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Periodic Trends and Chemical Reactivity

We have already studied the periodic trends in the various fundamental properties such as atomic and ionic radii, ionization energy, electron affinity, electronegativity and valence. The periodic trends in these properties can be explained on the basis of electronic configuration of the elements. The chemical reactivity of elements can be related to the fundamental properties of elements. As already discussed the ionization energy is least for the element at the extreme left of the period and the electron affinity is most negative for the element at the extreme right of the period (For group-17 elements). The elements of group-18 have positive electron affinity due to their stable electronic configurations. As a result, the chemical reactivity is maximum at the two extremes and lowest in the centre. The extreme reactivity of group-1 elements is due to the ease with which these elements can lose an electron leading. to the formation of corresponding cation. On the other hand, the reactivity of halogens is due to the ease with which these elements can gain an electron to form the corresponding anion. Thus, elements at the extreme left exhibit strong reducing behaviour whereas the elements at the extreme right exhibitstrong oxidizing behaviour.

 

Since ionization energy of alkali metals decreases on moving down the group their reactivity increases from lithium to caesium. For example, lithium reacts with water slowly, sodium reacts vigorously whereas potassium reacts almost violently. Reaction of caesium with water is explosive

2M + 2H2O                2MOH + H2

(M = Li, Na or K)

 

On the other hand among halogens, the reactivity decreases on going down the group because tendency to gain electron becomes less on descending the group.

Noble gases are almost inert due to their stable electronic configuration.

 

EVALUATE YOURSELF

Objective type Questions

Select the most appropriate choice from the options given as

(a), (b), (c) and (d) after each question.

 

1. In the long form of periodic table the elements are arranged according to their

(a) Mass number (b) Atomic number

(c) Neutron number (d) Atomic mass.

 

2. Which of the following sets of elements have the similar electronic configuration?

(i) H, He, Be               (ii) Li, Be, B

(iii) He, Ne, Ar            (iv) Li, Na, K

(a) (iii)                         (b) (iii), (iv)

(c) (iv)                         (d) (ii), (iii)

 

3. Transition metals are:

(a) found between groups 2 and 13

(b) all solids

(c) metals and semi-metals

(d) found between groups 1 and 13.

 

4. Hydrogen can be placed along with group 17 elements because it

(i) can aquire noble gas configuration by gaining one electron.

(ii) exhibits oxidation state of+ 1.

(iii) has one electron in the outermost shell.

(iv) can exhibit oxidation state of -1.

(a) (ii) and (iii) (b) (i) and (iv)

(c) (i) (d) (iii).

 

5. number of elements in the 5th period of the periodic table is

(a) 3                 (b) 9

(c) 8                  (d) 18.

 

6. In the modern periodic table, the period indicates the value of:

(a) atomic number                   (b) atomic mass

(c) principal quantum number

(d) azimuthal quantum number.

 

7. The correct order of size among Cl, Cl+ and CI- is

(a) CI+ < CI- < Cl (b) Cl+ > CI- > Cl

(c) Cl+ < Cl < CI- (d) CI- < Cl < CJ+.

 

8. Considering the elements B, Al, Mg, and K, the correct order of their metallic character is

(a) B > Al > Mg > K               (b) Al > Mg > B > K

(c) Mg > Al > K > B                (d) K > Mg > Al > B.

 

9. Which of the following groups consists entirely of metals?

(a) 18               (b) 2

(c) 14               (d) 15.

 

10. Which of the following represents the correct order of ionic radii?

(a) s2- > Cl- > K+ > Ca2+      (b) Ca2+ > K+ > CI- > s2-

(c) CI- > S2- > K+ > Ca2+      (d) S2- > Ca2+ > K+ > CI-.

 

11. The outermost electronic configuration of the most electronegative element is

(a) ns2 np3 . (b) ns2 np4

(d) ns2 np6.

 

12. The first ionization energies of Na, Mg, Al and Si are in the order

(a) Na < Mg > Al < Si             (b) Na > Mg > Al > Si

(c) Na < Mg < Al > Si              (d) Na > Mg > AI < Si.

 

13. Keeping in view, the periodic law and periodic table, suggest which of the following elements should have maximum electronegative character?

(a) Oxygen      (b) Nitrogen

(c) Fluorine      (d) Astatine.

 

14. An element with atomic number 20 is placed in which period of the periodic table?

(a) 4                 (b) 3

(c) 2                  (d) 1.

 

15. Which of the following structures is associated with the biggest jump between the second and the third ionization energies?

(a) 1s22s22p6              (b) 1s22s22p63 s1

(c) ls22s22p63s2         (d) 1s22s22p1.

 

 

16. The statement that is not correct for periodic classification of elements is

 

(a)    the properties of elements are a periodic function of their atomic numbers.

(b)   non-metallic elements are less in number than metallic elements.

(c)    the first ionization energies of elements along a period do not vary in a regular manner   with increase in atomic number.

(d)   for transition elements, the d-subshells are filled with electrons monotonically with increase in atomic number.

 

17. Which one of the following statements is incorrect in relation to ionization energy?

(a)    Ionization energy increases for each successive electron.

(b)   The greatest increase in ionization energy is experienced on removal of electron from core noble gas configuration.

(c)    End of valence electrons is marked by a big jump in ionization energy.

(d)   Removal of electron from orbitals bearing lower n value is easier than from orbital having higher n value.

 

18. The incorrect statement among the following is

(a) The first ionization energy of Al is less than the first ionization energy of Mg

(b) The second ionization energy of Mg is greater than the second ionization energy of Na

(c) The first ionization energy of Na is less than the first ionization energy of Mg

(d) The third ionization energy of Mg is greater than the third ionization energy of AL

 

19.  Which of the following ions has the highest value of ionic radius?

(a) Li+ (b) BJ+

(c) oz- (d) p-

 

20.  Which one of the following sets of ions represents the collection of isoelectronic species?

(a) K+, Ca2+, Sc3+, Cl-          (b) Na+, Ca2+, Sc3+, p-

(c) K+, Cl-, Mg2+, Sc3+         (d) Na+, Mg2+, Al3+, Cl-

(Atomic numbers: F= 9, Cl = 17, Na = 11, Mg = 12, Al = 13,

K = 19, Ca = 20, Sc = 21).

 

II. Fill in the blanks.

21 . Complete the following sentences by supplying appropriate words.

(i) The most electropositive element in second period is …… whereas the most electronegative element is …….

(ii) The ionization energy of nitrogen is …… than that of oxygen.

(iii) There are …… periods in long form of periodic table and …… groups.

(iv) The ions Ca2+ and sz- are isoelectronic with …… noble gas.

(v) The d-block elements are known as …… metals.

(vi) Ca2+ has a smaller ionic radius than K+ because it has …… .

(vii) Among halogens, the most negative electron affinity is of

(viii) The element with highest value of first ionization energy is …… .

(ix) Metallic radius is …… than the covalent radius.

(x) Modem periodic law was proposed by ……

(xi) Metallic character …… from top to bottom in a group.

(xii) Amongst the elements Na, Mg and Al the highest value of second ionization energy is of …… .

 

 

III. Discussion Questions

22. What is modem periodic law? What is periodicity and what is its cause?

23. What is the total number of groups in. the. long form of periodic table?

24.  An element has atomic number 34. Deduce its period, group and block.

25. Write the general outer electronic configuration of s-, p-, d and /-block elements.

26.  Which groups constitute d-block of the periodic table?

27. An element has valence shell electronic configuration as ns2, np3  To which group does this element belong?

28. In what group of the periodic table is each of the following elements found?

(i) 1s2, 2s2, 2p6, 3s2, 3d1, 4r  (ii) [Xe] 4f4 Set 6s2

(iii) [Ar] 3d10 4s2                     (iv) [Ar] 3d10, 4s2, 4p4.

29. Out of metallic radius and covalent radius of an element, which is larger and why?

30. Why van der Waals’ radius of an element is always larger than the covalent radius?

31. Out of the following ions which has the smallest ionic size?

Li+, Na+, K+

32. Arrange the following ions in the increasing order of their sizes.

CI-, P3-, s2-, F -

33. Arrange the following sets of ions in the decreasing order of their sizes

(i) Al3-, Mg2+, Na+,O2-, F(

(ii) Na+, Mg2+, K+.

34. Out of I and I+ which has larger size and why?

35. Which element has the highest ionization energy?

36. Among the alkali metals which element has the highest ionization energy?

37. Arrange each of the following sets of elements in the increasing order of their ionization energies:

(i) 0, N, S                    (ii) C, N, 0

(iii) Li, Be, Na             (iv) Ne, He, Ar.

38. Out of Na (Z = 11) and Mg (Z = 12), which has higher second ionization energy and why?

39. Explain why

(i) Be has higher ionization energy than B

(ii) 0 has lower ionization energy than N and F?

40. Why halogens have highest negative electron affinities in their respective periods?

41. Why noble gases have largest positive electron affinity in their respective periods?

42.  Comment on the statement that “all elements having high ionization energies also have high negative electron affinities.”

43. Out of oxygen and sulphur, which has greater negative electron affinity and why?

44. Consider the electronic configurations of the elements X, Y and Z and answer.

X : ls2, 2s2, 2px1, 2py1

Y : ls2 2 s2 2p 2 2p 1 2p 1

Z: 1s2, 2s2, 2px1, 2py1, 2pz1

(i) Which element has highest negative electron affinity?

(ii) Which element has lowest negative electron affinity?

 

45. In each of the following sets, arrange the elements in the increasing order of their negative electron affinities:

(i)C,N,O          (ii) 0, N, S

(iii) S, Cl, Ar    (iv) F, Cl, Br.

 

46. What is the trend of metallic character on going down from top to bottom in a group?

47. Define electronegativity. How does it vary along a period and along a group?

48. What is the basic difference between the terms electron affinity and electronegativity?

49. Why second ionization energy is always higher than first ionization energy?

 

50. A, B, C are three elements with atomic numbers, Z- 1, Z, Z + 1 respectively. B is an inert gas.

Answer the following questions:

(i) Predict the group of A and C.

(ii) Which out of the three has positive electron gain energy and why?

(iii) Which of the three has least value of ionization energy?

 

51. The electronic configurations of some elements are given as:

(a) [Ne] 3s2 3p3          (b) [Ne] 3s2 3p4

(c) [Ne] 3s2 3r             (d) [Ne] 3s2 3p63tf’ 4s1

(i) Which element will be most metallic ?

(ii) Which element will have most negative electron affinity?

(iii) Which element belongs to d-block?

(iv) Which element belongs to group 17?

(v) Out of a, b and c which will have least ionization energy?

 

52. Account for the following:

(i) Be has slightly higher value of ionization energy than that of boron (B).

(ii) The ionization energy of Na+ is more than that of Ne although they have same configuration.

(iii) Electron affinity of Cl is more negative than that of F. Among the elements of second period Li to Ne pick out the element

(i) with the highest ionization energy

(ii) with highest negative electron affinity

(iii) with largest covalent radius

(iv) that is most reactive non-metal

(v) that is most reactive metal.

 

54. The first (IE1) and the second (IE2) ionization energies (in kJ moJ-1) and the electron affinity (in kJ mol-1) of a few elements are a given as follows:

Which of the above element is likely to be:

(i) the least reactive element.

(ii) the most reactive metal.

(iii) the most reactive non-metal.

(iv) the least reactive non-metal.

(v) the metal which can form a stable binary halide of the formulae