The dichromate ion, Cr2O72-, has neither Cr¬Cr nor O¬O bonds. (b) How many outer-shell electrons does each Cr atom have in your electron-dot structure? What is the likely geometry around the Cr atoms?

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Identify the oxidation state of chromium (Cr) in the dichromate ion, Cr2O7^2-. Since the overall charge of the ion is -2 and each oxygen (O) typically has a charge of -2, calculate the total charge contributed by the oxygen atoms and use this to find the oxidation state of Cr.

Determine the electron configuration of chromium in its oxidation state. Subtract the number of electrons lost during oxidation from the total number of valence electrons in a neutral chromium atom to find the number of outer-shell electrons around each Cr in the dichromate ion.

Draw the Lewis structure of the dichromate ion, Cr2O7^2-. Start by placing the Cr atoms and connecting them to the appropriate number of O atoms based on typical bonding patterns, ensuring the overall charge of the ion is maintained.

Predict the molecular geometry around each Cr atom using VSEPR theory (Valence Shell Electron Pair Repulsion theory). Consider the arrangement of the shared and lone electron pairs around the Cr atom to determine the most likely geometry.

Confirm the absence of Cr-Cr and O-O bonds in your structure by checking that each Cr is bonded only to O atoms and each O is bonded only to Cr atoms, consistent with the information given in the problem statement.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Electron Configuration

The electron configuration of an atom describes the distribution of its electrons among the various orbitals. For chromium (Cr), the electron configuration is [Ar] 3d5 4s1, indicating that it has six outer-shell electrons in its valence shell. Understanding this configuration is essential for determining the number of outer-shell electrons in the dichromate ion.

Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms and the lone pairs of electrons in a molecule. In the case of the dichromate ion, drawing the Lewis structure helps visualize the arrangement of electrons around the chromium atoms, which is crucial for answering how many outer-shell electrons each Cr atom has.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. The geometry around the chromium atoms in the dichromate ion can be predicted using VSEPR theory, which states that electron pairs will arrange themselves to minimize repulsion. In this case, the geometry is typically described as tetrahedral due to the presence of four oxygen atoms surrounding the chromium atoms.

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