Molecular Shape Fundamentals
The core idea behind predicting molecular geometry is counting electron domains around a central atom. Each bond — single, double, or triple — counts as one domain. Each lone pair also counts as one. The domains repel each other, and they settle into whatever arrangement minimizes that repulsion. That arrangement determines the geometry you see. I spent years grading lab reports on this exact topic, and I can tell you what most students get wrong. They confuse electron geometry with molecular geometry. Electron geometry includes lone pairs. Molecular geometry describes where the atoms actually sit. For water, the electron geometry is tetrahedral because oxygen has four domains, but the molecular geometry is bent because two of those domains are lone pairs. Students write tetrahedral and lose points for not reading the question carefully.
Worksheet 15 Molecular Shapes Answers
If you are looking for answer keys to a specific worksheet labeled Worksheet 15 on molecular shapes, you need to understand what that worksheet covers before you use any answers. Typical assignments in this section ask you to draw Lewis structures, count domains, identify geometry, and predict bond angles. The answers follow a predictable pattern once you know the counting method. Here is how the method works in practice. Take carbon dioxide. Carbon has two double bonds and zero lone pairs. That gives two electron domains. Two domains arrange themselves 180 degrees apart. The geometry is linear. The bond angle is exactly 180 degrees. Now take ammonia. Nitrogen has three single bonds to hydrogen and one lone pair. That is four domains. Four domains adopt tetrahedral electron geometry. But the molecular geometry is trigonal pyramidal because the lone pair is invisible in the shape. The bond angles are slightly less than 109.5 degrees, usually around 107 degrees, because lone pairs push bonding pairs closer together. The tricky case that catches everyone up is xenon tetrafluoride. Xenon is a noble gas, which makes students suspicious immediately. They assume it cannot form compounds. It does. Xenon has four bonds to fluorine and two lone pairs. That is six domains total. Six domains give octahedral electron geometry. The two lone pairs occupy opposite positions to minimize repulsion, leaving a square planar molecular geometry. The bond angles are exactly 90 degrees. I had a student argue that square planar was impossible for a period 5 element because he had never seen it in nature. It exists in synthesis labs. The worksheet answer is square planar with 90-degree angles, and there is no controversy about it.
Another common pitfall involves resonance structures. Some molecules have multiple valid Lewis structures, and students often draw one and forget the other. Resonance does not change the geometry. The electron domains remain the same regardless of which resonance form you draw. Benzene is a classic example. The bond angles stay at 120 degrees in every resonance structure because the electron domain count does not change. When predicting bond angles, remember that lone pairs compress adjacent angles. A perfect tetrahedral angle is 109.5 degrees. Add one lone pair and the remaining bond angles shrink to about 107 degrees. Add two lone pairs and they shrink further to about 104.5 degrees in water. Triple bonds also compress adjacent angles slightly because they contain more electron density and exert greater repulsion. This is why acetylene, with its triple bond, maintains 180 degrees while allene, with two double bonds flanking a central carbon, has 180-degree angles at the center but 120-degree angles at the terminal carbons. If you need to verify your answers, draw the Lewis structure first. Count domains. Match the domain count to the geometry chart: two is linear, three is trigonal planar, four is tetrahedral, five is trigonal bipyramidal, six is octahedral. Then remove the lone pairs from the geometry name to get the molecular shape. The bond angles follow from there, with adjustments for lone pair compression and multiple bond repulsion.
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