Ch. 8 Basic Concepts of Chemical Bonding

Ionic b

Electrostatic attraction between ions

Covalent

Sharing of electrons

Metallic bond

Free electron hold metal atoms together

Lewis Symbols

•G. N. Lewis developed a method to denote potential bonding electrons by using one dot for every valence electron around the element symbol.

•When forming compounds, atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons (the octet rule).

Ionic Bonding

-Metals and nonmetals bond (except for group 8A)

-Electron transfer

-Exothermic

•One element readily gives up an electron (has a Low ionization energy).

•Another element readily gains an electron (has a High electron affinity).

Arrow(s) indicate the transfer of the electron(s).

Properties of Ionic Substances

•Evidence of well-defined 3-D structures:

–Brittle

–High melting points

–Crystalline (well ordered)

–Cleave along smooth lines

Lattice Energy

•Energy required to completely separate one mole of a solid ionic compound into its gaseous ions.

•That amount of energy is Released to Make the ionic compound (in the Born–Haber cycle).

•Lattice energy increases with:

–Increasing charge on the ions

–Decreasing size of ions

Formation of salts from the elements (endo or exo?)

Exothermic

Electron Configuration of Ions

•Main group metals lose electrons, resulting in the electron configuration of the previous noble gas.

•Nonmetals gain electrons, resulting in the electron configuration of the nearest noble gas.

•Transition metals do Not follow the Octet rule.

Transition metals lose the Valence electrons First, Then lose the d- electrons necessary for the given ion charge

Covalent Bonding

•In covalent bonds, atoms share electrons.

•There are several electrostatic interactions in these bonds:

–Attractions between electrons and nuclei

–Repulsions between electrons

–Repulsions between nuclei

•For a bond to form, the attractions must be greater than the repulsions.

Number of Bonds for Nonmetals

•The group number is the number of valence electrons.

•To get an octet, like the nearest noble gas, in the simplest covalent molecules for nonmetals, the number of bonds needed will be the same as the electrons needed to complete the octet.

A bond contains 2 electrons

Lone pairs

electrons located on only one atom in a Lewis structure

Bonding pairs

Shared electrons in a Lewis structure; they can be represented by two dots or one line, Not both!

Multiple Bonds

•Some atoms share only one pair of electrons. These bonds are called single bonds.

•Sometimes, two pairs need to be shared. These are called double bonds.

•There are even cases where three bonds are shared between two atoms. These are called triple bonds.

Bond polarity

A measure of how equally or unequally the electrons in a covalent bond are shared.

Nonpolar covalent bond,

The electrons are shared equally.

Polar covalent bond,

One of the atoms attracts electrons to itself with a greater force.

Electronegativity

•Electronegativity is the ability of an atom in a molecule to attract electrons to itself.

•We use a quantity called electronegativity to estimate whether a given bond is nonpolar covalent, polar covalent, or ionic

•On the periodic table, electronegativity generally increases as you go:

–from left to right across a period.

from the bottom to the top of a group

•When two atoms share electrons unequally, a _____ results.

polar covalent bond

Polar Covalent Bonds

The greater the difference in electronegativity, the more polar is the bond.

Dipoles

•When two equal, but opposite, charges are separated by a distance, a dipole forms.

Localized electrons

Localized electrons are specifically on one atom or shared between two atoms;

Delocalized electrons

Delocalized electrons are shared by multiple atoms.

Exceptions to the octect rule

•There are three types of ions or molecules that do not follow the octet rule:

1.Ions or molecules with an odd number of electrons

2.Ions or molecules with less than an octet

3.Ions or molecules with more than eight valence electrons (an expanded octet)

•Bond enthalpies are ____, because bond breaking is an endothermic process.

Positive