Q.State Mendeleev Periodic Law.
Ans: The chemical and physical properties of the elements are periodic functions of their atomic
masses.
Q State modern periodic law.
Ans-The chemical and physical properties of the elements are periodic functions of their atomic
numbers.Mosley state this law.
Q.Why are isotopes not considered in Mendeleev’s periodic table?Name the elements which are
predicted by Mendeleev as eka –silicon and eka –aluminum ?
Ans.Because Mendeleev gave priority to similarities in properties.
Germanium and gallium.
Q. What are Dobereiner’s triads? Give example of 2 triads ?
Ans: he arranged the elements in several sets of 3 elements .these sets were called triads which
resembled with each other in chemical properties and atomic mass of the middle element was
approx the mean of the atomic masses of the remaining two.
Example (i) chlorine, bromine and iodine A.T of cl =35.5 and iodine =127 .middle element bromine
has (35.5 +127)/2 =81.25
(ii) Calcium (40) ,strontium(88.5) and barium (137)
Q Write the general E.C of s, p, d and f Block elements.(L2)
Ans: (i) s block- ns
1-2
(ii) p block – ns
2
np
1-6
(iii) d block – (n-1)d
1-10
ns
1-2
(iv) f block -- (n-2) f
1-14
(n-1)d
0-1
ns
Q Assign the position of the element having outer electronic configuration
(i)ns
2
np
4
for n=3 , {L3]
(ii) (n-1) d
2
ns
2
for n=4
(iii) (n-2)f
7
(n-1)d
1
ns
2
for n=6
Ans: (i) n= principal no. which shows no of periods. N=3 so E.C =3s23p4=elements having position 3
rd
period 16
th
group.
(ii) n=4 so elements belong in 4
th
period .it is d block element .d2= 2 electron in d orbital it belongs to
4
th
group in d block.
(iii) n=6 and electron goes in fshell, so it is f block element having 6
th
period and 7
th
element of
lanthanides series.
Q. Which out of F or Cl has a more negative electron gain enthalpy?
Ans. Cl has more negative electron gain enthalpy. It is because there is more inter electronic
repulsion between valence electrons of F, due to smaller size than Cl.
Q.1 Consider the following species
N
-3
, O
-2
, F-, Na
+
, Mg
+2
and Al
+3
(a). what is common in them?
(b). arrange them in the order of increasing ionic radii.
Ans. (a). They have same number of electrons.
(b). Al
3+
<Mg
2+
<Na
+
<F
-<O
2-<N
3
Q. Explain Screening effect ?
Ans..Screening effect. The inner electrons between valence electron and nucleus shield the
valence electrons from nucleus; it is called shielding effect.
Q.2 (a). Which is largest in size- Cu
+
, Cu
+2
, Cu and why?
Ans.. Cu is largest due to less effective nuclear charge. It has 29 electrons, 29 protons, Cu
+
has 28
electrons and 29 protons, Cu
2+
has 27 electrons and 29 protons.
Q. How does electro negativity vary down the group 17 and why? How does it vary from left to
right in the period? Name an element having highest electro negativity.
Ans. Electro negativity decreases down the group due to increase in atomic size. It increases along
the period from left to right due to decrease in atomic size.
Fluorine (F) has highest electro negativity.
Q.1 Would you expect the second electron gain enthalpy of O as positive, more negative or less
negative than the first? Justify your answer.
Ans. Second electron affinity of O is largely +ve because of repulsion between negatively charged
ions and second electron to be added. Energy required to overcome repulsion is more than the
energy released in gaining electron, so net energy is absorbed.
Q.Explain why Halogens have very high negative electron gain enthalpy?
Ans.The electron gain enthalpy for halogens is highly negative because they can acquire the nearest
stable noble gas configuration by accepting an extra electron.
Q. How would you explain the fact that the first ionization of Na is lower than that of Mg but its
second ionization enthalpy is higher than that of Mg? (L-3)
Ans. The electronic configurations of Na and Mg are: Na:1s
2
2s
2
2p
6
3s
1
And Mg: 1s
2
2s
2
2p
6
3s
2
thus, the
first electron in both the cases has to be removed from the 3s orbital, but the nuclear charge of Na is
lower than that of Mg, therefore, the first ionization energy of Na is lower than that of Mg. But after
the removal of the first electron, Na acquire the nearest noble gas configuration (Ne) which makes
Na highly stable, but an electron is still left in the p orbital of Mg. Hence second ionization energy of
Na is higher than that of Mg.
Q. Explain why Electron gain enthalpy of noble gases is positive. (L-1)
Ans. Noble gases have large positive electron gain enthalpies because the extra electron has to be
placed in the next higher principal quantum energy level, thereby producing highly unstable
electronic configuration.
Q. Explain why cations are smaller and anions are larger in size than their parent atom. (L-2)
Ans.The ionic radius of cation is always smaller than the parent atom because the loss of one or
more electrons increases the effective nuclear charge. As a result, the force of attraction of nucleus
for the electrons increases and hence, the ionic radii decreases. In contrast, the ionic radius of an
anion is always larger than its parent atom because the addition of one or more electrons decreases
the effective nuclear charge. As a result, the force of attraction of the nucleus for the electrons
decreases and hence the ionic radii increases.
Q Arrange the following order of the property indicated:
(a). F, Cl, Br and I (negative electron gain enthalpy)
(b). Mg, Al, Si and Na (ionization enthalpy)
(c). C, N, O and F (second ionization enthalpy)
Q. Arrange the following elements in increasing order of metallic character: B, Al, Mg and K.
Q. Why is fluorine (F2) more reactive than chlorine (Cl2)?
Q.Explain why
(i). Be has higher first ionization enthalpy than B?
(ii). O has lower first ionization enthalpy than N and F?
Q.3 ∆iH1 value of Mg is more as compared to that of Na while its ∆
iH2 value is less. Explain.
Ans. ∆iH1 value of Mg is more than that of Na due to greater symmetry and smaller size. But ∆iH2
value of Na is higher because Na
+
ion has the configuration of noble gas element neon while Mg
+
ion
does not have a symmetrical configuration.
Q. The increasing order of reactivity among group 1 elements is Li<Na<K<Rb<Cs whereas among
the group 17 elements, it is F>Cl>Br>I. Explain
Ans: The chemical and physical properties of the elements are periodic functions of their atomic
masses.
Q State modern periodic law.
Ans-The chemical and physical properties of the elements are periodic functions of their atomic
numbers.Mosley state this law.
Q.Why are isotopes not considered in Mendeleev’s periodic table?Name the elements which are
predicted by Mendeleev as eka –silicon and eka –aluminum ?
Ans.Because Mendeleev gave priority to similarities in properties.
Germanium and gallium.
Q. What are Dobereiner’s triads? Give example of 2 triads ?
Ans: he arranged the elements in several sets of 3 elements .these sets were called triads which
resembled with each other in chemical properties and atomic mass of the middle element was
approx the mean of the atomic masses of the remaining two.
Example (i) chlorine, bromine and iodine A.T of cl =35.5 and iodine =127 .middle element bromine
has (35.5 +127)/2 =81.25
(ii) Calcium (40) ,strontium(88.5) and barium (137)
Q Write the general E.C of s, p, d and f Block elements.(L2)
Ans: (i) s block- ns
1-2
(ii) p block – ns
2
np
1-6
(iii) d block – (n-1)d
1-10
ns
1-2
(iv) f block -- (n-2) f
1-14
(n-1)d
0-1
ns
Q Assign the position of the element having outer electronic configuration
(i)ns
2
np
4
for n=3 , {L3]
(ii) (n-1) d
2
ns
2
for n=4
(iii) (n-2)f
7
(n-1)d
1
ns
2
for n=6
Ans: (i) n= principal no. which shows no of periods. N=3 so E.C =3s23p4=elements having position 3
rd
period 16
th
group.
(ii) n=4 so elements belong in 4
th
period .it is d block element .d2= 2 electron in d orbital it belongs to
4
th
group in d block.
(iii) n=6 and electron goes in fshell, so it is f block element having 6
th
period and 7
th
element of
lanthanides series.
Q. Which out of F or Cl has a more negative electron gain enthalpy?
Ans. Cl has more negative electron gain enthalpy. It is because there is more inter electronic
repulsion between valence electrons of F, due to smaller size than Cl.
Q.1 Consider the following species
N
-3
, O
-2
, F-, Na
+
, Mg
+2
and Al
+3
(a). what is common in them?
(b). arrange them in the order of increasing ionic radii.
Ans. (a). They have same number of electrons.
(b). Al
3+
<Mg
2+
<Na
+
<F
-<O
2-<N
3
Q. Explain Screening effect ?
Ans..Screening effect. The inner electrons between valence electron and nucleus shield the
valence electrons from nucleus; it is called shielding effect.
Q.2 (a). Which is largest in size- Cu
+
, Cu
+2
, Cu and why?
Ans.. Cu is largest due to less effective nuclear charge. It has 29 electrons, 29 protons, Cu
+
has 28
electrons and 29 protons, Cu
2+
has 27 electrons and 29 protons.
Q. How does electro negativity vary down the group 17 and why? How does it vary from left to
right in the period? Name an element having highest electro negativity.
Ans. Electro negativity decreases down the group due to increase in atomic size. It increases along
the period from left to right due to decrease in atomic size.
Fluorine (F) has highest electro negativity.
Q.1 Would you expect the second electron gain enthalpy of O as positive, more negative or less
negative than the first? Justify your answer.
Ans. Second electron affinity of O is largely +ve because of repulsion between negatively charged
ions and second electron to be added. Energy required to overcome repulsion is more than the
energy released in gaining electron, so net energy is absorbed.
Q.Explain why Halogens have very high negative electron gain enthalpy?
Ans.The electron gain enthalpy for halogens is highly negative because they can acquire the nearest
stable noble gas configuration by accepting an extra electron.
Q. How would you explain the fact that the first ionization of Na is lower than that of Mg but its
second ionization enthalpy is higher than that of Mg? (L-3)
Ans. The electronic configurations of Na and Mg are: Na:1s
2
2s
2
2p
6
3s
1
And Mg: 1s
2
2s
2
2p
6
3s
2
thus, the
first electron in both the cases has to be removed from the 3s orbital, but the nuclear charge of Na is
lower than that of Mg, therefore, the first ionization energy of Na is lower than that of Mg. But after
the removal of the first electron, Na acquire the nearest noble gas configuration (Ne) which makes
Na highly stable, but an electron is still left in the p orbital of Mg. Hence second ionization energy of
Na is higher than that of Mg.
Q. Explain why Electron gain enthalpy of noble gases is positive. (L-1)
Ans. Noble gases have large positive electron gain enthalpies because the extra electron has to be
placed in the next higher principal quantum energy level, thereby producing highly unstable
electronic configuration.
Q. Explain why cations are smaller and anions are larger in size than their parent atom. (L-2)
Ans.The ionic radius of cation is always smaller than the parent atom because the loss of one or
more electrons increases the effective nuclear charge. As a result, the force of attraction of nucleus
for the electrons increases and hence, the ionic radii decreases. In contrast, the ionic radius of an
anion is always larger than its parent atom because the addition of one or more electrons decreases
the effective nuclear charge. As a result, the force of attraction of the nucleus for the electrons
decreases and hence the ionic radii increases.
Q Arrange the following order of the property indicated:
(a). F, Cl, Br and I (negative electron gain enthalpy)
(b). Mg, Al, Si and Na (ionization enthalpy)
(c). C, N, O and F (second ionization enthalpy)
Q. Arrange the following elements in increasing order of metallic character: B, Al, Mg and K.
Q. Why is fluorine (F2) more reactive than chlorine (Cl2)?
Q.Explain why
(i). Be has higher first ionization enthalpy than B?
(ii). O has lower first ionization enthalpy than N and F?
Q.3 ∆iH1 value of Mg is more as compared to that of Na while its ∆
iH2 value is less. Explain.
Ans. ∆iH1 value of Mg is more than that of Na due to greater symmetry and smaller size. But ∆iH2
value of Na is higher because Na
+
ion has the configuration of noble gas element neon while Mg
+
ion
does not have a symmetrical configuration.
Q. The increasing order of reactivity among group 1 elements is Li<Na<K<Rb<Cs whereas among
the group 17 elements, it is F>Cl>Br>I. Explain
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