Chemical Bonding Flashcards
What do material properties depend on?
Chemical composition, crystal structure, microstructure, defect population, operating environment
What is crystal structure?
The arrangement of atoms at the
microscopic level
Microstructure
Spatial arrangement of different phases
What is defect population linked to?
It is linked to manufacturing process
(critical in fatigue, which can arise during cyclic loading)
What factors could be affected in an operating environment
Temperature, Chemical Activity, and Pressure.
Define atom
Basic unit of chemical element
What is inter-atomic bonding governed by?
The electron structure of the atoms (particularly valence electrons)
Name two models covered in class that explain the atomic structure
Bohr atomic model and the wave-mechanical model
What are the three characteristics of the Bohr atomic model?
- Electrons revolve around
nucleus in discrete orbitals - Electrons can only have
specific energy levels - Orbitals are separated by
finite energies
What type of numbers describes electrons in the wave-mechanical model?
- Electrons are described by four
quantum numbers
What type of behaviour do electrons exhibit in the wave-mechanical model?
Electrons exhibit both wave and
particle behavior
What is the position of an electron described by in the wave-mechanical model?
The position of an electron is described
as a probability distribution at
various locations around the nucleus.
What do elements in a group (column) have in common/are similar to each other?
Valence electron structures
What is the difference between alkaline earth metals and halogens in terms of donating and accepting electrons?
Alkaline earth metals readily donate electrons and are electropositive.
Halogens readily accept electrons and are electronegative.
When do atoms accept electrons more readily?
When their valence shell is nearly full and when they are less shielded from the nucleus.
What is the trend of electronegativity across the periodic table?
Electronegativity increases from left to right and from the bottom to the top across the periodic table.
What is the difference between primary and secondary bonds?
Primary bonds are strong while secondary (inter-molecular) bonds are weak.
What are the three primary bonds?
Ionic bonds, covalent bonds, and metallic bonds.
What are the types of secondary bonds?
Van der Waals bonds and hydrogen bonds.
Where are ionic bonds found?
In compounds of metals and non-metals
How does electron transfer in ionic bonds work?
Metal atoms (electropositive) give up their valence electron
to the non-metal (electronegative)
Is the ionic bond directional or non-directional?
Ionic bonds are non-directional
Describe the relative bonding energies of ionic bonds.
- Bond energies can be strong, between 600-1500 kJ/mol
Where are covalent bonds found?
In materials whose atoms have small differences in
electronegativity (close to each other in the periodic table)
Do covalent bonds share or transfer valence electrons?
They share valence electrons.
What properties have a wide range regarding covalent bonds?
- Wide range in bonding energies, melting temperatures and
mechanical properties
Is the covalent bond directional or non-directional?
- Directional bond
What is the strength and rigidity of diamond attributed to?
The tetrahedral structure of carbon atoms (extends infinitely in space)
How is graphite arranged and what is it made out of?
Graphite is also comprised of carbon atoms, but arranged in
hexagonal sheets that are held weakly by a type of secondary bond.
Why is graphite soft and ductile?
Because the hexagonal sheets are held by weak secondary bonds.
Where are metallic bonds found?
In metals and their alloys
Why do metals have excellent electrical and thermal conductivity?
Because they have freely moving electrons.
How do metallic bonds form?
Metal atoms give up their valence electron to form a “sea of
electrons” or “electron cloud”
What type of bond (non-directional or directional) is a metallic bond?
- Non-directional bond
Describe the relative bonding energies of metallic bonds.
- Range of bond energies
Where does van der Waals bonding exist?
It exists between virtually all atoms or molecules
Why does van der Waals bonding occur?
Due to random fluctuations in polarization.
What is polarization?
Spatial arrangement
of electrons in the atom
What is the weakest type of secondary bonding?
Van der Waals bonding
What is the range of bond energies of van der waals bonding?
Low bond energies of 4-
30 kJ/mol
What are hydrogen bonds?
Special type of secondary bond when Hydrogen is
covalently bonded to Oxygen, Nitrogen or Fluorine
How does hydrogen bonding affect DNA?
Hydrogen bonding
governs interactions
between nitrogenous
bases in DNA
What is the strongest type of secondary bonding?
Hydrogen bonding
Why does hydrogen bonding occur?
Due to asymmetric distribution of charge, as a result of
the difference in electronegativities of atoms and their spatial
arrangement
What are the 5 bonding controls?
- Stiffness
- Melting temperature
- Density
- Thermal and Electrical Conductivity
- Coefficient of Thermal Expansion
What primary bonds are present in metals?
Metallic bonding
What primary bonds are present in polymers?
Covalent bonding
What primary bonds are present in ceramics?
Ionic and covalent bonding
What correlation is consistent within a bond type?
A correlation between bond energy and melting temperature.
What is the thermal expansion coefficient?
The change in size of a material per degree temperature change.
Which type of materials have lower expansion coefficients?
The more strongly bonded, higher-melting point materials.
What is the relationship between bonding energy, the difficulty of breaking a bond, the energy required to break a bond, and melting temperature?
More bond energy = more difficult to break a bond = more energy required to break bond = higher melting temperature
What does the percent ionic character of a bond depend on?
Percent ionic character depends on the atoms’ electronegativity.
For materials of a comparable structure or certain bond type, what is the relationship between melting temperature and the linear expansion coefficient?
Inverse relationship; the linear expansion coefficient decreases with increasing melting temperature.
What does non-directional mean?
The magnitude is equal in all
directions around an ion
Graphite is a soft material, while diamond is very hard. What are the key contributing factors that explain these differences?
- Secondary atomic bonding
- Geometry of primary atomic bonding
Why can geckos cling to any surface?
Due to the Van der Waals forces that can interact via the tiny hairs.
