Answer to Question #88067 in General Chemistry for Siddhi Mahajan

Question

Hello, I am a grade 12 student chemistry and was wondering if you could help me with some chemistry questions: 1)the anomalous configuration of Cr is 1s^2 2s^2 3s^ 3p^6 3s^1 3d^5 using this anomalous configuration, predict the two most common valences of chromium. Explain how these make sense with the electron configuration. 2) Why is the second ionization energy of Al larger than the second ionization energy of Mg, and larger than the second ionization energy of Si? explain using the mechanical model of an atom.

Answer 1

1)    Cr 1s²2s²2p⁶3s²3p⁶4s¹3d⁵  There are two main exceptions to electron configuration: Cr and Cu. A completely full or half full d sub-level is more stable than a partially filled d sub-level, so in case of Cr an electron from the 4s orbital is excited and rises to a 3d orbital.

The most common oxidation states of chromium are +6 and +3

2)     The second ionization energy (IE₂) is the energy required to remove an electron from a 1+ cation in the gaseous state.

X⁺(g) → X²⁺(g) + e^-

IE₂ is affected by size, effective nuclear charge, and electron configuration.

We would expect second ionization energies to increase from left to right as the ionic size decreases. Al has a greater IE₂ than Si. This is probably due to the extra stability of the s² subshell in the Al⁺ ion.