4 electron groups (3 bonding + 1 lone pair) produce a tetrahedral electron pair geometry. But molecular geometry describes only where the atoms are — the lone pair is invisible in the shape name. With 3 bonded atoms arranged around a central atom with one lone pair pushing down, you get a tripod-like structure — trigonal pyramidal. Ammonia (NH₃) is the classic example. Option 2 (tetrahedral molecular geometry) is wrong because molecular geometry cannot include the lone pair in its shape description.
Question 2 Multiple Choice
Water has a bond angle of approximately 104.5° rather than the ideal tetrahedral 109.5°. What best explains this compression?
AWater has only 2 bonding pairs, so the bond angle is naturally smaller than in molecules with 4 bonds
BOxygen is electronegative, pulling bonding electrons toward itself and compressing the angle between the bonds
CThe two lone pairs repel the bonding pairs more strongly than bonding pairs repel each other, pushing the H–O–H angle inward
DWater's geometry is trigonal planar, which has a smaller ideal angle than tetrahedral
The repulsion hierarchy is: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair. Lone pairs spread out more than bonding pairs because they are held by only one nucleus rather than pinned between two atoms. In water, two lone pairs squeeze the two O–H bonding pairs closer together, compressing the angle from the ideal 109.5° to ~104.5°. Option 0 misidentifies the cause — the tetrahedral electron pair geometry is set by all 4 groups; the compression comes from the lone pair's stronger repulsion.
Question 3 True / False
Water and methane both have 4 electron groups around their central atom, but they have different molecular geometries.
TTrue
FFalse
Answer: True
Methane (CH₄) has 4 bonding pairs and 0 lone pairs — both its electron pair geometry and its molecular geometry are tetrahedral. Water (H₂O) also has 4 electron groups (2 bonding + 2 lone pairs), giving it a tetrahedral electron pair geometry, but its molecular geometry is bent because only the 2 bonded hydrogen atoms define the shape. Lone pairs are not atoms and do not contribute to the molecular geometry name — their presence changes the shape without appearing in it.
Question 4 True / False
In VSEPR theory, a double bond counts as two electron groups because it contains two pairs of electrons.
TTrue
FFalse
Answer: False
In VSEPR theory, each bond — single, double, or triple — counts as exactly ONE electron group, regardless of how many electron pairs it contains. All the electrons in a double bond are concentrated in the same region between the two atoms, so they repel neighboring groups as a single unit. CO₂, with two double bonds on carbon, has only 2 electron groups and a linear geometry — not 4 groups giving a tetrahedral shape.
Question 5 Short Answer
Why do lone pairs on a central atom cause actual bond angles to be smaller than the ideal electron pair geometry predicts?
Think about your answer, then reveal below.
Model answer: Lone pairs are held by only one nucleus rather than shared between two atoms, so their electron cloud spreads out more in space. This makes lone pairs stronger repellers of neighboring electron groups than bonding pairs are. When lone pairs push on adjacent bonding pairs, they squeeze those bonds closer together, compressing the angle below the ideal value. The more lone pairs present, the greater the compression — water (two lone pairs) has a smaller angle than ammonia (one lone pair), both relative to the ideal tetrahedral 109.5°.
The repulsion hierarchy — LP-LP > LP-BP > BP-BP — is the quantitative expression of this effect. Understanding it lets you not just name molecular geometries but also predict whether bond angles will be above or below ideal values, which directly affects molecular polarity: the next concept that builds on this one.