MTP M - Ch. 2 Atomic Structure and Bonding

8/11/2019 MTP M - Ch. 2 Atomic Structure and Bonding

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Materials and Technological

Processes (MTP M1E)

Lesson 2


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ISSUES TO ADDRESS

What promotes bonding?

What types of bonds are there?

What properties are inferred from bonding?

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CHAPTER 2:

BONDING AND PROPERTIES


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ATOMIC STRUCTURE AND

BONDING IN SOLIDS

The name atom comes from the Greek ""tomos (from -, "un-" + temno, "to cut"), which means uncuttable, or

undividable, something that cannot be divided further.

Discovered approx. 6th century BCE

atom is a basic unit of matter that consists of a dense, centralnucleus surrounded by a cloud of negatively charged electrons.

Atomic nucleus = protons + neutrons

Protons positively charged

Neutrons electrically neutral

Electrons negatively charged

Atomic number = Number of protons in the nucleus
http://en.wikipedia.org/wiki/Greek_languagehttp://en.wikipedia.org/wiki/Greek_language


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ATOMIC STRUCTURE AND BONDING

IN SOLIDS PERIODIC TABLE OF ELEMENTS

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CHECK OUT THIS SITE!!!

www ptable com

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http://www.ptable.com/http://www.ptable.com/http://www.ptable.com/


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BONDING IN SOLIDS

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Shells

Bohr model


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BONDING IN SOLIDS

Bohr versus

wave

mechanical

atom models


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ELECTRON ENERGY STATES

1s

2s2p

K-shell n = 1

L-shell n = 2

3s3p M-shell n = 3

3d

4s

4p4d

Energy

N-shell n = 4

Electrons... have discrete energy states tend to occupy lowest available energy state.

Adapted from Fig. 2.4,

Callister & Rethwisch 3e.


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SURVEY OF ELEMENTS

Why? Valence (outer) shell usually not filled completely.

Most elements: Electron configuration not stable.

Electron configuration

(stable)

...

...

1s22s22p 63s23p 6 (stable)...

1s22s22p 63s23p 63d10 4s24p 6 (stable)

Atomic #

18...

36

Element1s11Hydrogen

1s22Helium

1s22s13Lithium

1s22s24Beryllium

1s22s22p 15Boron

1s22s22p 26Carbon...

1s22s22p 6 (stable)10Neon1s22s22p 63s111Sodium

1s22s22p 63s212Magnesium

1s22s22p 63s23p 113Aluminum...

Argon...

Krypton

Adapted from Table 2.2,



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BONDING IN SOLIDS

Shells (Energy level) : K, L, M, N, O (1, 2, etc.)

Sub-shells (Orbitals) : s, p, d, f (0, 1, 2 n-1)

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ELECTRON CONFIGURATIONS

Valence electrons those in unfilled shells

Valence electrons can participate in the formation ofchemical bonds with other atoms.

Filled shells more stable Valence electrons are most available for bonding and

tend to control the chemical properties

An ion is an atom or molecule in which the total numberof electrons is not equal to the total number of protons,giving it a net positive or negative electrical charge.

example: C (atomic number = 6)

1s2 2s2 2p2

valence electrons


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ATOMIC STRUCTURE - HYDROGEN


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ATOMIC STRUCTURE

Valence electrons determine all of thefollowing properties

1) Chemical

2) Electrical

3) Thermal

4) Optical


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Potential energy at

minimum

Bond length

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0

POTENTIAL ENERGY AND BOND LENGTH


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PROPERTIES FROM BONDING: TM

Bond length, r

Bond energy, Eo

Melting Temperature, Tm

Tm is larger if Eo is larger.

ro r

Energy

r

larger Tm

smaller Tm

Eo =

bond energy

Energy

ro r

unstretched length


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ror

larger a

smaller a

Energy

unstretched length

Eo

Eo

PROPERTIES FROM BONDING:

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a - coefficient of thermal expansion

a ~ symmetric at ro

a is larger if Eo is smaller.

= a (T2 -T1)D L

L o

coeff. thermal expansion

D L

length, Lo

unheated, T1

heated, T2

a


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INTERATOMIC BONDS

Primary (chemical) bonds: Ion bonding

Covalent bonding

Metal bonding

Secondary bonds:

Van der Waals, dipole,

hydrogen bonding

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Ionic Bonding

Occurs between + and ions

Both metallic and non-metallic elements

Requires electron transfer

Large difference in electronegativity required

Example: NaCl

INTERATOMIC BONDS

Na (metal)unstable

Cl (nonmetal)unstable

electron

+ -CoulombicAttraction

Na (cation)stable

Cl (anion)stable


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donates acceptselectrons electrons

Dissimilar electronegativities

ex: MgO Mg 1s2 2s2 2p6 3s2 O 1s2 2s2 2p4

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IONIC BOND METAL + NONMETAL

Mg2+ 1s2 2s2 2p6 O2- 1s2 2s2 2p6


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IONIC BONDING

Energy minimum energy most stable Energy balance of attractive and repulsive terms

Attractive energy EA

Net energy EN

Repulsive energy ER

Interatomic separation r

EN= EA + ER

Adapted from Fig. 2.8(b),



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EXAMPLES: IONIC BONDING

Predominant bonding in Ceramics

Adapted from Fig. 2.7, Callister & Rethwisch 3e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the

Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.

Give up electrons Acquire electrons

NaCl

MgO

CaF2

CsCl


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Share electrons Bonds determined by valence s & p orbitals dominate bonding

Example: CH4

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COVALENT BONDING

C: has 4 valence e-,

needs 4 more

H: has 1 valence e-,

needs 1 more

Electronegativitiesare comparable.

Adapted from Fig. 2.10, Callister & Rethwisch 3e.

shared electronsfrom carbon atom

shared electrons

from hydrogenatoms

H

H

H

H

C

CH4


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METALLIC BONDING

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Sea of electrons =electron cloud

The electron cloud gives the metals good electrical and thermal conductivity.


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THE PERIODIC TABLE

Columns: Similar Valence Structure

Adapted from

Fig. 2.6,

Callister &

Rethwisch 3e.

Electropositive elements:

Readily give up electrons

to become + ions.

Electronegative elements:

Readily acquire electrons

to become - ions.

give

up

1e-

give

up

2e-

give

up

3e- i

nertgases

accept1e-

accept2e-

O

Se

Te

Po At

I

Br

He

Ne

Ar

Kr

Xe

Rn

F

ClS

Li Be

H

Na Mg

BaCs

RaFr

CaK Sc

SrRb Y


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SUMMARY: BONDING

Type

Ionic

Covalent

Metallic

Secondary

Bond Energy

Large!

Variable

large-Diamond

small-Bismuth

Variable

large-Tungsten

small-Mercury

smallest

Comments

Nondirectional (ceramics)

Directional

(semiconductors, ceramics

polymer chains)

Nondirectional (metals)

Directional

inter-chain (polymer)

inter-molecular


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SUMMARY: PRIMARY BONDS

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Ceramics

(Ionic & covalent bonding):

Large bond energy

large Tmlarge Esmall a

Metals

(Metallic bonding):

Variable bond energy

moderate Tm

moderate Emoderate a

Polymers

(Covalent & Secondary):

Directional Properties

Secondary bonding dominates

small Tmsmall E

large a

MTP M - Ch. 2 Atomic Structure and Bonding

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