16 3.6 Predicting Molecular Shape

Figures

Figure 3.6.7: Methane ammonia and water

Three molecular diagrams of methane, ammonia and water, each with different shapes due to varying numbers of lone pairs.

Methane (CH4) has no lone pairs, and is a tetrahedral structure drawn as a central carbon surrounded by four hydrogen. Single bonds are in the frontal plane and the back plane, represented by a thick wedge and dashes, respectively.

In the center, ammonia (NH3) has a trigonal pyramidal shape with one lone pair above its chemical symbol. Single bonds are again in the frontal plane and the back plane, represented by a thick wedge and dashes, respectively.

On the right, water (H20) has a bent structure due to two lone pairs on the oxygen atom, and a single wedge for one of the hydrogen bonds with other being drawn as a single line.

Figure 3.6.8: Table of Molecular Geometry

Table illustrating VSEPR theory molecular geometries based on the number of electron groups and lone pairs around a central atom. Each cell contains a 3D model, and each model uses a dark sphere to represent the central atom, red spheres for bonded atoms, and light spheres with dots for lone pairs. The geometries are shown transitioning from fully bonded atoms to those with increasing lone pairs, altering the molecule’s shape.

In the table, the heading for the first column is Number of Electron Groups.

• In row one, with 2 electron groups: a linear molecular shape with 0 lone pairs.
• In row two with 3 electron groups: a trigonal planar with 0 lone pairs, then in column 2 a either angular or bent in shape, with 1 lone pair.
• In row three with 4 electron groups: a tetrahedral with 0 lone pairs, then in column two, a trigonal pyramidal with 1 lone pair, then in column 3, an angular or bent shape with 2 lone pairs.
• In row four with 5 electron groups: a trigonal bipyramidal with 0 lone pairs, then in column 2, a seesaw with 1 lone pair, then in column 3, a t-shape with 2 lone pairs, then in column 4, linear with 3 lone pairs.
• In row five with 6 electron groups: an octahedral with 0 lone pairs, then in column 2, square pyramidal with 1 lone pair, then in column 3, a square planar with 2 lone pairs, then in column 4, a t-shape with 3 lone pairs, and finally in column 5, a linear shape with 4 lone pairs.

Figure 3.6.9

A lewis dot diagram of carbon dioxide, with a central carbon atom double-bonded to two oxygen atoms, each with two pairs of lone electrons. Arrows indicate the polar nature of the carbon-oxygen bonds with the arrow heads pointing towards oxygen, denoting the negative pole, and tails at the carbon, the positive pole. Large cross marks over the arrows illustrate that the opposing polarities cancel out leading to no net dipole moment for the molecule, rendering it nonpolar.

Figure 3.6.10

Four lewis dot diagrams depicting the polarity of water molecules (H₂o) from top to bottom:

1. An oxygen atom in the center with two lone pairs of electrons and single bonds to two hydrogen atoms. Two crossed arrows above the oxygen atom point towards the hydrogen atoms, indicating the polar nature of the O-H bonds.
2. An oxygen atom at the center bonded to two hydrogen atoms. The diagram includes four arrows, two either side of the oxygen molecule, pointing inwards from the hydrogen atoms and again towards the oxygen, representing the electronegativity difference.
3. An oxygen atom at the center bonded to two hydrogen atoms.  The diagram shows the molecule’s angular shape with two lone pairs of electrons on the oxygen. Horizontal arrows with plus and minus signs indicate the horizontal components of the polar covalent bonds, which are canceled out due to their opposite directions. The vertical components, represented by vertical arrows, do not cancel, resulting in a net dipole moment pointing upwards.
4. An oxygen atom at the center bonded to two hydrogen atoms.  A large vertical arrow above the oxygen indicates the net dipole moment, resulting from the molecule’s bent shape which doesn’t cancel out the vertical components of the polar bonds, thus making water polar.

Practice questions

Multiple-choice questions

1. Predict the geometry of Phosphorous trifluoride PF3. Is this molecule polar or non-polar? (Hint: Draw the Lewis Structure to determine the number of electron-dense areas.)
   1. Phosphorous trifluoride is a compound with 4 electronegative regions and 1 lone pair, making it a trigonal pyramid. It is a polar compound.
   2. Phosphorous trifluoride is a compound with 3 electronegative regions and 2 lone pairs, making it a trigonal pyramid. It is a polar compound.
   3. Phosphorous trifluoride is a compound with 4 electronegative regions and 2 lone pairs, making it a trigonal pyramid. It is a non-polar compound.
   4. Phosphorous trifluoride is a compound with 4 electronegative regions and 1 lone pair, making it a trigonal pyramid. It is a non-polar compound.

Short-answer questions

1. Fill in the missing words/numbers.
   SCl₂ Sulphur dichloride is a compound with ______ electronegative regions and ______ lone pairs, making it angular or bent. It is a _______ compound.
2. Fill in the missing numbers in the first two boxes. Write down the polarity in the last two boxes.
   SiBr₄ Silicon tetrabromide is a compound with ______ electronegative regions and ________ lone pairs, making it a ____________ compound. It is ________________.

Solutions

Multiple-choice questions

1. a

Short-answer questions

Fill in the missing words/numbers

1. 4/ four; 2/ two; polar
2. 4/ four; 0/ zero; tetrahedral/ non-polar; non-polar/ tetrahedral