Chapter 2: Atomic Structure & Interatomic Bonding ISSUES TO ADDRESS... • What promotes bonding? • What types of bonds are there? • What properties are inferred from bonding?

Chapter 2 - 1

Atomic Structure (Freshman Chem.) • atom –

electrons – 9.11 x 10-31 kg protons -27 kg 1.67 x 10 neutrons

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• atomic number = # of protons in nucleus of atom = # of electrons of neutral species

• A [=] atomic mass unit = amu = 1/12 mass of 12C Atomic wt = wt of 6.022 x 1023 molecules or atoms

1 amu/atom = 1g/mol C H

12.011 1.008 etc. Chapter 2 - 2

Atomic Structure • Valence electrons determine all of the following properties 1) 2) 3) 4)

Chemical Electrical Thermal Optical

Chapter 2 - 3

Electronic Structure • Electrons have wavelike and particulate properties. – This means that electrons are in orbitals defined by a probability. – Each orbital at discrete energy level is determined by quantum numbers. Quantum #

Designation

n = principal (energy level-shell) l = subsidiary (orbitals) ml = magnetic

K, L, M, N, O (1, 2, 3, etc.) s, p, d, f (0, 1, 2, 3,…, n -1) 1, 3, 5, 7 (-l to +l)

ms = spin

½, -½ Chapter 2 - 4

Electron Energy States Electrons...

• have discrete energy states • tend to occupy lowest available energy state. 4d 4p

N-shell n = 4

3d 4s Energy

3p 3s

M-shell n = 3 Adapted from Fig. 2.4, Callister & Rethwisch 8e.

2p 2s

L-shell n = 2

1s

K-shell n = 1 Chapter 2 - 5

Chapter 2 - 6

Chapter 2 - 7

SURVEY OF ELEMENTS • Most elements: Electron configuration not stable. Element Hydrogen Helium Lithium Beryllium Boron Carbon ...

Atomic # 1 2 3 4 5 6

Electron configuration 1s 1 1s 2 (stable) 1s 2 2s 1 1s 2 2s 2 1s 2 2s 2 2p 1 1s 2 2s 2 2p 2 ...

Adapted from Table 2.2, Callister & Rethwisch 8e.

Neon Sodium Magnesium Aluminum ...

10 11 12 13

1s 2 2s 2 2p 6 (stable) 1s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 3s 2 1s 2 2s 2 2p 6 3s 2 3p 1 ...

Argon ... Krypton

18 ... 36

1s 2 2s 2 2p 6 3s 2 3p 6 (stable) ... 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 (stable)

• Why? Valence (outer) shell usually not filled completely. Chapter 2 - 8

Electron Configurations • Valence electrons – those in unfilled shells • Filled shells more stable • Valence electrons are most available for bonding and tend to control the chemical properties – example: C (atomic number = 6) 1s2 2s2 2p2 valence electrons

Chapter 2 - 9

Electronic Configurations ex: Fe - atomic # = 26 1s2 2s2 2p6 3s2 3p6 3d 6 4s2 4d 4p

N-shell n = 4 valence electrons

3d

4s Energy

3p 3s

M-shell n = 3 Adapted from Fig. 2.4, Callister & Rethwisch 8e.

2p 2s

L-shell n = 2

1s

K-shell n = 1 Chapter 2 - 10

give up 1egive up 2egive up 3e-

• Columns: Similar Valence Structure

accept 2eaccept 1einert gases

The Periodic Table

H

He

Li Be

O

F Ne

Na Mg

S

Cl Ar

K Ca Sc Rb Sr

Y

Cs Ba

Se Br Kr Te

I

Adapted from Fig. 2.6, Callister & Rethwisch 8e.

Xe

Po At Rn

Fr Ra

Electropositive elements: Readily give up electrons to become + ions.

Electronegative elements: Readily acquire electrons to become - ions. Chapter 2 - 11

Electronegativity • Ranges from 0.7 to 4.0, • Large values: tendency to acquire electrons.

Smaller electronegativity

Larger electronegativity

Adapted from Fig. 2.7, Callister & Rethwisch 8e. (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. Chapter 2 - 12

Chapter 2 - 13

Chapter 2 - 14

Ionic bond – metal

+

donates electrons

nonmetal accepts electrons

Dissimilar electronegativities ex: MgO

Mg

1s2 2s2 2p6 3s2 [Ne] 3s2

Mg2+ 1s2 2s2 2p6 [Ne]

O

1s2 2s2 2p4

O2- 1s2 2s2 2p6 [Ne] Chapter 2 - 15

• • • •

Ionic Bonding

Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required. Example: NaCl Na (metal) unstable

Cl (nonmetal) unstable electron

Na (cation) stable

-

+ Coulombic Attraction

Cl (anion) stable

Chapter 2 - 16

Ionic Bonding • Energy – minimum energy most stable – Energy balance of attractive and repulsive terms

EN = EA + ER =

-

A r

+

B rn

Repulsive energy ER

Interatomic separation r Net energy EN Adapted from Fig. 2.8(b), Callister & Rethwisch 8e.

Attractive energy EA Chapter 2 - 17

Chapter 2 - 18

Chapter 2 - 19

Examples: Ionic Bonding

• Predominant bonding in Ceramics NaCl MgO CaF 2 CsCl

Give up electrons

Acquire electrons

Adapted from Fig. 2.7, Callister & Rethwisch 8e. (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. Chapter 2 - 20

Covalent Bonding • similar electronegativity  share electrons • bonds determined by valence – s & p orbitals dominate bonding • Example: CH4 C: has 4 valence e-, needs 4 more H: has 1 valence e-, needs 1 more Electronegativities are comparable.

H

CH 4 H

C

H

shared electrons from carbon atom

H shared electrons from hydrogen atoms

Adapted from Fig. 2.10, Callister & Rethwisch 8e.

Chapter 2 - 21

Chapter 2 - 22

Chapter 2 - 23

Primary Bonding • Metallic Bond -- delocalized as electron cloud • Ionic-Covalent Mixed Bonding

% ionic character =

 (X A -X B )2    4 1e  x (100%)    

where XA & XB are Pauling electronegativities Ex: MgO

XMg = 1.2 XO = 3.5

( 3.5-1.2 )2     4 % ionic character  1 - e x (100%)  73.4% ionic      Chapter 2 - 24

Chapter 2 - 25

SECONDARY BONDING Arises from interaction between dipoles • Fluctuating dipoles asymmetric electron clouds

+

-

+ secondary bonding

-

ex: liquid H 2 H2 H2

H H

H H secondary bonding

Adapted from Fig. 2.13, Callister & Rethwisch 8e.

• Permanent dipoles-molecule induced -general case: -ex: liquid HCl -ex: polymer

+

-

H Cl

secondary bonding

+

secondary bonding

H Cl

Adapted from Fig. 2.15, Callister & Rethwisch 8e.

secondary bonding Chapter 2 - 26

Summary: Bonding Comments

Type

Bond Energy

Ionic

Large!

Nondirectional (ceramics)

Covalent

Variable large-Diamond small-Bismuth

Directional (semiconductors, ceramics polymer chains)

Metallic

Variable large-Tungsten small-Mercury

Nondirectional (metals)

Secondary

smallest

Directional inter-chain (polymer) inter-molecular Chapter 2 - 27

Properties From Bonding: Tm • Bond length, r

• Melting Temperature, Tm Energy

r

• Bond energy, Eo

ro

Energy

r smaller Tm

unstretched length ro

r

Eo = “bond energy”

larger Tm Tm is larger if Eo is larger.

Chapter 2 - 28

Properties From Bonding : a • Coefficient of thermal expansion, a length, L o coeff. thermal expansion unheated, T1

DL = a(T2 -T1) Lo

DL

heated, T 2

• a ~ symmetric at ro Energy unstretched length ro

E o

E o

r

a is larger if Eo is smaller.

larger a smaller a Chapter 2 - 29

Summary: Primary Bonds Ceramics (Ionic & covalent bonding):

Metals (Metallic bonding):

Polymers (Covalent & Secondary):

Large bond energy large Tm large E small a

Variable bond energy moderate Tm moderate E moderate a

Directional Properties Secondary bonding dominates small Tm small E large a

Chapter 2 - 30

Chapter 2 - 31