The dichromate ion, Cr2O72-, has neither Cr¬Cr nor O¬O bonds. (a) Taking both 4s and 3d electrons into account, draw an electron-dot structure that minimizes the formal charges on the atoms.

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Draw the skeletal structure of the dichromate ion, \( \text{Cr}_2\text{O}_7^{2-} \), with two chromium atoms connected by an oxygen bridge and each chromium atom bonded to three other oxygen atoms.

Assign the total number of valence electrons. Chromium (Cr) has 6 valence electrons (4s^1 3d^5), and oxygen (O) has 6 valence electrons. The ion has a 2- charge, adding 2 more electrons. Calculate the total: \(2 \times 6 + 7 \times 6 + 2 = 42\) valence electrons.

Distribute the electrons to satisfy the octet rule for the oxygen atoms first. Place lone pairs on the oxygen atoms to complete their octets.

Assign formal charges to each atom. The formal charge is calculated as: \( \text{Formal charge} = \text{Valence electrons} - \text{Non-bonding electrons} - \frac{1}{2} \times \text{Bonding electrons} \). Adjust the structure to minimize formal charges, typically by forming double bonds between chromium and oxygen atoms.

Ensure that the overall charge of the ion is \(-2\) and that the formal charges are minimized, ideally with the central chromium atoms having a formal charge of zero.

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Electron-Dot Structures

Electron-dot structures, also known as Lewis structures, represent the valence electrons of atoms within a molecule. They illustrate how electrons are shared or transferred between atoms, helping to visualize bonding and lone pairs. In the case of the dichromate ion, drawing the correct electron-dot structure involves placing electrons around the chromium and oxygen atoms to minimize formal charges.

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04:28

Lewis Dot Structures: Ions

Formal Charge

Formal charge is a theoretical charge assigned to an atom in a molecule, calculated based on the number of valence electrons, the number of non-bonding electrons, and half the number of bonding electrons. It helps in determining the most stable structure of a molecule. For the dichromate ion, minimizing formal charges on chromium and oxygen is crucial for achieving a stable electron-dot structure.

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Formal Charge

Oxidation States

Oxidation states indicate the degree of oxidation of an atom in a compound, reflecting the number of electrons lost or gained. In the dichromate ion, understanding the oxidation states of chromium and oxygen is essential for constructing the electron-dot structure and ensuring that the overall charge of the ion is accurately represented. The oxidation state of chromium in Cr2O72- is +6, which influences its bonding and electron distribution.

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