화학14. VSEPR

Elemental analysis of Compound X is shows its mass percent composition to be 87.419% nitrogen and 12.581% hydrogen. The density of X at 1atm pressure and 25℃ is 1.31gL^-1

(1) Determine the empirical formular for X

 

(2) Determine the molecular formula for X

 

(3) Write down its Lewis diagram

 

(4) Predict the structure of X using VSEPR

 

(5) What is the Hybridzation of the nitrogen atoms?

 

 

---

Simple Bonding Theory

 

Chemical Bond : driving force is stabilization of energy

• covalent bond: between nonmetals (polar, nonpolar)
• coordinate bond
• ionic bond: between metal & nonmetal
• metallic bond

 

Molecular Models

Main group chemistry

• Valence Bond Theory (VBT) = localized bond theory > Lewis structure, hybridization concept
• Molecular Orbital Theory (MOT)

Transition metal chemistry

• VBT
• Crystal Field Theory (CFT)
• MOT
• Ligand Field Theory (LFT = CFT + MOT)

 

14.1. Valence Bond Theory (VBT)

1916 electron pairing model by Lewis("An electron may form a part of the shell of two different atoms and cannot be said to belong to either one exclusively.")

1927 Q. M. method by W. Heitler & F. London

1930s valence bond approach by L. Pauling & J. C. Slater

 

Valence state electron configurations and Promotion Energies

The promotion energy is the energy required to promote electrons from the ground state to a “valence state”, which is one type of excited state configuration that is used for bonding.

 

Localized Bonding Models

Localized implies that electrons are confined to a particular bond or atom.

 

The Lewis approach to bonding : Pairs of electrons are localized in bonds or as non-bonding “lone pairs” on atoms. Each bond is formed by a pair of electrons shared by two atoms.

Octet rule: most main group atoms will tend to end up with an ns 2 np 6 electron configuration.

 

Rules for drawing Lewis diagrams

• Pick the central atom. (Atoms that are present only once in the formula, especially heavy elements and metals, tend to be at the center of the structure. Oxygen is often terminal and hydrogen almost always is. Often the formula is written with the central atom first. (Sometimes there may be more than one central atom.))
• Write out the valence shell electron configurations for the neutral central atom and the "terminal" atoms in their ground states.
• If there is a negative charge distribute it among the terminal atoms in the first instance. Bear in mind that all the terminal atoms must make at least one covalent bond with the central atom, so do not create any noble gas configurations on them. Positive charge is best initially assigned by removing electrons from the central atom.
• The total number of unpaired electrons on the terminal atoms will have to match the number of unpaired electrons on the central atom to account for the bonds and leave no unpaired electrons. If this is not the case, once the first three steps have been carried out, there are two strategies available:
• Move electrons between the central atom and the terminal atoms as necessary. Make sure you keep track of the formal charges because you must be specific about their location. Enclosing a Lewis structure in brackets with the charge outside is not acceptable.
• If and only if the central atom comes from the second period or below (Na onwards, n=3 and up), electrons can be placed into the nd subshell. (Whether the d orbitals play a significant role in bonding in main group compounds is debatable, but they do help to predict correct structure without invoking canonical structures with unreasonable charge separations.)

 

General rules for a Lewis structure

• max. # of bonds
• reasonable location of the atoms in the actual molecule
• reasonable distribution of formal charges

 

 

Electronegativity

• The tendency of an atom or a functional group in a molecule to attract electrons (or electron density) towards itself
• dipole moment < bond polarity
• bond energy, heat of formation
• molecular polarity

 

Guidelines For Predicting the Most Favorable Lewis Structures of Compounds Containing Only Main Group Elements

• Whenever possible, the least electronegative atom will usually bond to the largest number of other atoms or groups
• The most stable Lewis structure or resonance form will usually have:(The minimum number of formal charges; no formal charges larger than ±1 for triatomic or larger molecules; no adjacent formal charges of the same sign, in so far as possible. Negative formal charges on the most electronegative atom and positive formal charges on the least electronegative atom. . Maximum bond order – i.e., the number of σ and π bonds should be maximized provided no other important rules are violated. No highly strained rings (e.g., 3 and 4 membered ringsespecically containing the elements B through F) – unless they can’t be avoided. No multiple bonds (of the Pπ-Pπ variety) between heavier main group atoms and other atoms – unless there is no alternative usually the alternative will be a polymer containing only single bonds unless large substituents are present to prevent polymerization. No O-O, O-F, N-O or N-N single bonds unless absolutely necessary. (These bonds are very weak and are generally unstable relative to other bonding arrangements.) )
• Often isoelectronic molecules having the same number of atoms are also isostructural. This generalization holds very well for triatomic molecules, but there are often exceptions for molecules with more than three atoms.