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Home , Chemical bond, Lattice energy, Single bond, Triple bond

AP NOTES 14-1 TYPES OF CHEMICAL BONDS

Chemical Bond – attractive that hold together

TYPES OF CHEMICAL BONDS

*IONIC BOND – generally involves from the left side of the interacting with nonmetals from the right side of the table through a transfer of

This generally results in the formation of a solid, regular array of positive and negative called a lattice:

The strength of the attraction between these ions is known as the crystal lattice . (Related to the of dissociation). The likelihood of this type of bond forming can be predicted using the following variation of Coulomb’s Law:

where E = crystal k = a proportionality constant Q1 = charge of first Q2 = charge of second ion d = distance between ion centers

A negative crystal lattice energy indicates that the compound is lower in energy than the individual atoms, so it favors the formation of the compound.

QUESTION: Discuss the formation of a compound containing potassium and barium

QUESTION: Which would have the higher , or potassium chloride? Why?

Which would be more soluble in water? Why?

QUESTION: Which would have the higher melting point, sodium chloride or magnesium ? Why?

Which would be more soluble in water? Why?

Characteristics of Ionic Compounds

1. They are solids with high melting points (typically >400oC)

2. Many are soluble in polar such as water.

3. Most are insoluble in nonpolar solvents.

4. Molten compounds conduct well because they contain mobile charged particles (ions).

5. Aqueous conduct electricity well because they contain mobile charged particles (ions).

6. They tend to be brittle and shatter when hammered.

7. The formulas are usually given in simplest ratio of elements (empirical)

* – Involves the sharing of electrons between two atoms, usually between two nonmetals

Characteristics of Covalent Compounds:

1. They are gases, liquids, or solids with low melting points (typically <300oC)

2. Many are insoluble in polar solvents.

3. Most are soluble in nonpolar solvents

4. Liquid and molten compounds do not conduct electricity.

5. Aqueous solutions are usually poor conductors of electricity because most do not contain charged particles.

6. The formulas are usually given in the true ratios of atoms (molecular).

*Metallic Bond – formed between identical atoms when a “sea” of electrons surrounds an array of positively charged “core” ions

*Characteristics of Metals:

1. They are usually crystalline solids.

2. They have a range of melting points usually dependant upon the number of valence electrons.

3. They are good conductors of electricity in both the solid and molten states.

4. They are malleable and ductile.

5. They have a luster. ALLOYS

Larger metals Smaller metals Substitute in the “holes”

AP Chemistry NOTES 14-2

TYPES OF COVALENT BONDS

* – consists of 1 pair of shared electrons forming a bond (σ) (2 “p” orbitals overlap “head-to-head”)

* – consists of 2 pairs of shared electrons *1 *1 (π) – formed when 2 “p” orbitals overlap “side-to-side”

* – consists of 3 pairs of shared electrons *1 sigma bond 2 pi bonds

RELATED FACTS CONCENRING COVALENT BOND TYPES

1. A single sigma bond is stronger than a pi bond

2. The triple bond is the shortest of the bonds and is stronger than the rest

3. The single bond is the longest of the bonds and is weaker than the rest *A single bond has a of “1” *A double bond has a bond order of “2” *A triple bond has a bond order of “3”

4. Bond Dissociation Energy – the energy required to break a (also referred to as “” for short); increases with bond order

The molecular geometry is determined by the number of pairs on the central , and whether these electron pairs are bonding pairs or lone pairs (non-bonding pairs).

The geometry can be determined using the table below:

TOTAL ELECTRON PAIRS ON ELECTRON PAIR OR STRUCTURAL CENTRAL ATOM GEOMETRY HYBRIDIZATION 2 linear sp 3 trigonal planar sp2 4 tetrahedral sp3 5 trigonal bipyramidal sp3d 6 octahedral sp3d2

MOLECULAR GEOMETRIES

Linear Trigonal Planar

Tetrahedral Pyramidal

Trigonal Bipyramidal Bent

See-Saw T-Shaped

Linear Octahedral

Square Pyramidal Square Planar

BOND POLARITY A polar bond forms when two atoms of different values bond together covalently. This results in pair of electrons that is shared “un-equally” between the two atoms. The diagram below represents a atom (on the left) bonded with a atom (on the right):

hydrogen fluorine

Since fluorine in the above example has a higher electronegativity value (4.0) than hydrogen (2.1), their shared electron pair is displaced more toward fluorine, resulting in a partial negative “pole” on the fluorine end and a partial positive pole on the hydrogen end – hence, a polar bond. may be polar or non-polar, depending upon their molecular structure. Water, for instance, has an asymmetrical structure that consists of two hydrogen atoms and two “lone pairs” of electrons in a tetrahedral orientation around a single atom. This gives the water an uneven distribution of electric charge, resulting in a molecule that has a partial positive pole and a partial negative pole – a polar molecule:

Molecules like tetrachloride consist of polar bonds. However, the symmetrical structure of the molecule results in an even distribution of charge – the molecule as a whole is nonpolar in nature:

In general, molecules are nonpolar if

*there are no lone pairs on the central atom and *all peripheral atoms are the same

Websites to view molecular geometries: http://intro.chem.okstate.edu/1314F97/Chapter9/VSEPR.html http://www.chemmybear.com/shapes.html http://people.bath.ac.uk/ch3tam/inorganic/vsepr/html/vseprmolecularshapes.htm

EXAMPLES:

Lewis Structure Electron Pair Molecular Polar or Drawing Molecule Geometry Geometry Nonpolar?

BeCl2

BF3

CH4

NH3

H2O

Lewis Structure Electron Pair Molecular Polar or Drawing Molecule Geometry Geometry Nonpolar?

PCl5

SeF4

IBr3

- ICl2

SCl6

ClF5

XeF4

RESONANCE STRUCTURES AND BOND ORDER

Resonance – when two or more plausible structures can be drawn for one molecule; the shared electrons are delocalized among the bonding sites

Bond Order – the bond order of a molecule can be determined using the following equation:

EXAMPLE: Draw the possible resonance structures of the carbonate ion and determine the bond order of the molecule.

FORMAL CHARGE

The use of oxidation numbers is actually artificial – more or less o bookkeeping method for keeping track of the electrons. A more realistic method is that of formal charges. The is the difference between the number of valence electrons on the free atom and the number of electrons assigned to the atom in the molecule. (A formal charge of + 2 is about as large as it gets.)

(Note: The sum of the formal charges in a species must equal the total charge of the species)

Formal Charge = (# of val e- on the free atom) – (# of val e- assigned in molecule)

EXAMPLE: Determine the formal charge on both oxygen and in the perchlorate ion.