Answer is: Cl and Na.
sodium and chlorine are in third period and they have very different properties. Sodium is solid metal and chlorine is gaseous nonmetal.
They form compound NaCl (Sodium chloride), because sodium lost one valence electron and form cation Na⁺, chlorine gain one electron and form anion Cl⁻.
Electron configuration of sodium atom: ₁₁Na 1s² 2s² 2p⁶ 3s¹.
Electron configuration of chlorine atom: ₁₇Cl 1s² 2s² 2p⁶ 3s² 3p⁵.
Other examples are metal-metal pairs and they do not form cation and anion.
Reduction Reaction:
Ag⁺ + 1e⁻ → Ag ------ (1)
Oxidation Reaction:
NO₂⁻ → NO₃⁻ + 2e⁻
Balance Oxygen atoms, by adding H₂O to side having less number of O atoms, and H⁺ to side having greater number of O atoms.
NO₂⁻ + H₂O → NO₃⁻ + H⁺ + 2e⁻
Balance the H⁺'s,
NO₂⁻ + H₂O → NO₃⁻ + 2 H⁺ + 2e⁻ ------ (2)
Multiply eq. 1 by 2 to balance the number of electrons in overall reaction.
2 Ag⁺ + 2e⁻ → 2 Ag ----- (3)
Compare eq. 2 and 3,
NO₂⁻ + H₂O → NO₃⁻ + 2 H⁺ + 2e⁻
2 Ag⁺ + 2e⁻ → 2 Ag
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NO₂⁻ + H₂O + 2 Ag⁺ → NO₃⁻ + 2 Ag + 2 H⁺
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