Apotassium (k) atom attains a stable electron configuration when it combines with. one chlorine atom two chlorine atoms one calcium atom two calcium atoms

Answer is: one chlorine atom.
They form ionic compound potassium-chloride (KCl).
Electron configuration of potassium: ₁₉K 1s²2s²2p⁶3s²3p⁶4s¹.
Electron configuration of chlorine: ₁₇Cl 1s²2s²2p⁶3s²3p⁵.
Potassium lose one electron to attains a stable electron configuration of noble gas argon (Ar).

Carbon dioxide. 1 carbon is just carbon because you don’t put mono in-front. 2 oxygens is dioxide.