An atom with a formal charge does not necessarily have more or less electron density than the atoms in the molecule without formal charges. We can see this by examining the potential maps for H₂O, H₃O⁺, and HO⁻.
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a. Which atom bears the formal negative charge in the hydroxide ion?
b. Which atom has the greater electron density in the hydroxide ion?

Verified step by step guidance

1. Step 1: To determine which atom bears the formal negative charge in the hydroxide ion (HO⁻), recall that formal charge is calculated using the formula: Formal Charge = (Valence Electrons) - (Non-bonding Electrons) - (Bonding Electrons/2). In HO⁻, the oxygen atom has a formal negative charge because it has one extra electron compared to its neutral state.
2. Step 2: To identify which atom has the greater electron density in the hydroxide ion, consider the electronegativity of the atoms involved. Oxygen is more electronegative than hydrogen, meaning it attracts electrons more strongly. This results in a higher electron density around the oxygen atom in HO⁻.
3. Step 3: Examine the molecular electrostatic potential map (if provided). In such maps, regions of high electron density are typically represented by red or orange colors, while regions of low electron density are represented by blue or green colors. For HO⁻, the oxygen atom would appear in the red/orange region, indicating higher electron density.
4. Step 4: Compare the electron density distribution in HO⁻ with other molecules like H₂O and H₃O⁺. In H₂O, oxygen also has high electron density due to its electronegativity, but it does not bear a formal charge. In H₃O⁺, the oxygen atom has reduced electron density due to the positive formal charge.
5. Step 5: Summarize the findings: The oxygen atom in HO⁻ bears the formal negative charge and has the greater electron density compared to the hydrogen atom in the same molecule. This aligns with the principles of electronegativity and formal charge distribution.

Key Concepts

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 bonds, and the number of non-bonding electrons. It helps in understanding the distribution of electrons within a molecule and can indicate the stability of different resonance structures. A formal charge does not necessarily correlate with the actual electron density around an atom.

Electron Density

Electron density refers to the probability of finding electrons in a given region of space around an atom or molecule. It is visualized using electron density maps, which illustrate areas of high and low electron concentration. Understanding electron density is crucial for predicting molecular behavior, reactivity, and the distribution of charges within a molecule, especially in ions like hydroxide (HO−).

Molecular Geometry and Hybridization

Molecular geometry describes the three-dimensional arrangement of atoms in a molecule, which is influenced by the hybridization of atomic orbitals. Different hybridization states (e.g., sp3, sp2) lead to distinct shapes and bond angles, affecting the overall electron distribution and reactivity. In the context of hydroxide and related ions, understanding geometry helps explain the formal charges and electron density distribution.