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You arrange the bonding pairs and lone pairs around the central atom in a way that minimizes electron pair repulsion. For molecules where there is no lone pair around the central atom, the bond angles are 180 degrees for molecules with 2 bonding pairs around the central atom, 120 degrees for 3 bonding pairs, 109.5 degrees for 4 bonding pairs, 120/90/180 degrees for 5 bonding pairs, and 90 degrees for 6 bonding pairs.
@taywebb Yes if there is a lone pair on the central atom it would make the bond angles slightly less than the standard. This is because the lone pairs repel the other atoms away from itself making their bond angles slightly smaller
This chart also sums up the point made above. The presence of lone pairs typically alters the bond angles to less/greater than expected, except in the two cases seen (steric number 5, linear shape and steric number 6, square planar shape) as these electrons can be placed anywhere on the axis, so they repel equally.
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