Bonding

Structure and Bonding Fundamental Concepts

Table of Contents

2Chapter 9: Structure and Bonding Fundamental Concepts

2Structure

2Bonding

3Bonding Types

3Ionic Bond

3Covalent Bond

3Electronegativity

4Bonding - Ionic Bonds

4Why do ionic bonds form?

5Crystalline Substances

8Covalent Bonding

8Electron Dot Structures (a.k.a. Lewis Dot Structures)

9Lewis Dot Structures For Ions

10Lewis Dot structures for Molecules

14Resonance Structures

15Features of Resonance

16Bond Order

17Formal Charge

17Definition:

17Counting Rules

18Electroneutrality Principle

18Formal Charge Examples

20VSEPR Theory

20Valence Shell Electron Pair Repulsion Theory

21VSEPR Geometries

26VSEPR for Multiple central Atoms

27Molecular Polarities

28Dipole Moment

Chapter 9: Structure and Bonding Fundamental Concepts

Structure

arrangement of atoms in space.Bonding

forces that hold atoms together in a molecule.A Bond: a force of attraction between atoms.These are both determined based on arrangements of electrons.

Electrons are subdivided into 2 categories:

1. Core Electrons inner e-s that are not involved in chemical behavior.2. Valence Electrons outermost shell electrons. They determine chemical behavior. For main group elements, the number of valence electrons is equal to the old group #. (The sum of the _________s and p electrons)

NaKRb[Ne]3s1[Ar]4s1[Kr]5s1All in Group 1A have 1 valence electron

FClBr[He]2s22p5[Ne]3s23p5[Ar]4s24p5All in Group 7A (or 7B) have 7 valence electrons

Bonding Types

The attractive forces that hold atoms together to form a chemical bond come in 2 general types:

Ionic Bond

One or more valence e-s are transferred from one atom to another (e.g. Na [Ne]3s1 + Cl [Ne]3s23p5 ( Na+ [Ne] + Cl- [Ar])Covalent Bond

All covalently bonded atoms _____________ share e-s Non-polar covalent bonds _______________________________ shares evenly (e.g., H:H or Cl:Cl) Polar covalent bonds___________________________________ share unevenly (more electronegative (electron loving)hogs e-s (e.g. H :Cl)) Picture form http://www.avon-chemistry.com/chem_bond_explain.html

Electronegativity

Is: the ability of an atom in a molecule to attract a shared pair of electrons. It is a continuum. Most bonds are partially ionic/partially covalent. Electronegativity follows the general trends of I.E. and E.A.(EN < 0.4 considered equal sharing (non-polar covalent bond)

X---:---Y(EN 0.5-1.6 considered unequal sharing (polar covalent)

(+ X-----:-Y (-(EN >1.6 (some say 2) considered e- transfer (ionic bond)(metal plus nonmetal: always considered ionic)____F is the most electronegative (4.0)Chart of electronegativities form http://www.avon-chemistry.com/chem_bond_explain.html Bonding - Ionic Bonds

Why do ionic bonds form?

Na has a low ionization energy (tends to loose e-) andCl has a high electron affinity soYou would expect that the formation of NaCl would be thermodynamically favored (which it is). (H = -411 kJ/molHOWEVER, the change in energy upon formation of a salt cannot be accounted for just by the ___IE for the cation + the ___EA for the anion.

I.E. for Na(g) ( Na+(g) + e-(H = +495.8 kJ/molE.A. for Cl(g) + e- ( Cl-(g)(H = -348.6 kJ/mol

(H = +147.2 kJ/molThis would predict that energy is needed, but the Rxn is spontaneous.

The difference is the stability coming from the electrostatic attractive forces.

The attractive force is governed by _____________________ Coulombs Law:

This means:

Higher charges have _________________greater attractive forces. Larger ions have _______greater distances between the (+) and (-) charges leading to smaller attractive forces.For NaCl in the gas phase, the ion pair energy is 498 kJ/mol

+147 kJ/mol (from electron transfer)

-498 kJ/mol (ion pair energy Coulomb forces)

(H=-351 kJ/mol (for the reaction in the gas phase.)for the gas phase reaction: Na(g) + Cl(g) ( NaCl(g) Crystalline Substances

So far we have assumed that all substances are in the gas phase because that is the way that ionization energy and electron affinity are defined.

Additional energy is released and stability is gained when a crystalline solid is formed.Picture from McMurry Fay 3rd Ed.

Lattice Energy: is the energy of formation of 1 mole of a solid crystalline ionic compound when ions in the gas phase are combined.

Na+(g) + Cl-(g) ( NaCl(s)

(H = -787 kJ/mol

Additional Coulombic forces are at play.

Each charged ion is not attracted to multiple opposing ions in the lattice structure.picture from http://www.le.ac.uk/eg/spg3/NaCl.gif

Lattice E for NaCl:Na+(g) + Cl-(g) ( NaCl(s) = -787 kJ/mol

Na+(g) + Cl-(g) ( NaCl(g) = -498 kJ/mol

Additional Stabilization

-298 kJ/molAdditional energy for the overall cycle is required to calculate the net energy for the formation of NaCl from room temperature reactants:

You need to get atoms of Cl in the gas phase (break Cl-Cl bond)

Cl2 ( 2 Cl(H= 243 kJ/mol

Cl2 ( Cl(H= 122 kJ/mol

You need to have Na in the gas phase (vaporize Na)

Na(s) ( Na(g) = (H=107 kJ/molThe complete cycle is called a Born-Haber Cycle

The Born-Haber Cycle for Sodium Chloride. Picture from McMurry Fay 3rd Ed.Picture from McMurry Fay 3rd Ed.

If time Permits, work out Born-Haber Cycle for MgCl2

Example:

What is the (E for the formation of magnesium chloride from magnesium metal and chlorine gas given the following information:

The lattice energy for magnesium chloride is 2542 kJ/mol

The first ionization energy for magnesium is 737.7 kJ/mol

The second ionization energy for magnesium is 1450.7 kJ/mol

The electron affinity for chlorine is -384.6 kJ/mol (x2 = -697.2 kJ/mol)

The (H of formation (sublimation) of Mg(g) is 147.7 kJ/molThe Cl-Cl bond energy is 243 kJ/mol

Covalent Bonding

Typically takes place when ____________(EN