Molecular Structure and Shape Chapter 6 Sections 2 and 5

Lewis Structures of Second-Period Elements

Vocabulary for Lewis Structures Unshared pairs (lone pairs): a pair of valence electrons not involved in bonding to another atom Single bond: a covalent bond in which one pair of electrons is shared between two atoms Multiple bonds: double bond shares 2 pairs of electrons triple bond shares 3 pairs of electrons

Steps for Drawing Lewis Structures

Drawing Lewis Structures for Molecules The atom with the lowest electronegativity is usually the central atom If carbon is present, it is usually the central atom If hydrogen is present, it is never in the center can only have 2 e-

Drawing Ions You must consider the absence of an electron or the extra electrons If the charge is positive, subtract an absent electron from the total If the charge is negative, add any extra electrons to the total Place the entire structure in brackets and put the charge in the upper right corner outside the brackets

Resonance Structures Resonance Structures: bonding in molecules or ions that cannot be correctly represented by a single Lewis structure Draw each possible structure and the hybrid structure

VSEPR Theory Electron groups around the central atom will be most stable when they are as far apart as possible. We call this Valence Shell Electron Pair Repulsion theory. Because electrons are negatively charged, they should be most stable when they are separated as much as possible. The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule.

Electron Groups The Lewis structure predicts the number of valence electron pairs around the central atom(s). Each lone pair of electrons constitutes one electron group on a central atom. Each bond constitutes one electron group on a central atom, regardless of whether it is single, double, or triple. O N • There are three electron groups on N: One lone pair One single bond One double bond

Molecular Shapes Three-dimensional shape helps determine physical and chemical properties Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular shapes based on the idea that electrons repel one another

Molecular Shapes Lewis structures show which atoms are connected, where they are connected, and by how many bonds, but they don’t properly show the 3-D shapes of molecules To find the geometric shape: draw the Lewis structure then use VSEPR theory

VSEPR Rules To apply VSEPR theory: Draw the Lewis structure of the molecule and identify the central atom Count the number of electron charge clouds (lone and bonding pairs) surrounding the central atom. Predict molecular shape by assuming that clouds orient so they are as far away from one another as possible

(Could include 180º) (Could include 180º)

Polarity of Molecules Molecules of: Two atoms: bond polarity is the molecular polarity More than 2 atoms: the geometry of the molecule must be considered If the bonds are identically non-polar, the molecular is non-polar Some molecules with polar bonds can be non-polar

More Sometimes the partial charges cancel each other out because they are directly opposite each other Consider CO2 and CCl4 The symmetrical distribution of the bonds leads to cancellation of the charges.

Polar and Non-polar Molecules

1. Linear with More than 2 Atoms Sometimes molecules made of more than 2 atoms are shaped linearly Example: CO2 The oxygens pull the electrons in equal, but opposite, directions The molecule is sometimes nonpolar and sometimes polar

2. Trigonal Planar This shape resembles a triangle Example: CH2O and BF3 Boron is an exception to the octet rule The bond angle is 120° The molecule can be polar or nonpolar depending on the nature of the atoms bonded to the central atom

2. Bent There are two shapes that are called bent The first one has one lone pair on the central atom and two bonded atoms (trigonal planar family) Example: NOCl (nitrosyl chloride) Bond angle: 117° The molecule is polar

3. Tetrahedral This shape usually occurs when 4 atoms are bonded to the central atom Examples: CF4, CH4 (methane) If the 4 bonded atoms are identical, and the central atom obeys the octet rule, the molecule is nonpolar Bond angle is 109.5°

3. Trigonal Pyramidal There is one lone pair on the central atom and three bonded atoms Examples: NH3, PCl3 Bond angle is 107° The molecule is polar

3. Bent The second “bent” molecular geometry has two lone pairs on the central atom Example: H2O The bond angle is 105° (some books report 104.5°) The molecule is polar