tema 2 ciència de materials

7/24/2019 Tema 2 Cincia de Materials

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TEMA 2.ENLLA QUMIC

2.1 Estructura atmica

2.2 Enlla inic, nombre de coordinaci

2.3 Enlla covalent

2.4 Enlla metllic

2.5 Forces febles tipus Van der Waals

2.6 Materials: classificaci segons tipus denlla


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Atom the smallest unit of matterindivisible

Heliumatom


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a)

Atomic number = number of Electrons

b)

Electrons vary in the amount of energy

they possess, and they occur at certainenergy levels or electron shells.

c) Electron shells determine how an atom

behaves when it encounters other atoms


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Fig. 2.1


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Comparison of the(a)

Bohr and

(b)

(b) wavemechanical

atom models in terms ofelectron distribution.


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Fig. 2.3


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Schematicrepresentationof the relativeenergies of the

electrons for thevarious shellsand subshells.


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Electrons are placed in shells

according to rules:


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Why are electrons important?

1)

Elements have different electronconfigurations!

different electron configurations meandifferent levels of bonding


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The electronegativity values for the elements


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Chemical bonds:an attempt to fill electron shells

1.

Ionic bonds 2. Covalent bonds

3. Metallic bonds


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IONIC BOND

Bond formed between

two ions by the

transfer of electrons


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Fig. 2.4


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Formation of Ions from Metals

"

Ionic compoundsresult when metalsreact with

nonmetals

"

Metals loseelectrons to match the number of valence

electrons of their nearest noble gas"

Positive ionsformwhen the number of electrons are

lessthan the number of protons

Group 1 metals ion 1+




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Formation of Sodium Ion

Sodium atom Sodium ion

Na e- Na +

2-8-1 2-8 ( = Ne)

11 p+ 11 p+

11 e- 10 e-0 1+


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Fluoride Ion

unpaired electron octet

1 -

:F + e- : F :

2-7 2-8 (= Ne)

9 p+ 9 p+

9 e- 10 e-0 1 -

ionic charge


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Ionic Bond

Between atoms of metals and nonmetalswith very different electronegativity

Bond formed by transfer of electrons Produce charged ions all states. Conductors

and have high melting point.

Examples; NaCl, CaCl2, K

2O


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1). Ionic bond electron from Na is transferred to Cl, this causes

a charge imbalance in each atom. The Na becomes (Na+) and theCl becomes (Cl-), charged particles or ions.


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Fig. 2.6


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Fig. 2.7


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Fig. 2.8


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Fig. 2.9


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Fig. 2.10


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Schematic representation of ionic bonding insodium chloride (NaCl).


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Fig. 2.11

Coordination number


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Fig. 2.12


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COVALENT BOND

bond formed by the sharing ofelectrons

Between nonmetallic elements of similarelectronegativity.

Formed by sharing electron pairs

Stable non-ionizing particles, they are notconductors at any state

Examples; O2, CO2, C2H6, H2O, SiC


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Covalent bonds- Two atoms share one ormore pairs of outer-shell electrons.


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NONPOLAR

COVALENT BONDS

when electrons are shared equally

H2or Cl2

POLAR COVALENT BONDSwhen electrons are shared but shared unequally

H2O


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- water is apolarmoleculebecause oxygen is more electronegativethan hydrogen, and therefore electrons are pulled closer to oxygen.


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Fig. 2.14


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Fig. 2.15


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Fig. 2.16


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Fig. 2.17


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Fig. 2.18


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Fig. 2.19


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Bond Properties

bond order, bond length, bondenergy, bond polarity


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Bond OrderFractional bond ordersin resonance structures.Consider NO2

-


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Bond OrderBond order is proportional to two important bond

properties:(a) bond strength

(b) bond length


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Bond Length

Bond length is the distance between the nuclei of twobonded atoms.


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Bond Length

Bond length depends on

bond order.


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Using Bond Energies

Estimate the energy of the reaction

HH + ClCl ----> 2 HCl

Net energy = "Hrxn =

= energy required to break bonds

- energy evolved when bonds are made


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Using Bond Energies

Estimate the energy of the reaction

HH + ClCl ----> 2 HCl


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METALLIC BOND

bond found in metals; holds metal

atoms together very strongly Formed between atoms of metallic elements

Electron cloud around atoms

Good conductors at all states, lustrous, veryhigh melting points

Examples; Na, Fe, Al, Au, Co


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Fig. 2.20


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A Sea of Electrons


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Metals Form Alloys

Metals do not combine with metals. They formAlloys which is a solution of a metal in a metal.

Examples are steel, brass, bronze etc.


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Fig. 2.21

Dipole-Dipole interaction


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Fig. 2.22


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Fig. 2.23


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