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.