Atomic Structure & Bonding

TUTORIAL 2: Question vs Solution

ENT 112 – Materials Engineering


Q1:

A gold wire is 0.70 mm in diameter and 8।0 cm in length. How many atoms does it contain? The density of gold is 19.3 g/cm3.

Solution:

First determine the mass of gold based upon the wire volume and the density of gold.

Q2:

A solder contains 52 wt % tin and 48 wt % lead. What are the atomic percentages of Sn and Pb in this solder?

Solution:

Q3:

Describe the ionic bonding process between a pair of Na and Cl atoms.
i) Which electrons are involved in the bonding process?
ii) After ionization, why is the sodium ion smaller than the sodium atom?
iii) After ionization, why is the chloride ion larger than the chlorine atom?

Solution:

i) The ionic bonding process between a pair of Na and Cl atoms involves a transfer of the outer 3s1 electron of the Na atom to the 3p vacancy in the Cl atom. Thus, the Na ion formed has the Ne electron configuration while the Cl ion has the Kr electron configuration.

ii) After ionization to the Na+, the Na atom becomes smaller because the electron-to-proton ratio of the Na atom is decreased when the Na+ ion forms. Also, the outer third shell no longer exists once the 3s1 electron is lost by the Na atom.

iii) After ionization, the Cl- ion is larger because the electron-to-proton ratio of the chlorine atom is decreased by the ionization process.

Q4:

Briefly describe the following types of primary bonding: (a) ionic, (b) covalent, and (c) metallic।

Solution:

(a) Ionic bonding arises from the electrostatic attraction between oppositely charged ions. In the process of ion formation, an electron or a number of electrons may be transferred from a highly electropositive element to a highly electronegative one. The ionic bond in solids is nondirectional.

(b) Covalent bonding is a primary type of bonding which arises from the reduction in energy associated with the overlapping of half-filled orbitals of two atoms. In this bond, there is an electron exchange interaction. The covalent bond is a directional type of bond.

(c) Metallic bonding is a primary type of bonding involving the interaction of the valence electron or electrons of one atom with many surrounding atoms. This interaction leads to a reduction in energy of the system considered. The valence bonding electrons of these bonds are sometimes regarded as an ‘electron gas’ bonding the positive ion cores (atoms less their valence electrons) of atoms. The metallic bond is nondirectional.

Q5:
If the attractive force between a pair of Cs+ and I- ions is 2.83 × 10-9 N and the ionic radius of the Cs+ ion is 0.165 nm, calculate the ionic radius of the I- ion in nanometers.

Solution:

Q6:

How can the high electrical and thermal conductivities of metals be explained by the “electron gas” model of metallic bonding? Ductility?

Solution:

The high electrical and thermal conductivities of metals are explained by the mobility of their outer valence electrons in the presence of an electrical potential or thermal gradient.

The ductility of metals is explained by the bonding .electron gas. which enables atoms to pass over each other during deformation, without severing their bonds.

Introduction to Materials Science & Engineering

TUTORIAL 1: Question vs Solution

ENT 112 – Materials Engineering


1. What are materials? List eight commonly encountered engineering materials.

Materials are substances of which something is composed or made. Steels, aluminium, alloys, concrete, wood, glass, plastics, ceramics and electronic materials.


2. Define materials science and materials engineering.

Materials science is the subject which is concerned with basic knowledge of the internal structure, properties, and processing of materials. Materials engineering is the subject which is concerned with the use of fundamental and applied knowledge of materials so that materials can be converted into the products necessary to and desired by society.


3. What are the main classes of engineering materials?

Metallic, polymeric, ceramic, composite, and electronic materials are the five main classes.

4. What are some of the important properties of each of these engineering materials?

Metallic Materials
§ many are relatively strong and ductile at room temperature
§ some have good strength at high temperature
§ most have relatively high electrical and thermal conductivities

Polymeric Materials
§ generally are poor electrical and thermal conductors
§ most have low to medium strengths
§ most have low densities
§ most are relatively easy to process into final shape
§ some are transparent

Ceramic Materials
§ generally have high hardness and are mechanically brittle
§ some have useful high temperature strength
§ most have poor electrical and thermal conductivities

Composite Materials
§ have a wide range of strength from low to very high
§ some have very high strength-to-weight ratios (e.g. carbon-fiber epoxy materials)
§ some have medium strength and are able to be cast or formed into a variety of shapes (e.g. fibreglass-polyester materials)
§ some have useable strengths at very low cost (e.g. wood and concrete)


Electronic Materials
§ able to detect, amplify and transmit electrical signals in a complex manner
§ are light weight, compact and energy efficient

5. Define a composite material. Give an example of a composite material.

A composite material is a materials system composed of a mixture or combination of two or more materials. Two examples are carbon-fiber epoxy and fiberglass polyester materials.

6. List some materials usage changes which you have observed over a period of time in some manufactured products. What reasons can you give for the changes that have occurred?

The modern automobile is being constructed with more and more plastic materials and less metallic due to the lower cost and weight of plastics. The modern airplane is using more composite materials and plastics and less metallic materials to reduce plane weight. Modern electronics equipment uses a great number of solid state devices made with electronic materials. These materials are more compact, weigh less, and provide higher overall and energy efficiency. In many cases, they are the only type of material that can be used for specific applications such as complex computer memories.

7. What factors might cause materials usage predictions to be incorrect?
*If a war breaks out and, as a consequence, a raw material’s supply is cut off. For example, if a major war broke out in the Middle East, the price of oil would increase, and hence the price of plastic materials would also increase.
*If a major new discovery is made, some materials’ usage may change.
*If defects show up in a specific material after a certain length of its service, the material’s usage may decrease. For example, a high strength composite material used for aircraft may start showing some delimitation defects.