Chemical Bonding

Question 6.19: Why the electron affinities of II–A elements are less than those of I–A group elements?
Answer: This can be explained on the basis of electronic distribution in valence subshell. The II–A group elements have relatively stable electronic configuration due to completely filled s-sub shell. So extra electron has to be accommodated in the higher orbital. So, their electron affinities are lower. On the other hand, I–A group elements have partially filled s-sub shell, so incoming electron can be added to partially filled orbital. Thus, their electron affinities are higher.

Question 6.20: First electron affinity for the most of the elements is negative, while the second electron affinity for all elements is positive.
Answer: Energy is usually released when elements like halogens absorb an electron and E.A. is expressed in negative figures. However, when a second electron is added to a uni-negative ion, the incoming electron is repelled by already present negative.

Question 6.21: Why does fluorine has electron affinity less than that of chlorine?
Answer: As fluorine atom has very small size and outer 2p subshell is attracted very close to nucleus as well as 2s subshell of fluorine atom. So in fluorine atom two electrons in 2s subshell and seven electrons in 2p subshell make a thick electronic cloud. This thick electronic cloud repels the incoming electrons greatly as compared to electronic cloud of chlorine atom. So incoming electron is less attracted by nucleus of fluorine atom. Hence, its electron affinity is lower.

Question 6.23: How does the electronegativity difference decide the nature of ionic bond?
Answer: When the electronegativity difference between two bonded atoms is 1.7 or more than that, then the bond is said to be ionic. If the difference is less than 1.7, then bond is said to polar covalent bond. For example, in NaCl the electronegativity difference between Na and Cl is 2.3 is bond between Na and Cl is ionic. On the other hand, in H-Cl electronegativity difference between H and Cl is 1, hence, bond between H and Cl is polar covalent.

Question 6.24: No bond in chemistry is 100% ionic. Justify it.
Answer: In the periodic table, F atom has maximum electronegativity of value 4 and Cs has minimum ionization energy. So, the electronegativity difference between F and Cl is maximum than the electronegativity difference between any two elements. The percentage of ionic character for CsF is 92. This indicates that there is no ionic bond with 100% ionic character.

Question 6.25: Why NH3 and PH3, give coordinate covalent bonds with H+.
Answer: Let us consider NH3 and H+. There is a lone pair of electrons on nitrogen atom of ammonia. On the other hand, H+ is electron deficient and it needs two electrons to attain stable electronic configuration of He. As a result, it accepts electron pair from donor NH3 and makes coordinate covalent bond. Similarly, PH3 has a lone pair on P atom that forms a coordinate covalent bond with H+.i.e.

Question 6.26: A coordinate covalent bond is produced between NH3 and BF3, although the BF3 does not have positive charge. Why?
Answer: Nitrogen atom of ammonia has a lone pair of electrons, while boron atom is electron deficient. Actually, Boron atom needs two electrons to complete its octet and to attain stable electronic configuration of Ne. Therefore, it accepts pair of electron from donor NH3 and results in formation of coordinate covalent bond.
Question 6.27: The distinction between a co-ordinate covalent bond and covalent bond vanishes after bond formation in NH4+, H3O+ and CH3NH3+. Why?
Answer: As, NH3, H2O and CH3NH2 donate a pair of electrons to H+ ion to form NH4+, H3O+ and CH3NH3+, respectively, the electron pair becomes a common property of the two bonded atom. So, the distinction between covalent and coordinate covalent bond vanishes.

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