The VSEPR (Valence Shell Electron Pair Repulsion) theory describes the arrangement of electron pairs around a central atom in a molecule, and the VSEPR number represents the total number of electron pairs around the central atom, including both bonding pairs and lone pairs.The VSEPR number can be determined from the Lewis structure of the molecule, which shows the arrangement of atoms and lone pairs around the central atom. The first two digits of the VSEPR number correspond to the number of bonding pairs and lone pairs, respectively, around the central atom.To determine the third digit of the VSEPR number, you need to consider the shape of the molecule. The shape is determined by the repulsion between electron pairs, which is strongest between lone pairs and decreases in the order lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair.The third digit of the VSEPR number indicates the shape of the molecule according to the following scheme:1: linear shape2: trigonal planar shape3: tetrahedral shape4: trigonal bipyramidal shape5: octahedral shapeThus, to determine the third digit of the VSEPR number, you need to determine the shape of the molecule based on the number of electron pairs and their relative positions. This can be done by applying the VSEPR theory and considering the repulsion between electron pairs. Alternatively, you can consult a table or chart that lists the shapes associated with different VSEPR numbers.