Chemistry ch 9

Table 9.3 Comparison of Some General Properties of an Ionic Compound and a Covalent Compound

*Molar heat of fusion and molar heat of vaporization are the amounts of heat needed to melt 1 mole of the solid and to vaporize 1 mole of the liquid, respectively.

Table 9.1 Lattice Energies and Melting Points of Some Alkali Metal and Alkaline Earth Metal Halides and Oxides

*Na2O sublimes at 1275°C

Electron Affinity

- measurable, Cl is highest

Electronegativity

- relative, F is highest Fluorine is the most electronegative element, so it attracts the electron in the covalent bond more strongly than the H does. This leads to an electron rich region on the F side of the molecule and an electron poor region on the H side of the molecule

Resonance

Actual structure is an average of the resonance structures. -Electrons are really delocalized - they can move around the entire molecule.

Why should two atoms share electrons?

Bonding pairs - pairs of electrons found in the space between the atoms. Lone pairs - pairs of electrons localized on an atom.

Octet Rule

Elements form stable molecules when surrounded by eight electrons.

Variation of Electronegativity with Atomic Number

Generally, electronegativity increases across a row and decreases down a column. -There are several exceptions to these general trends, though, especially throughout the transition metals.

Duet Rule

Hydrogen forms stable molecules where it shares two electrons.

Coulomb's Law:

Lattice energy increases as Q increases and/or as r decreases

Lewis dot formula=

Lewis formula = Lewis dot structure = Lewis structure

Consider the Lewis structure for POCl3 . Assign the formal charge for each atom in the molecule.

Rule: The sum of the formal charges of all atoms in a given molecule or ion must equal the overall charge on that species.

Bond Enthalpies

Single bond < Double bond < Triple bond

There are three possible structures for SO3

The double bond can be placed in one of three places

Born-Haber Cycle for LiCl

The energy changes involved in the formation of solid lithium fluoride from its elements. -The formation of solid LiF from its gaseous ions is the lattice energy of the compound

bond enthalpy or Bond energy

The enthalpy change required to break a particular bond in one mole of gaseous molecules is the bond enthalpy or Bond energy

Comparing Energy Changes

The large lattice energy for magnesium oxide (doubly charged ions are combining) compared with that for sodium fluoride (singly charged ions are combining).

Rules Governing Formal Charge

The sum of the formal charges of all atoms in a given molecule or ion must equal the overall charge on that species

Lewis Dot Symbols for the Representative Elements & Noble Gases

The symbol is the elemental symbol surrounded by up to eight dots, each dot representing a valence electron. -By convention, the dots appear on the left, right, top, and bottom of the elemental symbol.

Sodium Chloride: A Common and Important Compound

The use of NaCl in the chlor-alkali process accounts for 50%, 10% is used in the production of sodium carbonate, and 4% is used in other chemical manufacturing processes. -The use of NaCl as a deicer on roads accounts for 17% of its use, and 4% is used in animal feed. -The remaining 12% of NaCl is used in meat processing, food canning, water softening, paper production, textiles and dyeing, and the rubber and oil industries.

Bond Energies

To break bonds, energy must be added to the system (endothermic, energy term carries a positive sign). -To form bonds, energy is released (exothermic, energy term carries a negative sign).

Born-Haber Cycle for Determining Lattice Energy

To produce one mole of LiF from its elements, first lithium must be sublimed and fluorine must dissociate from its molecular to elemental form. -Once both Li and F atoms are in the gaseous state, an electron is transferred from Li to F, forming lithium and fluoride ions. -The formation of solid LiF from its gaseous ions is the lattice energy of the compound. -The heat of formation of LiF is considered the overall heat of the reaction. -The overall heat of reaction is equal to the enthalpy of sublimation of Li, the bond dissociation energy of fluorine, the ionization of Li, the electron affinity of F, and the lattice energy of LiF. -Both the electron affinity and ionization energy are negative values, according to the direction of the equation that represents each process

Formal Charge

Used to evaluate nonequivalent Lewis structures. -Atoms in molecules try to achieve formal charges as close to zero as possible. -Any negative formal charges are expected to reside on the most electronegative atoms.

Practice writing many Lewis formulas

When it is necessary to exceed the octet rule for one of several third-row (or higher) elements, place the extra electrons on the central atom

A covalent bond is

a chemical bond in which two or more electrons are shared by two atoms. - Covalent Bonding - electrons are shared equally by nuclei.

Polar covalent bond or polar bond is

a covalent bond with greater electron density around one of the two atoms.

Generally, electronegativity increases across

a row. -As the metallic character of an element decreases across a row, electronegativity, the desire of an atom for a shared pair of electrons, increases. -Electronegativity generally decreases down a column. As the metallic character of an element increases down a column, electronegativity decreases

Electronegativity is

ability of an atom to attract toward itself the electrons in a chemical bond.

Unequal sharing of electrons between

atoms in a molecule. -The molecule is linear, but the electron density is greater about the F atom instead of equally shared between the two atoms. -Because the F atom has more electron density, the F side of the molecule is said to be electron rich, and the H side of the molecule is said to be electron poor

As the size of the atom increases,

bond energy decreases. -When two atoms are bonded, a triple bond is stronger than a double bond, and a double bond is stronger than a single bond

An atom's formal charge is

difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure

Bond Length:

distance between the nuclei of two covalently bonded atoms in a molecule. Bond Lengths Triplebond < Double Bond < SingleBond

Lattice energy (U) is

energy required to completely separate one mole of a solid ionic compound into gaseous ions

Lewis structures with large formal charges are

less plausible or feasible than those with small formal charges

For neutral molecules, a Lewis structure in which there are

no formal charges is preferable to one in which formal charges are present

A resonance structure is

one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure. -More than one valid Lewis structure can be written for a particular molecule.

A plot of percent ionic character versus electronegativity difference shows that

pure covalent bonds (such as the Cl-Cl bond in Cl2 ) have zero percent ionic character, but no bonds have 100% ionic character. -Many covalent compounds have some ionic character (less than 25%), and many ionic compounds have at least 75% ionic character

A Lewis dot symbol consists of

symbol of an element and one dot for each valence electron in an atom of the element -Shows how valence electrons are arranged among atoms in a molecule -Reflects central idea that stability of a compound relates to noble gas electron configuration

Ionic bond:

the electrostatic force that holds ions together in an ionic compound. -Ionic Bonding - electrons are transferred

Among Lewis structures having similar distributions of formal charges,

the most plausible structure is the one in which negative formal charges are placed on the more electronegative atoms

For elements in Groups 1A through 8A,

the number of valence electrons for the atom is the same as the group number. -For example, sodium is a Group 1A element and has one valence electron. -Sulfur is a Group 6A element and has six valence electrons.

Triple bond -

two atoms share three pairs of electrons

Double bond -

two atoms share two pairs of electrons