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Review of APSC 131

Atomic Structure and Inter-atomic Bonding (Chapter 2 Callister text)

- consider the schematic representation of the Bohr atom:

- electrons (-ve) orbit around a nucleus (+ve) at different energy levels or states (arranged

into shells)

- w.r.t. inter-atomic bonding, the most important electrons are the valence electrons,referring to the electrons which occupy the outermost shell

- when a large number of atoms bond together, a solid is formed

- the inter-atomic spacing of atoms within a solid is governed by a simple force balancebetween the atomic forces of attraction and repulsion:

RANFFF +=

Note: Electrons (-ve) from one atom are attracted to the nucleus (+ve) of its neighbour,

but repelled by the neighbours electron field

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- sinceFA andFR vary as a function of inter-atomic separation, r, the equilibrium spacing

between atoms, r0, occurs where there is no net force, i.e.FN= 0 orFA= -FR, as shown inFigure 2.8(a):

- similarly, the balance between attractive and repulsive forces can be presented in terms

of energy, where:

= drFE

therefore, for a given inter-atomic distance, r,

R

r

A

r

R

r

ANNEEdrFdrFdrFE +=+==

- an energy curve for inter-atomic spacing is shown above in Figure 2.8(b)

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Notes: Due to the characteristic shape of theENcurve, it is often called an energy

well.

The equilibrium inter-atomic spacing, r0, corresponds to the lowest possible

level of total energy within the system,E0. E0 refers to the conditions at

absolute zero (0 K) therefore, when heat (energy) is added to the system, thervalue varies between a theoretical min. and max. (i.e. sides of the well),

causing the atoms to vibrate.

- there are 3 types of primary bonds that can be present in solid materials:

i) ionic bonds

- formed between a metallic and non-metallic element- metallic atoms give up their valence electrons to the non-metallic atoms, such

that charged ions are produced, eg. NaCl => Na+

+ Cl-

(see Figure 2.9)- the attractive forces are coulombic, meaning that the positive and negative ions

naturally attract one another

- ionic bonds are non-directional, i.e. bonding forces are equal in all directions- to maintain stability, a +ve ion must have ve ions for all of its nearest

neighbours (in 3-D) this characteristic has serious implications with respect to

deformation and crystallographic slip!

- ionic materials tend to be very hard and brittle, but are very good thermal andelectrical insulators

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ii) covalent bonds

- formed by relatively equal sharing of pairs of valence electrons

- bonds are directional, ie. bonding only occurs between pairs of atoms sharing

an electron

- the strength of covalent bonds varies widely, from diamond (i.e. a 3-D carbonnetwork), which is the hardest known material, to bismuth, which has a

relatively low melting temperature (270 C )- covalent bonds comprise the backbone of polymer chains (carbon chains);however, the chains are held together by weak secondary bonds

- similar to ionic materials, purely covalent solids tend to be hard and brittle

Note: Very few bonds are purely covalent or purely ionic, instead they are usually a

mixture of the two.

iii) metallic bonds

- formed in metals in their alloys

- valence electrons form a non-localized electron cloud (or sea of valence

electrons) around a regular array of ion cores, i.e. valence or not attached to anyone atom, but instead free to roam throughout structure

- metallic bonds are non-directional

- given the non-localized nature of free electrons, metals and their alloys arevery good conductors of electricity and heat

- in addition to primary bonds, it is also possible to have secondary bonding in a solid

(eg. van der Waals bonding), caused by molecular and/or atomic dipoles secondarybonds are quite weak in comparison to primary bonds, which is the main reason for the

relatively low strength of most polymers