linear
trigonal planar
The shape of a molecule only describes the arrangement of bonds around a central atom. The arrangement of electron pairs describes how both the bonding and nonbonding electron pair are arranged. For example, in its molecular shape, a water molecule is describes as bent, with two hydrogen atoms bonded to an oxygen atom. However, the arrangement of electron pairs around the oxygen atom is tetrahedral as there are two bonding pairs (shared with the hydrogen) and also two nonbonding pairs.
Consider: Number of bonding domains on the central atom Number of non-bonding electron pairs (lone pairs) on the central atom
Lone electron pairs give the geometry a triangular base.
The molecular geometry of a molecule can be determined using the VSEPR theory. VSEPR (Valence Shell Electron Pair Repulsion) Theory: The basic premise of this simple theory is that electron pairs (bonding and nonbonding) repel one another; so the electron pairs will adopt a geometry about an atom that minimizes these repulsions. Use the method below to determine the molecular geometry about an atom. Write the Lewis dot structure for the molecule. Count the number of things (atoms, groups of atoms, and lone pairs of electrons) that are directly attached to the central atom (the atom of interest) to determine the overall (electronic) geometry of the molecule. Now ignore the lone pairs of electrons to get the molecular geometry of the molecule. The molecular geometry describes the arrangement of the atoms only and not the lone pairs of electrons. If there are no lone pairs in the molecule, then the overall geometry and the molecular geometry are the same. If the overall geometry is tetrahedral, then there are three possibilities for the molecular geometry; if it is trigonal planar, there are two possibilities; and if it is linear, the molecular geometry must also be linear. The diagram below illustrates the relationship between overall (electronic) and molecular geometries. To view the geometry in greater detail, simply click on that geometry in the graphic below. Although there are many, many different geometries that molecules adopt, we are only concerned with the five shown below.
Just as the valence electrons of atoms occupy atomic orbitals (AO), the shared electron pairs of covalently bonded atoms may be thought of as occupying molecular orbitals (MO).
trigonal planar
The shape of a molecule only describes the arrangement of bonds around a central atom. The arrangement of electron pairs describes how both the bonding and nonbonding electron pair are arranged. For example, in its molecular shape, a water molecule is describes as bent, with two hydrogen atoms bonded to an oxygen atom. However, the arrangement of electron pairs around the oxygen atom is tetrahedral as there are two bonding pairs (shared with the hydrogen) and also two nonbonding pairs.
In predicting molecular geometries, unshared electron pairs and double bonds influence the overall shape of a molecule. Unshared electron pairs tend to repel bonding pairs, causing distortions in the molecular geometry. Double bonds restrict rotation around the bond axis, affecting the spatial arrangement of the surrounding atoms and leading to a fixed geometry for the molecule.
One can predict molecular geometry by considering the number of bonding and non-bonding electron pairs around the central atom, using VSEPR theory. The arrangement of these electron pairs determines the shape of the molecule.
The molecular geometry for a molecule with two electron groups and only bonded pairs is linear.
Sulfur dioxide is an example of a molecule that has a tetrahedral arrangement of electron pairs due to its VSEPR geometry, but it is not a tetrahedral molecule. This is because it has a bent molecular shape, with two bonding pairs and one lone pair of electrons around the central sulfur atom.
There are two electron pairs shared between carbon atoms in a molecule of C2H4. This is because each carbon atom forms a double bond with the other, consisting of one sigma bond and one pi bond, sharing a total of two electron pairs.
Electron pairs
Consider: Number of bonding domains on the central atom Number of non-bonding electron pairs (lone pairs) on the central atom
Check the link, it is a sheet describing the different types of electron and molecular geometry. It helped me a lot. ^^ electron pair geometry and molecular geometry won't be the same if there are lone pairs involved.
Without given a specific molecule there is not any way to determine the shape. Beryllium chloride consists of beryllium in the middle and a chlorine on each side, and is in the shape of a straight line.