Three of the following molecular models have a tetrahedral central atom, and one does not. Which is the odd one? (There may be a 'hidden' atom directly behind a visible atom in some cases.) (a)
(b)
(c)
(d)

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Identify the central atom in each molecular model (a, b, c, d). The central atom is typically the one bonded to multiple other atoms.

Count the number of atoms directly bonded to each central atom. A tetrahedral geometry typically involves a central atom bonded to four other atoms.

Consider the possibility of lone pairs on the central atom. A central atom with four bonds and no lone pairs generally adopts a tetrahedral shape.

Visualize or sketch the molecular geometry based on the number of bonds and lone pairs. A tetrahedral shape has bond angles of approximately 109.5 degrees.

Compare the geometries of the central atoms in options (a), (b), (c), and (d). The one that does not fit the criteria of having four bonds and no lone pairs on the central atom is the odd one out.

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Key Concepts

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

Tetrahedral Geometry

Tetrahedral geometry occurs when a central atom is bonded to four other atoms, forming a shape where the bonds are directed towards the corners of a tetrahedron. This arrangement minimizes electron pair repulsion according to VSEPR theory, resulting in bond angles of approximately 109.5 degrees. Understanding this geometry is crucial for identifying which molecular models exhibit a tetrahedral structure.

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the geometry of individual molecules based on the repulsion between electron pairs in the valence shell of the central atom. According to this theory, the spatial arrangement of atoms is determined by the need to minimize repulsion between electron pairs, which helps in predicting molecular shapes, including tetrahedral configurations.

Coordination Number

The coordination number refers to the number of atoms, ions, or molecules that a central atom can bond with in a complex or molecule. In the context of tetrahedral geometry, a coordination number of four indicates that the central atom is bonded to four other atoms. Identifying the coordination number is essential for determining whether a molecular model fits the tetrahedral description or is the odd one out.

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Textbook Question

The following ball-and-stick molecular model is a representation of acetaminophen, the active ingredient in such over-thecounter headache remedies as Tylenol. (Red = O, gray = C, blue = N, ivory = H.) (a) What is the formula of acetaminophen?

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