The lowest oxidation number of an atom in a compound A₂ B is -2 . The number of electrons in its valence shell is _______.

Solution

Chemistry ma’am · 4 steps

A lowest oxidation state of −2 means the atom is 2 electrons short of an octet, so it already holds 6.

1. Meaning of lowest. An atom shows its most negative oxidation number when it gains as many electrons as it can. A main-group atom stops gaining once its valence shell reaches 8 electrons.
2. Link to valence. So the lowest oxidation number is -(8 - valence electrons). Here that gives 8 - v = 2.

   v = 8 - 2 = 6

3. Check with A2B. In A₂B the atom B takes -2 and each A takes +1, so the total charge is zero. Real examples are Na₂O and H₂S, where O and S are group 16 atoms with 6 valence electrons.
4. So. The atom has 6 electrons in its valence shell.

The common slip: 2. Taking the size of the oxidation number as the valence electron count gives 2. The 2 is the number of electrons still missing from the octet, so the shell already has 8 - 2 = 6.

The idea to keep. For a main-group atom, lowest oxidation number = v - 8, where v is its number of valence electrons. Work back from the octet: −3 means 5, −2 means 6, −1 means 7.

Answer from the NTA final answer key. Solution by PrepWiser.