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ENGINEERING MATERIALS > ATOMIC STRUCTURE AND BONDING IN SOLIDS PART 2 > Flashcards

ATOMIC STRUCTURE AND BONDING IN SOLIDS PART 2 Flashcards

1
Q

What are the main type of bonds?

A

-primary
-secondary

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2
Q

what are examples of primary bonds

A
  • ionic bonds
    -covalent bonds
    -metallic bonds
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3
Q

what are the characteristics of primary bonds?

A
  • relaticely strong bonds
    -bond energies range from 68kj/mol - 850kj/mol
    -have chemical bonds (between atoms in compounds or in elemental solids)
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4
Q

what are the types of forces an atom can exert on each other?

A

Attractive forces (fa) and repulsive forces (fr)

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5
Q

what do Fr arises from?

A

When negative charged electrons interact with each other

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6
Q

what is net force fn?

A

the sum of the attractive and repulsive forces (fn = fa +fr)

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7
Q

when is equilibrium achieved?

A

when the net force is 0/ or when the repulsive and attractive forces are equal

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8
Q

what happens at the equilibrium state?

A

atoms will resist attempts to pull them apart or push them closer together

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9
Q

what affects the magnitude of the forces

A

the interatomic distance

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10
Q

at original distance (r0) of the atomic bond what is equal to it?

A

the energy required to separate the atoms
e0

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11
Q

ionic bond formula

A

F = (Z1e)(Z2e)/4piE0r^2
Z1,Z2: valences of the two ions
e: electron charge
E0: the permittivity of vacuum
r: separation distance of the two
ions

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12
Q

what is bond length?

A

this is the distance between the nuclei of two bonded atoms. it is influenced by the size of the atoms involved and the type of bond(single, double, triple)

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13
Q

what is bond energy?

A

This is the amount of energy required to break a chemical bond and separate the bonded atoms. the higher the bond energy the higher the bond strength

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14
Q

what is electronegativity?

A

The ability of an atom to attract electrons. it ranged from 0.7 to 4.0

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15
Q

what factors affect bond strength and give a reason for each

A
  • electronegativity: the greater the difference in electronegativity the two atoms in the bond have the greater the bond strength.

-number of bonds : multiple bonds (double and triple) are stronger then single bonds as they tend to involve more shared more electrons.

-electrostatic forces: the attractive forces between oppositely charged particles/ions(in ionic) forms strong bonds.

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16
Q

is bond formation endo or exo thermic?

A

exothermic as energy is released to the surrounding

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17
Q

is bond breakage endo or exothermic?

A

endothermic as energy is absorbed from the surrounding

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18
Q

what is attractive energy formula?

A

-A/R where A is a constant and r is the separation

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19
Q

constant A formula?

A

1/4piE0 (|Z1|e) (|Z2|e)

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20
Q

what is repulsive force formula?

A

B/r^n where B and n are constants

21
Q

Properties of bonding

A
  • the larger the bond energy e0 the larger the melting temp.
  • the smaller the bond length r0 the larger the bond energy
  • the smaller the bond energy e0 the larger the thermal expansion a
22
Q

what is thermal expansion(a)?

A

this is a phenomenon where materials expand or contract in response to a change in temperature.

23
Q

what is the formula for thermal expansion (a)?

A

/\ L / L0 = a(T2-T1)
the change in length at T2 / the original length at T1 = thermal expansion ( temp 2 - temp 1)

24
Q

What is ionic bonding?

A

It involves the transfer of electrons from a metal to a non metal resulting in the formation of ions. the cation from the stable metal and the anion from the stable non metal have coulombic attraction holding them together

25
Q

What is coulombic attraction?

A

This is the electrostatic force of attraction between the positively charged ion (cation) and the negatively charged ion (anion).

26
Q

properties of ionic bonding

A
  • form ions
    -is non directional
  • the coulombic force is the same in all 3 dimensional directions
27
Q

examples of ionic bonding and here it is found

A

-predominantly found in ceramics
-NaCl
-MgO
-CaF2
-CsCl

28
Q

Coulomb’s Law

A

-can be used to describe the relationship between the electrostatic force (F), the charges (q₁ and q₂), and the distance between the charges (r):
F = K(q1.q2)/r2

where:
F is the electrostatic force,
k is Coulomb’s constant,
q1 and q2 are the magnitudes of the charges, and
r is the distance between the charges.

29
Q

what is a covalent bond

A

It involves the sharing of electrons between atoms.

30
Q

covalent bond properties

A
  • can be between non metals that are same eg cl2 and diamond or not same like CH4
    -can be between metal and non metal like H2O
  • the strength of the covalent bond depends on the number of shared electrons and the distance between the nuclei of the bonded atoms

-material have to have small differences in electronegatively

  • covalent bonds are directional
  • there is an overlap of electron orbital between the two bonding atoms
  • bonds are determined by valence and the s & p orbitals dominate the bonding
  • electrons are shared
  • covalent bonds may be strong have high m.p like in diamond or weak like in bismuth an have low m.p
31
Q

uses of covalent bonds?

A

electrical insulators and semiconductors

32
Q

diamond (covalent bond) is strong then ionic bonding true or false?

A

-true!

33
Q

In an ionic covalent mixed bond what benchmark can one consider the compound to be either covalent and ionic

A

50% below 50 its covalent above 50% its ionic

34
Q

%ionic character formula

A
  • ([Xa - Xb]^2/ 4)
    1 - e x 100%

where Xa and Xb are electronegativities

35
Q

what is metallic bonding?

A

This type of bonding occurs in metals and their alloys, electrons are delocalized and free to move throughout the structure. The positive metal ions are held by a “sea” of electrons

36
Q

characteristics of metallic bonding

A
  • non directional
    -good conductors of heat and electricity
    -bonds my be weaker or strong then covalent
37
Q

why are metallic bonds better conductors then ionic or covalent bonds

A
  • metallic bonds have delocalized electrons that can move freely where are ionic and covalent bonds have localized electrons, when an electrical potential is applied to a metal the delocalized electrons can easily move through the metal lattice creating an electrical current.
    ionic bonds can only be good conductors in molten or dissolved states. as electrons have some freedom to move.
    in covalent bond electrons are localized between the two atoms sharing them thus there is no freedom of movement and an electrical current does not form.
38
Q

metallic vs ionic which can deform and which cant? and why?

A

it is much easier to deform a metallic bond then an ionic bond this is because of sliding atom planes slide over each other during deformation. this is easier for metallic bonds as they have delocalized electrons where as with ionic bonds this is not energetically favourable.

39
Q

metals are……. while ceramics are…..
fill in the blank.

A

ductile brittle

40
Q

Secondary bond / van der waals

A

-These are weak forces that exist between molecules and atoms
-there are 3 main types
london dispersion forces
dipole - dipole forces and
hydrogen forces

41
Q

London dispersion forces

A

these are present in all molecules. they occur due to momentary fluctuations in electron distribution around an atom or molecule forming temporary dipoles.
These are present in noble gases such as helium and argon

42
Q

Dipole - Dipole interactions

A

Present only in polar molecules and occur due to the difference in electronegativity between two atoms in a molecule
aka as permanent dipoles.
stronger then london forces
found in polymers
Eg HCL

43
Q

Hydrogen bonds

A

a specific type of dipole - dipole interaction that occurs when hydrogen bonds with highly electronegative atoms (N, O and F)
- hydrogen becomes partial postive
- strongest of the secondary forces.
e.g H2O

44
Q

polymers contain both what bonds?

A

secondary and covalent

45
Q

secondary bond are non direction true or false?

A

false

46
Q

polymers have high thermal expansion properties as well as small melting points and small bond energy . True or false

A

All true

47
Q

metals have variable bond energy and moderate melting temp and thermal expansion. true or false?

A

True

48
Q

Ceramics are predominantly covalent bonds. true or false?

A

False they are ionic predominantly

ENGINEERING MATERIALS (4 decks)

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