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Why is the water molecule bent while the carbon dioxide molecule is linear?

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representations of molecular structure
molecule Several methods of representing a molecule's structure. In Lewis structures, element symbols represent atoms, and dots represent electrons surrounding them. A pair of shared electrons (covalent bond) may also be shown as a single dash. The ball-and-stick model better illustrates the spatial arrangement of the atoms. For aromatic compounds, the Kekulé structure is common, in which each bond is represented by a dash, carbon atoms are implied where two or more lines meet, and hydrogen atoms are usually omitted. Bond-line formulas, similar to the Kekulé structure, are often used for complex nonaromatic organic compounds. Sugars are often drawn as Fischer projections, in which the carbon “backbone” is drawn as a straight vertical line, with carbon atoms implied where horizontal lines intersect the vertical one.

The difference in shape between water (H2O) and carbon dioxide (CO2) is explained by valence-shell electron-pair repulsion (VSEPR) theory, which posits that electron pairs around a central atom arrange themselves to minimize mutual repulsion.

Water (H2O)

The oxygen atom in water is surrounded by four electron pairs: two bonding pairs connected to hydrogen atoms and two lone pairs. These four pairs adopt a tetrahedral arrangement to minimize repulsion. Because the molecular shape is defined by the position of the atoms rather than the electron pairs, the presence of the two lone pairs forces the bonding pairs closer together, resulting in an angular or "bent" structure with a bond angle of approximately 104.5°.12

Carbon Dioxide (CO2)

In contrast, the central carbon atom in carbon dioxide is bonded to two oxygen atoms via double bonds. In VSEPR theory, these double bonds act as regions of electron density that repel each other. To minimize this repulsion, the two regions of electron density position themselves on opposite sides of the carbon atom, resulting in a linear molecular geometry.32