Answer:
To determine the first digit of the VSEPR number of a molecule, you need to count the number of electron groups around the central atom in the molecule. Electron groups can either be lone pairs or bonding pairs.
Once you have counted the number of electron groups, you can use the following guidelines to determine the first digit of the VSEPR number:
If there are two electron groups, the first digit is 2.
If there are three electron groups, the first digit is 3.
If there are four electron groups, the first digit is 4.
If there are five electron groups, the first digit is 5.
If there are six electron groups, the first digit is 6.
Step-by-step explanation:
For example, consider the molecule CH4 (methane). The carbon atom has four bonding pairs, so there are four electron groups around the central atom. Therefore, the first digit of the VSEPR number is 4. The VSEPR notation for CH4 is AX4, where A represents the central atom and X represents the bonding pairs.