Atomic Structure and Bonding: Electronegativity and Ionic Bonding, Exams of Chemical Principles

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ATOMIC STRUCTURE AND

INTERATOMIC BONDING

Chapter 2

Electronegativity

• Electronegativity , symbol χ, is a chemical property that describes the ability of an atom to attract electrons towards itself in a covalent bond. First proposed by Linus Pauling in 1932 as a development of valence bond theory, it has been shown to correlate with a number of other chemical properties.
• Electronegativity cannot be directly measured and must be calculated from other atomic or molecular properties. Several methods of calculation have been proposed and, although there may be small differences in the numerical values of the electronegativity, all methods show the same periodic trend between elements.
• The difference in electronegativity between atoms A and B is given by:

where the dissociation energy, E d, of the A–B, A–A and B–B bonds are expressed in electron volts, the factor (eV)−½ being included to ensure a dimensionless result. Hence, the difference in Pauling electronegativity between hydrogen and brome is 0. (dissociation energies: H–Br, 3.79 eV; H–H, 4.52 eV; Br–Br 2.00 eV)

- Ionic bonding energy is relatively large: 600-1500 kJ/mol (3-8 eV/atom; 1eV=1.602 x 10-19^ J)

• The ionic materials typically have high melting point , are hard and brittle as well as electrically and thermally insulators
• Example: Ceramics (MgO, SiO 2 etc.)

Cs Cl

MgO

CaF 2

NaCl

EXAMPLES: IONIC BONDING

Na (metal)

unstable

Cl (nonmetal)

unstable

electron

Coulomb

Attraction

Na (cation)

stable

Cl (anion)

stable

• Occurs between + and - ions.
• Requires electron transfer.
• Large difference in electronegativity required.
• Example: NaCl

IONIC BONDING (1)

r

1

4

(Z e)(Z e) E 0

1 2 A ^  

Z 1

Z 2

 0 – a vacuum permittivity (8.85 • 10 -12^ F/m)

• Nonmetallic Molecules: H 2 , Cl 2 , F 2
• Molecules with dissimilar atoms : CH 4 , HNO 3 , HF
• Elemental solids ( e.g. column IVA: C, Si, Ge, etc,)
• Compound solids (columns IIIA, IVA, VA: SiC, GaAs, InSb)
• Polymeric Materials

Si C

C(diamond)

C 2.

H 2

Cl (^2)

F 2

Si 1.

GaAs

Ge 1.

column IVA

**Sn

Pb 1.**

EXAMPLES: COVALENT BONDING

• Requires shared electrons
• Electronegativities are comparable

shared electrons

from carbon atom

shared electrons

from hydrogen atoms

H

H

H

H

C

COVALENT BONDING (1)

• The number of covalent bonds that is possible for a specific atom is determined by number of valance electrons (Nv): Number of covalent bonds = 8-Nv

Examples: Carbon – Nv=4  Number of covalent bonds = 8 – 4 = 4

Diamond: each carbon atom covalently bonds with 4 other C atoms.

Methane (CH 4 ): Carbon has 4 valence e and needs 4 more, while H has 1 valence e, and needs 1 more

COVALENT BONDING (3)

Mixing Bonds

• Very few compounds exhibit pure ionic or covalent bonding
• The degree of either bond type depends on the different in their

electronegativities (i.e. relative element position in the periodic table):

• the greater the difference in electronegativity – the more ionic bond
• the smaller the difference in electronegativity (i.e. closer the atoms together in periodic table) – the greater the degree of covalent bonds.

To estimate the % of ionic bonds between elements A and B one can use the following expression:

% ionic character = {1-exp [-0.25(XA-XB)^2 ] }x 100

where Xa and XB are elements electronegativity.

http://www.youtube.com/watch?v=Ibr63AjnEoQ

METALLIC BONDING (2)

EXAMPLES: METALLIC BONDING

- Permanent dipoles - molecule induced

H Cl^ secondaryH^ Cl bonding

Examples:

polymer

SECONDARY (van der Waals) BONDING (2)

liquid HCl

Asymmetrical arrangement of the electrical field for polar molecules

• Special and the strongest (E~0.5eV/atom) type of polar molecule bond is so-called hydrogen bond

In molecule where H is covalently bonded (i.e. shared electron) to Fluorine (e.g. HF), Oxygen (e.g. H 2 O) and nitrogen (e.g. NH 3 ). The H end of the molecule is highly positively charged – provides strong attractive interaction with the negative end of the adjacent molecule.

SECONDARY (van der Waals) BONDING (3)

THE STRUCTURE of POLYMERS:

Hydrocarbon Molecules

• Most polymers are organic, and formed from hydrocarbon molecules
• Each C atom has four e- that participate in bonds, each H atom has one bonding e-
• Attachment of different organic groups to the hydrocarbon backbone offers wide variety of possible polymers
• Examples of saturated (all bonds are single ones) hydrocarbon molecules (of type CnH2n+2)

Comparison of Different Atomic Bonds