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MCAT Chemistry Question — Alkali Metals

Similarities in chemical behavior can be observed between elements within the same family in the periodic table. For instance, sodium and potassium react with oxygen to produce a metal oxide. Which of the following gives the balanced reaction for the oxidation of an alkali metal (M)?


  1. 2M(s) + O2(g) → 2MO(s)
  2. 4M(s) + O2(g) → 2M2O(s)
  3. 3M(s) + O2(g) → M3O2(s)
  4. M(s) + O2(g) → MO2(s)
Click for Explanation

Sodium and potassium are congeners found in the first column of the periodic table. They are included in the alkali metal family and exhibit similar chemical reactivity. In the presence of oxygen, alkali metals are oxidized. To form a complete octet, alkali metals lose one valence electron and take on their preferred oxidation state of +1. Oxygen, on the other hand, completes its octet by gaining two electrons to form an anion with a charge of -2. Thus, in the reaction above, two metal (M) atoms must each lose one electron in order for each oxygen atom to gain two electrons. Of the reactions above, B is the only choice where the metal (M) has a charge of +1 and oxygen had a charge of -2 with a 2:1 coefficient ratio of metal to oxygen. Thus, B is the correct answer for the oxidation of alkali metals.

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