 p orbitals. As a result, these elements have a maximum covalency of four, whereas other members of the same or different group have a maximum covalency that is greater than four due to the availability of vacant d – orbitals.
11. p-Block elements form acidic, basic and amphoteric oxides. Explain each property by giving two examples and also write the reactions of these oxides with water.
Answer.
Due to their various properties, p – block elements produce acidic, basic, and amphoteric oxides:The higher an element’s electronegativity, the more acidic its oxide.For example- Boron has an electronegativity of -2, carbon has an electronegativity of 2.5, and nitrogen has an electronegativity of 3. As a result, the order of acidic character of B, C, and N oxides is B 2O3 < CO2 < N 2O3 If the ionisation enthalpy of an element is high, it will form acidic oxide; if it is intermediate, it will form amphoteric oxide; and if it is low, it will form basic oxides.For example, the ionisation enthalpy of boron is 800 while that of carbon is 1086.5, implying that carbon oxide is more acidic than boron oxide.The oxides of the first element in each group in the p – block are more acidic than the oxides of other elements. As we move down the group, the acidic character decreases, followed by elements that form amphoteric oxides and then basic oxides.For example- In the Boron family, B forms a weak acidic oxide, while Al, Ga, and In form amphoteric oxides, and Tl forms a strong basic oxide.Reactions of some of the oxides with water:Acidic Oxides:B2O3 + 3 H2O → 2H3BO3Basic Oxides:Tl2O + H2O → 2 TlOH Amphoteric Oxides are insoluble in water and thus reacts with acid and base:Al2O3 + 2 Na OH → 2 Na Al O2 + H2 O Al 2O3 + 6 HCl → 2AlCl3 + 3H2O
12. How would you explain the fact that first ionisation enthalpy of sodium is lower than that of magnesium but its second ionisation enthalpy is higher than that of magnesium?
Answer.
The electronic configuration of both the atoms are as follows:Na: [Ne] 3 s 1 Mg: [Ne] 3 s2 The ion formed after removing one electron from the sodium atom takes on the configuration of an inert gas, neon, whereas Mg retains one electron. As a result, the first ionisation energy of Na is lower than that of Mg. High energy is required to remove an electron from a noble gas configuration.As a result, the second ionisation enthalpy of calcium is greater than that of magnesium.
13. What do you understand by exothermic reaction and endothermic reaction?Give one example of each type.
Answer.
Exothermic reactions are those in which heat is produced. The formation of ammonia from nitrogen and hydrogen, for example, is an exothermic reaction.N2 + 3 H2 → 2 NH 3, Δ H = – 46.11KJ/mol.Endothermic reactions are ones in which heat is absorbed. The dissociation of ammonia into nitrogen and hydrogen, for example, is an endothermic reaction.2 NHv 3 → N2 + 3 H2, Δ H = +91.8KJ/mol.
14. Arrange the elements N, P, O and S in the order of-(i) increasing first ionisation enthalpy.
(i) increasing non-metallic character.Give reason for the arrangement assigned.
Answer.
(i) Due to the extra stable exactly half-filled 2 p- orbitals, the ionisation enthalpy of nitrogen (1s2,2s2,2p3) is greater than that of oxygen (1s2,2s2,2p4). Likewise, the ionisation enthalpy of phosphorous (1s2,2s2,2p6,3s2,3p3) is greater than that of sulphur (s2,2s2,2p6,3s2,3p3)Ionisation enthalpy decreases with decreasing atomic size as one moves down the group.As a result, the increasing order of first ionisation enthalpy is S < P < O < N(ii) Nonmetallic character increases across a period (left to right), but decreases as one moves down the group.As a result, the increasing non-metallic order is P < S < N < O.
15. Explain the deviation in ionisation enthalpy of some elements from the general trend by using Fig. 3.2.
Answer.
The ionisation enthalpy of some elements deviates from the general trend, as shown in the figure. The first ionisation enthalpy of B is lower than that of Be, whereas the first ionisation enthalpy of nitrogen is higher than that of O.
16. Explain the following:Electronegativity of elements increases on moving from left to right in the periodic table.Ionisation enthalpy decreases in a group from top to bottom?
Answer.
Due to the general decrease in atom size and increase in nuclear charge, the electronegative of elements increases as one moves from left to right in the periodic table.The ionisation enthalpy of a group decreases from top to bottom due to the increase in atomic size caused by the addition of a new shell.
17. How does the metallic and non metallic character vary on moving from left to right in a period ?
Answer.
Moving from left to right in a period, metallic character decreases and nonmetallic character increases. This is due to the increase in effective nuclear charge which causes an increase in ionisation enthalpy and electron gain enthalpy.
18. The radius of Na+ cation is less than that of Na atom. Give reason.
Answer.
Since Na+ is formed by losing one energy shell, its radius is smaller than that of Na atom.Na – 1 s 2, 2 s2, 2p6, 3s1Na+ – 1s2, 2s2, 2p6
19. Among alkali metals which element do you expect to be least electronegative and why?
Answer.
Due to the general increase in size, electronegativity decreases from top to bottom in a group. As a result, caesium is the least electronegative element.