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the enthalpy change for the reaction 2CO + O2 ® 2CO2

The electron-dot structure of NH3 places one pair of nonbonding electrons in the valence shell of the nitrogen atom. This means that there are three bonded atoms and one lone pair for a coordination number of four around the nitrogen, the same as occurs in H2O.

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The Lewis dot structure for ammonia, NH3.: The lone pair attached to the central nitrogen creates bond angles that differ from the tetrahedral 109.5 °.

We can therefore predict that the three hydrogen atoms will lie at the corners of a tetrahedron centered on the nitrogen atom. The lone pair orbital will point toward the fourth corner of the tetrahedron, but since that position will be vacant, the NH3 molecule itself cannot be tetrahedral; instead, it assumes a pyramidal shape, more specifically, that of a trigonal pyramid (a pyramid with a triangular base). The hydrogen atoms are all in the same plane, with the nitrogen outside of the plane. The non-bonding electrons push the bonding orbitals together slightly, making the H–N–H bond angles about 107°.

In 5-coordinated molecules containing lone pairs, these non-bonding orbitals (which are closer to the central atom and thus more likely to be repelled by other orbitals) will preferentially reside in the equatorial plane. This will place them at 90° angles with respect to no more than two axially-oriented bonding orbitals. We can therefore predict that an AX4E molecule (one in which the central atom A is coordinated to four other atoms X and to one nonbonding electron pair) such as SF4 will have a “see-saw” shape.

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Example of a see-saw structure: Try to imagine this molecule teetering on each end, and you will have a visual representation of a see-saw.

Substituting nonbonding pairs for bonded atoms reduces the triangular bipyramid coordination to even simpler molecular shapes.

Interactive: Unshared Electrons and the “Bent” Shape: Use the 3D model to see how unshared electrons repel those that are shared in the bonds between hydrogen and oxygen, causing the molecule to have a “bent” shape.