Materials 1: Atomic structure and bonding Flashcards
Identify the four main parts of an atom?
Electrons, Protons and Neutrons
Identify the 7 general types of materials
Metals, Ceramics, Polymers, Semi-conductors, Composites, Bio-materials and Nano-scale materials
Define metals:
Metals: valence electrons are detached from atoms, and
spread in an ‘electron sea’ that “glues” the ions together.
Dense, strong, and can be readily formed into practical
shapes ductile, conduct electricity and heat well, are
opaque, shiny if polished, some are magnetic.
Define Polymers:
Polymers: are bound by covalent forces and also by
weak van der Waals forces, and usually based on C and H.
They have large molecular structures often chain like. Have
low densities and electrical conductivities, can be formed in
complex shapes. They decompose at moderate
temperatures (100 – 400 C), and are lightweight and inert.
Examples: plastics, rubber (polyethylene, nylon, Poly vinyl
chloride
Define Ceramics:
Ceramics: atoms behave like either positive or negative
ions, and are bound by Coulomb forces. They are usually
combinations of metals or semiconductors with oxygen,
nitrogen or carbon (oxides, nitrides, and carbides). Hard,
brittle, insulators, resistant to high temperature and harsh
environment . Examples: glass, porcelain.
Define Composites:
Composites: Composites are combinations of two materials in
which one of the materials, called the reinforcing phase, is in the form
of fibers, sheets, or particles, and is embedded in the other materials
called the matrix phase. Composites are created in order to obtain
properties that cannot be achieved by any of the components acting
alone. Example of familiar composite is fiber glass in which small glass
fibers are embedded in a polymeric material epoxy or polyester. The
glass fibers are strong and stiff ( but also brittle) whereas the polymer
is ductile ( but also weak and flexible). Thus the resulting fiber is
relatively stiff strong flexible and ductile. Another technological
important material is the carbon fiber reinforced polymer- carbon fibers
are embedded in a polymer.
Define Advanced materials: Semi-conductor
Semiconductors: the bonding is covalent (electrons are
shared between atoms).
Semiconductors have electrical properties that are
intermediate between electrical conductors and insulators(
ceramics and polymers). Their electrical properties depend
strongly on minute proportions of contaminants. Examples:
Si, Ge, GaAs. Semiconductors have revolutionized the
electronics and computer industries
Define Advanced materials: Bio-materials
Biomaterials : Are employed in components implanted into the human
body for replacement of damaged body parts. These materials must not
produce toxic substances and must be compatible with body tissues, must
not cause adverse biological reactions.All the above materials –metal
ceramics polymers composites and semiconductors may be used as
biomaterial. For example biomaterials are utilized in artificial hip
replacement
Define Ionization potential
Energy required to pull out a valence electron (in vacuum). By convention, IP is positive (i.e you must put energy in to pull an electron out).
Define Electron affinity
Energy gained by putting an electron into an atomic structure. (from vacuum). By convention, EA is negative as it goes from a high energy state within a vacuum to a low energy state within an atom.
Identify the 3 types of Primary bonding
Ionic (transfer of valence electrons)
Covalent (sharing of valence electrons, directional)
Metallic (delocalization of valence electrons)
Identify the 4 major types of Secondary bonding (or Van der Waals Bonding).
Dipole-dipole
H-bonds
Polar molecule-induced dipole
Fluctuating dipole (weakest)
Define Electronegativity and give the values it typically ranges between
Electronegativity is a measure of how willing an atom is to accept more electrons.
With regards to Electronegativity:
Subshells with +1 electrons will have ______
Subshells with -1 electrons will have ________
Subshells with +1 electrons will have - Low Electronegativity
Subshells with -1 electrons will have - High Electronegatity
From a periodic table, what is a simple method to identify whether an element has a smaller or larger electronegatity
Elements further to the left of the periodic table will have lower electronegativity wheres as elements to the right side will have higher electronegativity, with the acception of inert gases.
Since metals prefer to give up their electrons to become positive ions, what is the term we would use to describe these materials.
Electropositive
What is an Electron volt
Electron volt - The energy unit of atomic bonding. it defines the energy lost / gained by an electron when it is taken through a potential difference of one volt.
Offer an explanation as to why covalently bonded materials are generally
less dense than ionically or metallically bonded ones
Covalently bonded materials are less dense than metallic or ionically
bonded ones because covalent bonds are directional in nature whereas
metallic and ionic are not; when bonds are directional, the atoms cannot pack
together in as dense a manner, yielding a lower mass density.
(a) Briefly cite the main differences between ionic, covalent and metallic
bonding
(b) State the Pauli exclusion principle
Answer:
(a) The main differences between the various forms of primary bonding are:
Ionic–there is electrostatic attraction between oppositely charged ions.
Covalent–there is electron sharing between two adjacent atoms
such that each atom assumes a stable electron configuration.
Metallic–the positively charged ion cores are shielded from one another,
and also “glued” together by the sea of valence electrons.
(b) The Pauli exclusion principle states that each electron state can hold
no more than two electrons, which must have opposite spins
Explain what is secondary bonding or van der Waals Bonding
• Both van der Waals and hydrogen bonds are termed secondary, being weak in comparison
to the primary ones. They result from attractive forces between electric dipoles, which may
be induced or permanent.
• For the hydrogen bond, highly polar molecules form when hydrogen covalently bonds to a
non-metallic element such as fluorine.
State the kinds of primary interatomic bonds, and briefly explain each one of
them and provide an example
Learn Diagrams related to these descriptions.
Atomic bonding in solids may be considered in terms of attractive and repulsive
forces and energies. The three types of primary bond in solids are ionic,
covalent, and metallic.
• For ionic bonds, electrically charged ions are formed by the transference
of valence electrons from one atom type to another; forces are coulombic.
There is a sharing of valence electrons between adjacent atoms when
bonding is covalent.
• With metallic bonding, the valence electrons form a “sea of electrons” that is
uniformly dispersed around the metal ion cores and acts as a form of glue for
them.
Which of the following best describes ionic bonding?
(a) The electrons of one atom join with electrons of other atoms to produce a “sea” of electrons around the ions.
(b) The ions of the positive atoms form dipoles which attract the dipoles of the negative atoms.
(c) Electrons from both ions join in the middle and are shared.
(d) One atom gives up electrons to become an anion and another atom accepts the electron to become a cation: these are then electrostatically attracted to each other
Solution D
One atom gives up electrons to become an anion and another atom accepts the electron to become a cation: these are then electrostatically attracted to each other
Which of the following best describes covalent bonding?
(a) One atom shares its outer electron(s) with a neighbouring atom(s) and they both become energetically more stable.
(b) One atom donates its outer electron(s) to a different atom and they are electrostatically attracted.
(c) Hydrogen molecules join the atoms together.
(d) The atoms plastically deform and necking occurs between them making them stick to each other.
(e) One atom forms a random dipole and attracts an atom with an induced dipole.
Solution A
One atom shares its outer electron(s) with a neighbouring atom(s) and they both become energetically more stable.
Briefly cite the main differences between ionic, covalent, and metallic bonding.
SOLUTION:
The main differences between the various forms of primary bonding are:
- Ionic: there is an electrostatic attraction between oppositely charged ions.
- Covalent: there is electron sharing between two adjacent atoms such that each atom assumes a stable electron configuration.
- Metallic: the positively charged ion cores are shielded from one another, and also “glued” together by the sea of valence electrons.
