Factors affecting metallic bond strength

Metallic crystal structures

Factors affecting metallic bond strength:

1. Atomic (metallic) radius

Na Mg Al

371K 922K 933K

K

336K Melting

Rb Point (K)

312K

2. No. of valence electrons

Decrease in radius

Increase

In radius

Metallic bondingis an electrostatic attraction between sea of electrons and positive ions.

Metallic radius:

as half the internuclear distance between two atoms in a metallic crystal

Metallic bondingFactors affecting the strength of metallic

bonding:

1. number of valence electrons (outermost electrons / bonding electrons) and charge on cations. e.g. b.p. : Al > Mg > Na

2. metallic radius. e.g. b.p. : Li > Na > K

i.e. larger no. of valence electrons, smaller metallic radius will result in stronger metallic bonds.

Metallic bonding10.4 Metallic crystals

Close-packed structure

Open structure

74 % of space occupied by atoms / co-ordination no. = 12

68 % of space occupied by atoms/ co-ordination no. = 8

Hexagonal close packed structure:“abab”

Cubic close-packed structure:”abcabc”

Body center cubic structure

Open Structure(1 Type)

Close-packed Structure(2 types)

Unit CellUnit Cell

Co-ordination Co-ordination number: number: 88

Number of atom Number of atom in a unit cell: in a unit cell:

1 + 8 ( 1/8)1 + 8 ( 1/8)= 2= 2

Open Structure:

Body-centered Body-centered cubiccubic

10.4 Metallic crystalsOpen structure

Open Structure:

Body-centered Body-centered cubiccubic

Examples:Examples:

Close-Packed Structure:

Hexagonal Hexagonal close-packedclose-packed

abababab…..

hexagonal

Co-ordination Co-ordination number: number: 1212(6 in its own layer, 3 (6 in its own layer, 3 in the layer above in the layer above and 3 below.)and 3 below.)

Hexagonal close-packed structure:“abab”

10.4 Metallic crystals

co-ordination no. = 12

Close-Packed Structure:

Hexagonal Hexagonal close-packedclose-packed

Example:Example:

Close-Packed Structure:

CubicCubicclose-packedclose-packed

abcabcab…..

Co-ordination Co-ordination number: number: 1212

(6 in its own layer, 3 (6 in its own layer, 3 in the layer above in the layer above and 3 below.)and 3 below.)

also called face-centered cubic

Cubic close-packed structure:”abcabc”

co-ordination no. = 12

10.4 Metallic crystals

Close-Packed Structure:

CubicCubicclose-packedclose-packed

Examples:Examples:

“abab” and “abcabc”

10.4 Metallic crystals

Tetrahedral hole

Octahedral hole

10.4 Metallic crystals