‘Atoms of element X have 6 unpaired electrons. What could be element X?” The answer is chromium

    Yes, chromium could be one.

    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 accomodate 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.

    So where will the sixth unpaied electron be placed?

    Take a look at the of manganese, ##”Mn”##

    ##”Mn: ” [“Ar”] 3d^5 4s^2##

    Manganese has five unpaired electrons in its five 3d-orbitals and two paired electrons in its 4s-orbital.

    This is in fact a clue – 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, ##”Cr”##

    ##”Cr: ” [“Ar”] 3d^5 4s^1##

    You can rad more about why chromium’s electron configuration looks like that here:

    http://socratic.org/questions/why-do-electron-configurations-of-chromium-and-copper-contradict-the-aufbau-prin

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