Q4. Write Lewis symbols for S and S2−, Al and Al3+, H and H− (NCERT Exercise 4.3, page 133).
S has 6 dots and S2− 8 (an octet, shown in brackets with the charge). Al has 3 dots and Al3+ none (just the symbol with its charge). H has 1 dot and H− 2 (a duplet).
Q5. Draw Lewis structures for H2S, SiCl4, BeF2, CO32− and HCOOH (NCERT Exercise 4.4, page 133).
H2S: S with two S–H bonds and two lone pairs (8 valence electrons). SiCl4: Si with four Si–Cl bonds, three lone pairs on each Cl (32). BeF2: F–Be–F, three lone pairs on each F and only 4 electrons round Be, an incomplete octet (16). CO32−: C with one C=O and two C–O bonds; two lone pairs on the C=O oxygen, three on each C–O oxygen; in brackets with charge 2− (24). HCOOH: H–C(=O)–O–H, two lone pairs on each O (18).
Q6. Explain the formation of a chemical bond (NCERT Exercise 4.1, page 133).
Atoms combine to reach a more stable, lower-energy arrangement, usually a noble gas configuration in the outer shell. They do it either by transferring electrons, giving ions held together by electrostatic attraction (ionic bond), or by sharing electron pairs (covalent bond). The attractive force that results is the chemical bond.
Q7. Find the formal charge on each atom of HNO3 in the structure with one N=O, one N–O and one N–OH bond.
N: 5 − 0 − ½(8) = +1. The double-bonded O (2 lone pairs): 6 − 4 − 2 = 0. The single-bonded O with no H (3 lone pairs): 6 − 6 − 1 = −1. The O of OH (2 lone pairs, 2 bonds): 6 − 4 − 2 = 0. H: 0. Total 0.
Q8. NCERT draws acetic acid, CH3COOH, with an H=C double bond, a single bond from the second C to its top O, a lone pair on that C and one lone pair on the OH oxygen. The skeleton is right; write the correct Lewis structure (NCERT Exercise 4.20, page 134).
CH3 group: C with three C–H single bonds and one C–C single bond (no C=H double bond). The second C: a C=O double bond and a C–O single bond to the OH. Two lone pairs on each O. The lone pair on the second C becomes the second bond of C=O, and the OH oxygen gets a second lone pair. Total 24 valence electrons, every C and O with an octet.
Q9. How many electrons surround the central atom in BCl3, PF5 and SF6? Which obey the octet rule?
BCl3: 6 (incomplete octet). PF5: 10 and SF6: 12 (expanded octets). None of the three obeys it.