Which of the following transition metals in its ground state have unpaired electrons in an s-orbital?
Correct answer: A. Cr
- A. Cr
- B. Co
- C. Fe
- D. Sn
Explanation
Right from the start, you can guess that the unknown element must be a d-block element, or a transition metal. That must be the case because in order to be able to accommodate six unpaired electrons you need to have access to the five d-orbitals transition metals have access to. As you know, each orbital can hold a maximum of two electrons of different spin. When degenerate atomic orbitals are filled, they follow Hund's Rule, which states that one electron is added to each degenerate orbital in a subshell before two electrons can be added to the same orbital in a subshell. This implies that you can have a maximum of five unpaired electrons, one in each of the five d-orbitals. You can have six unpaired electrons if you have five unpaired electrons in the 3d-orbitals and one unpaired electron in the 4s-orbital. The element that matches this electron configuration is chromium, CrCr: [Ar] 3d5 4s1
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About Transition Elements
Transition elements are identified by their partially filled d subshells in atoms or common ions. The topic covers the electronic structure of d-block elements, including the filling order of 4s and 3d orbitals, variable oxidation states and the distinction between transition elements and elements with completely filled d subshells.
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