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The Molecular Geometry Calculator determines the bond angle and molecular shape based on the number of electron domains (steric number) and lone pairs around a central atom. Understanding molecular geometry is essential for predicting a molecule's polarity, reactivity, color, biological activity, and physical properties. The shape of a molecule determines how it interacts with other molecules, how it fits into enzyme active sites, and whether it has a net dipole moment. This calculator uses VSEPR principles to provide both the ideal bond angle from the base electron geometry and an approximate actual angle accounting for lone pair compression. From linear to octahedral arrangements, select the steric number and lone pair count to instantly determine the geometry.
Molecular geometry depends on two factors: the steric number (total electron domains including bonds and lone pairs) and the number of lone pairs.
The ideal bond angles for each base geometry are:
When lone pairs are present, they compress the bond angles because lone pair-bonding pair repulsion is greater than bonding pair-bonding pair repulsion. As a rough approximation, each lone pair compresses bond angles by about 2-3 degrees from the ideal value. Key examples:
The calculator provides an approximate actual angle using a simple compression model. For precise angles, consult experimental data from microwave spectroscopy or X-ray crystallography.
The ideal bond angle is the theoretical value from pure geometric considerations with no lone pair effects. The approximate actual angle accounts for lone pair compression using a simplified model (about 2.5 degrees reduction per lone pair). Real molecules may deviate slightly from these estimates due to differences in atom size, electronegativity effects, and steric interactions between substituent groups. The bonding pairs output confirms how many bonds exist, helping identify the molecular shape name.
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NH3 has steric number 4 with 1 lone pair, giving 3 bonding pairs. The ideal tetrahedral angle is 109.5 degrees, compressed to approximately 107 degrees by the lone pair. Experimental value is 107.3 degrees.
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SF6 has 6 bonding pairs and 0 lone pairs. The geometry is octahedral with ideal 90-degree bond angles. With no lone pairs, the actual angles equal the ideal values.
The steric number (or number of electron domains) is the total count of bonding pairs plus lone pairs around a central atom. Each single, double, or triple bond counts as one electron domain. The steric number determines the base electron geometry.
The 2.5-degree-per-lone-pair approximation is a simplified model. Actual compression depends on the specific atoms involved. For example, water's experimental angle is 104.5 degrees (5-degree compression for 2 lone pairs), while NH3 is 107.3 degrees (2.2-degree compression for 1 lone pair).
With SN=5: 0 lone pairs gives trigonal bipyramidal, 1 lone pair gives seesaw (or sawhorse), 2 lone pairs gives T-shaped, and 3 lone pairs gives linear. Lone pairs preferentially occupy equatorial positions to minimize repulsion.
In a trigonal bipyramidal arrangement (SN=5), equatorial positions have only two 90-degree neighbors, while axial positions have three. Since lone pairs are larger, they experience less repulsion in equatorial positions.
The geometry determines whether individual bond dipoles cancel out. Symmetric geometries (linear with identical groups, trigonal planar, tetrahedral with identical groups, octahedral) produce nonpolar molecules even with polar bonds. Asymmetric geometries or lone pairs create net dipole moments.
Yes. Elements in period 3 and beyond (S, P, Xe, etc.) can have steric numbers of 5 or 6, involving d-orbital participation. The VSEPR model works well for these expanded-octet molecules.
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