Core Formula/Logic: Isoelectronic species have the same number of electrons. Count electrons = atomic number - charge for ions.
Step-by-Step Solution: 1. O2- has atomic number 8, charge -2: electrons = 8 - (-2) = 8 + 2 = 10 electrons.
2. F- has atomic number 9, charge -1: electrons = 9 - (-1) = 9 + 1 = 10 electrons.
3. Cl- has atomic number 17, charge -1: electrons = 17 - (-1) = 17 + 1 = 18 electrons.
4. Li+ has atomic number 3, charge +1: electrons = 3 - (+1) = 3 - 1 = 2 electrons.
5. K+ has atomic number 19, charge +1: electrons = 19 - (+1) = 19 - 1 = 18 electrons.
Common Pitfall: Forgetting that negative charge adds electrons and positive charge removes electrons gives wrong counts. Confusing O2- with O2 (molecule) gives 16 electrons. Miscalculating Cl- as 17 - 1 = 16 instead of 17 + 1 = 18 produces option B.
Shortcut/Takeaway: For main group ions, isoelectronic species often match noble gas configurations. O2- and F- both have 10 electrons like neon. Memorize common isoelectronic series: N3-, O2-, F-, Ne, Na+, Mg2+, Al3+ all have 10 electrons.

Why Correct: SF4 has a see-saw (or distorted tetrahedral) geometry due to 5 electron pairs around sulfur (4 bonding pairs with fluorine atoms and 1 lone pair), following VSEPR theory for AX4E type molecules.
Distractor Analysis: Square planar geometry occurs with 6 electron pairs and 2 lone pairs (AX4E2 type). Tetrahedral geometry requires 4 bonding pairs with no lone pairs (AX4 type). Octahedral geometry requires 6 bonding pairs with no lone pairs (AX6 type).
Takeaway: For molecules with 5 electron pairs, the geometries are: trigonal bipyramidal (AX5), see-saw (AX4E), T-shaped (AX3E2), and linear (AX2E3), where A is central atom, X is bonded atom, and E is lone pair.

Why Correct: H2S contains sulfur in -2 oxidation state, its minimum possible, so it can only lose electrons to oxidize to higher states like 0 or +6, making it exclusively a reducing agent.
Distractor Analysis: H2O2 acts as both oxidizing and reducing agent since oxygen has -1 oxidation state. MnO2 with Mn in +4 state primarily oxidizes but can reduce in strong oxidizing conditions. K2Cr2O7 (corrected from K2Cr2O2) with Cr in +6 is a strong oxidizing agent.
Takeaway: Substances with elements at their minimum oxidation state (like H2S, HI, SnCl2) act only as reducing agents; at maximum oxidation state (like KMnO4, K2Cr2O7) act only as oxidizing agents.

Why Correct: Decomposition reactions vary: thermal decomposition of calcium carbonate is endothermic and reversible, while potassium chlorate decomposition is exothermic and irreversible.
Distractor Analysis: Not all decompositions are reversible, like explosive decompositions. Many decompositions are exothermic, such as ammonium nitrate decomposition. Combining reversibility and endothermicity excludes valid exothermic cases.
Takeaway: Electrolysis decomposes compounds using electricity, always requiring external energy input.

Why Correct: Corrosion involves electrochemical oxidation reactions that degrade metals, exemplified by iron rusting (4Fe + 3O2 + 6H2O -> 4Fe(OH)3).
Distractor Analysis: Mechanical erosion involves physical forces like abrasion, impact, or wear. Biological erosion involves living organisms like bacteria, fungi, or plant roots. Biophysical erosion combines biological and physical processes but is not a standard classification for corrosion.
Takeaway: Galvanization applies a zinc coating that sacrificially corrodes before iron, while painting creates oxygen and moisture barriers to prevent corrosion.

Why Correct: Ions form when a neutral atom either loses electrons (becoming a cation) or gains electrons (becoming an anion), depending on whether the atom becomes positively or negatively charged.
Distractor Analysis: Gain of electron alone yields only anions, and loss of electron alone yields only cations; both are partial and not comprehensive. Sharing of electrons results in covalent bonding, not ion formation.
Takeaway: The octet rule drives ion formation: metal atoms tend to lose electrons, non-metal atoms tend to gain electrons to achieve a stable noble gas configuration.

Why Correct: KCN (potassium cyanide) contains an ionic bond between K+ and CN- and a covalent triple bond within the cyanide ion (C≡N).
Distractor Analysis: CH4 has only covalent bonds. H2 has only a single covalent bond. NaCl has only an ionic bond.
Takeaway: Polyatomic ions like CN-, NH4+, or SO42- are the hallmark of compounds that simultaneously exhibit ionic and covalent bonding.

Why Correct: Heating platinum wire in a Bunsen flame only causes it to glow and expand; no new substance forms. The change is reversible and involves only a change in state or appearance, making it a physical change.
Distractor Analysis: Magnesium reacts with oxygen to form magnesium oxide (chemical change). Copper oxidizes to black copper(II) oxide (chemical change). Quicklime (CaO) reacts with water to form slaked lime (Ca(OH)2) in an exothermic reaction (chemical change).
Takeaway: Physical changes are reversible and do not alter chemical composition. Noble metals like platinum resist chemical change under heat.

Why Correct: O2 has two unpaired electrons in its π* antibonding orbitals according to molecular orbital theory, making it paramagnetic.
Distractor Analysis: N2 has all electrons paired with a bond order of 3, making it diamagnetic. F2 has all electrons paired with a bond order of 1, diamagnetic. H2 has a bond order of 1 with paired electrons, diamagnetic.
Takeaway: Paramagnetism in diatomic molecules is predicted by molecular orbital theory; O2 is a classic example of a paramagnetic molecule.

Why Correct: Sodium chloride (NaCl) is an ionic compound formed by complete transfer of an electron from sodium to chlorine, resulting in electrostatic attraction between Na+ and Cl- ions. This is the hallmark of an electrovalent (ionic) bond.
Distractor Analysis: Oxygen molecule (O2) has a double covalent bond. Water (H2O) has polar covalent bonds. Ammonia (NH3) has polar covalent bonds. None involve complete electron transfer.
Takeaway: Electrovalent bonds form between metals and non-metals with large electronegativity difference. NaCl (ΔEN ≈ 2.1) is a classic example. Covalent bonds (non-metal + non-metal) are not electrovalent.

Why Correct: Hydrogen bonding occurs when hydrogen is bonded to a highly electronegative atom (F, O, or N). In H2O, hydrogen bonds to oxygen; in HF, hydrogen bonds to fluorine. In H2S, sulfur is less electronegative than oxygen, so the S-H bond is not polar enough to form hydrogen bonds. Thus, hydrogen bonding exists in H2O and HF but not in H2S.
Distractor Analysis: Covalent bonding exists in all three molecules (H2O, HF, H2S) because they all share electrons between atoms. Ionic bonding does not exist in any of these because they are all covalent compounds, not ionic salts. Metallic bonding occurs in metals, not in these molecular compounds.
Takeaway: Hydrogen bonding requires hydrogen attached to F, O, or N. Sulfur is in the same group as oxygen but has lower electronegativity, so H2S does not exhibit hydrogen bonding. This is why H2O has a higher boiling point than H2S despite similar molecular masses.