CHEMICAL BONDS

Chemical Bond Mutual electrical attraction between the

nuclei and valence electrons of different atoms that binds the atoms together.

Ionic BondBonding that results from the electrical attraction between large numbers of cations and anions

A large difference in electronegativity between two atoms in a bond will result in ionic bonding

Covalent (Molecular) BondSharing electrons pairs between

two atomsA small difference in

electronegativity between two atoms in a bond will result in covalent bonding.

ElectronegativityA measure of the ability of an atom in a chemical compound to attract electrons.

Valence Electrons

Electrons in the highest occupied energy level of an elements’ atom

Determines the chemical properties of an element

Examples:

The Octet Rule

Atoms tend to gain, lose, or share enough electrons to become surrounded by eight valence electrons

Non-metallic elements gain electrons or share them

Metallic elements lose electrons

Electron Dot Notation

An electron configuration notation in which only the valence electrons of an atom of a particular element are shown

Indicated by dots placed around the element’s symbol

○ Example:

Learning Check A. X would be the electron dot

formula for

1) Na 2) K 3) Al

B. X would be the electron dot

formula

1) B 2) N 3) P

Dot notation can be used to represent moleculesExample: (H : H) represents a shared

electron pair An unshared pair or lone pair is a

pair of electrons that is not involved in bonding and belongs exclusively to one atomExample: Lone Pair

Lewis Structures

Electron distribution is depicted with Lewis electron dot structures

Electrons are distributed as shared or Bond Pairs or unshared or Lone Pairs

Steps:

•Write electron dot notation for each atom in molecule.

•Determine total number of valence electrons.

•Arrange atoms to form skeleton structure for molecule.

•If Carbon is present it will go in the center. Otherwise least electronegative atoms will be in the center. Hydrogen never is in the center

Steps ContinuedConnect atoms by electron-pair bonds

Add unshared pairs of electrons so each nonmetal is surrounded by 8 electrons

•Count the electrons to see it matches the number of valence electrons

•If too many electrons create double or triple bonds.

Lewis Structure Bond Formula

N A

2 # bonds

•To determine how many bonds exist in a molecule, use the following formula: N-A = # of Bonds 2

•Electrons are shared and represented by a dash

• lone electrons are represented by dots.

N-A = # of Bonds 2 Where: N = # of needed electrons, ( 8 for all elements but H, which

is 2.) A = # of available electrons (the

number of valence electrons.)

The least electronegative element

is the central atom.

Example: Write the Lewis structure of NH3

The total number of valence electrons is:

The number of electrons needed is:

A = 8

1 x 8 = 8

3 x 2 = 6N = 14

The Skeleton structure is:

N – A

2

14 – 8

2

= 3 Bonds

Connect the atoms with electron pairs. Remember 8 electrons are needed to obey the octet rule

Finish the structure by using the remaining electrons as lone pairs

Check that the final Lewis structure has the correct number of valence electrons (8) except Hydrogen (2)

Write the Lewis structure of H2CO

2 (H) 2 x 1 = 2

1 (C) 1 x 4 = 4

1 (O) 1 x 6 = 6

A = 12

Needed (N)Available (A)

2 (H) 2 x 2

1 (C)1 x 8

1 (O)1 x 8

= 4

= 8

= 8

N = 20

Write the Lewis structure of H2CO cont.

H | C= O | H

N – A

2

20 – 12

2

= 4 Bonds

Cl ClThis is the chlorine molecule,

Cl2

Single Covalent BondCovalent bond produced by the sharing of one pair of electrons between two atoms

or

Multiple Covalent Bonds Double Bond

Covalent bond produced by the sharing of two pairs of electrons between two atoms○Shown by either two side-by-side

pairs of dots or by two parallel dashes

OO=For convenience, the double bond

can be shown as two dashes.

OO

Triple BondCovalent Bond produced by sharing of

three pairs of electrons between two atoms

Carbon forms a number of compounds containing triple bonds

Lewis structures for molecules that contain carbon, oxygen or nitrogen, remember that multiple bonds between pairs are possible

If too many electrons have been used, subtract one or more pairs until the total number of valence electrons is correct. Then move one or more lone electron pairs to existing bonds between non-hydrogen atoms until the outer shells of all atoms are completely filled.

Ionic Bonding & Ionic Compounds

Ionic CompoundComposed of positive

(cations) and negative (anions) ions that are combined so that the numbers of positive and negative charges are equal.

Most ionic compounds exist as crystalline solids.A crystal of any ionic compound is

a 3-D network of positive and negative ions mutually attracted to each other.

Formation of an ionic bond can be viewed as a transfer of electrons

Ionic Bonds: One Big Greedy Thief Dog!

Crystal Lattice

In an ionic compound, the ions minimize their potential energy by combining in an orderly arrangement.

The distance between ions and their arrangement in a crystal represents a balance among all forces.

Properties of Ionic Compounds

High melting points Solids at room temperature Soluble in polar solvents,

insoluble in Nonpolar solvents Molten compounds & aqueous

solutions conduct electricity

Metallic Bonding

Chemical bonding is different in metals than it is in ionic, molecular or covalent network compounds

Metallic Bonding

Chemical bonding that results from the attraction between metal atoms and the surrounding sea of electrons

An attraction of the free-floating valence electrons for the positively charged metal ions.

Metallic Properties

Good conductors of electricity and heatMalleability

○ Ability of a substance to be hammered or beaten into thin sheets

Ductility○ Ability of a substance to be drawn,

pulled, or extruded through a small opening to produce a wire.

The Properties of Molecular (Covalent)

Compounds

Intermolecular Forces Reviewing what we know

Low density Highly compressible Fill container

Solids

• High density • Slightly compressible • Rigid (keeps its shape)

Gases

Intermolecular forces – occur between molecules

Intramolecular forces – occur inside the molecules

Polar Molecule vs. Nonpolar Molecule

•One very important property of molecule is whether it is polar or nonpolar.

•If the electrons in a molecule are not evenly distributed, the molecule can have a negative and positive side.

•A polar molecule will dissolve in another polar substance (such as water).

•Nonpolar molecule will dissolve in another nonpolar substance (such as carbon tetrachloride).

Polar vs. Nonpolar Rules

1. If the central atom has no lone pairs and has all the same types of atoms attached to it, then the molecule is nonpolar.

2. If the central atom has no lone pairs but different atoms attached to it, the molecule is polar.

3. If the central atom has lone pairs, the molecule is polar.

Dipole – dipole attraction

A dipole is created by an uneven charge distribution (electronegativity)

Dipole-Dipole is an electrostatic attraction between polar molecules.

Hydrogen Bonding

Occurs between H and highly electronegative atom (for example N, O, F)

Hydrogen Bonding

Affects physical properties Boiling point

London Dispersion Forces

Attraction of instantaneous and induced dipoles; exist between all molecules.

Formation of instantaneous dipoles

London Dispersion Forces

Nonpolar molecules

London Dispersion Forces

Become stronger as the sizes

of atoms or molecules increase

VValencealenceSShellhellEElectronlectronPPairairRRepulsionepulsion Theory Theory

Planar triangular

OctahedralTrigonal bipyramidal

VSEPR theory

(Valence Shell Electron Pair Repulsion) states that repulsion between valence electrons causes these sets to be oriented as far apart as possible.

Molecular Shape

Atoms bonded to

central atom (B)

Lone pairs (E)

Type of Molecule

Linear 2 0 AB2

Bent or angular

2 1 AB2E

Trigonal-planar

3 0 AB3

Molecular ShapeAtoms

bonded to central atom

Lone pairs Type of Molecule

Tetrahedral 4 0 AB4

Trigonal- pyramidal

3 1 AB3E

Molecular ShapeAtoms

bonded to central atom

Lone pairs Type of Molecule

Bent or angular

2 2 AB2E2

Trigonal- bipyramidal

5 0 AB5

Octahedral 6 0 AB6