Atomic structure and bonding

1. Engineering
Materials I
By Tsegaye Che.
MSc. In Materials Science and Engineering
1/27/2024
Werabe University
Department of Mechanical Engineering

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CHAPTER
2 Atomic structure and bonding
Werabe University
Department of Mechanical Engineering
Lizards rapidly run up vertical walls and along the undersides of
horizontal surfaces. How?
The secret is the presence of an extremely large number of microscopically
small hairs on each of their toe pads.
When these hairs come in contact with a surface, weak forces of attraction
(i.e., van der Waals forces) are established between hair molecules and
molecules on the surface.
Using their knowledge of this mechanism of adhesion, scientists
have developed several ultra-strong synthetic adhesives.

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CHAPTER
2 Atomic structure and bonding
Werabe University
Department of Mechanical Engineering
WHY STUDY Atomic Structure and Bonding?
 For example, consider carbon, which may exist as both graphite and
diamond.
 Whereas graphite is relatively soft and has a “greasy” feel to it,
diamond is the hardest known material.
 In addition, the electrical properties of diamond and graphite are
dissimilar: diamond is a poor conductor of electricity, but graphite is
a reasonably good conductor.
 These disparities in properties are directly attributable to a type of
interatomic bonding found in graphite that does not exist in
diamond
Type of bond allows us to explain a
material’s properties.

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1.1. Atomic structure
 Atom is the basic and smallest structural unite of an
element
 Are the basis for everything in the Universe and Any
matter is composed of atoms.
 Subatomic particles of atoms are the electrons, protons,
and neutrons.
 There are over 100 elements in the periodic table.
 Each element different in the number of electrons,
protons, and neutrons.
 The protons and neutrons are in the center of the atom
( the nucleus)
 The electrons are found whizzing around the center
(orbital)

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Cont…
 Each piece has either a "+", "-", or a "0."
 The electron always has a "-" or negative charge.
 The proton always has a "+" or positive charge.
 The neutron has a neural charge( charge of zero
 If the charge of an entire atom is "0", there are equal numbers of positive and
negative pieces (electrons and protons).
 The electrons specially the outer ones, determine most of the electrical,
mechanical, chemical and thermal properties

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Cont…
 The atomic mass (A) of a specific atom may be expressed as the
sum of the masses of protons and neutrons within the nucleus.
 Although the number of protons is the same for all atoms of a
given element, the number of neutrons (N) may be variable.
 Thus atoms of some elements have two or more different atomic
masses, which are called isotopes.
 The relative atomic mass of an element is the mass in grams of
6.023𝑥〖10〗^23atoms ( Avogadro's number) of the element.
 Atomic number
 The atomic number (Z) of an atom indicates the number of
protons (positive charged particles ) that are in its nucleus.
 For an electrically neutral or complete atom, the atomic number
also equals the number of electrons.
 In neutral atom of the atomic number is also equal to the
number of electrons.
 Atomic mass

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Cont…
 Atomic weight
 The atomic weight of an element corresponds to the weighted
average of the atomic masses of the atom’s naturally occurring
isotopes.
 Atomic mass unit (amu)

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Cont…
 Mole
 For example, the atomic weight of iron is 55.85 amu/atom, or
55.85 g/mol.
 Sometimes use of amu per atom or molecule is convenient; on
other occasions grams (or kilograms) per mole is preferred.
 The atomic weight of an element or the molecular weight of a
compound may be specified on the basis of amu per atom (molecule)
or mass per mole of material.
 In one mole of a substance there are 6.023𝑥〖10〗^23( Avogadro's
number) atoms or molecules. These two atomic weight schemes are
related through the following equation:

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Cont…
 Most of the elements really come under the metal classification.
 These are sometimes termed electropositive elements,
 They are capable of giving their few valence electrons to become
positively charged ions.
 The elements situated on the right-hand side of the table are
electronegative; they accept electrons to form negatively charged ions,
or sometimes they share electrons with other atoms.
 Electro negativity increases from left to right and from bottom to top.

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Cont…

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1.3. Atomic Bonds in solid
 The physical properties of materials is predicated on the
interatomic forces that bind the atoms together.
 The principles of atomic bonding are illustrated by the interaction
between two isolated atoms
 The interactions are negligible; but as the atoms approach, each
exerts forces on the other.
 These forces are attractive and repulsive, and the magnitude of each
is a function of the separation or interatomic distance.
 An attractive force FA depends on the type of bonding that exists
between the two atoms. Its magnitude varies with the distance.
 Ultimately, the outer electron shells of the two atoms begin to
overlap, and a strong repulsive force FR comes into play.

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Figure 2.8 (a) The dependence of
repulsive, attractive, and net
forces on interatomic separation
for two isolated atoms.
(b) The dependence of repulsive,
attractive, and net potential
energies on interatomic
separation for two isolated atoms.
 The net force FN is just the sum
of both attractive and repulsive
components; that is,
Cont…

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 Chemical bonds occur when there is a net decrease in the
potential energy. Atoms are more stable energy condition
 Chemical bonds divided into primary and secondary
bonds
Bonding energy
I. Primary Bonds(strong bonds): are the strongest bonds
which hold atoms together.
 The three types are: Covalent Bonds, Ionic Bonds, and
Metallic Bonds

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 The charged atoms or ions will be
attracted to each other, and form bonds.
 The compound NaCl, or table salt, is the
most common example.
 To have a filled outer shell of electrons
by transferring electrons from one atom
to another.
 The donor atom will take a positive
charge, and
 The acceptor will have a negative charge.
A. Ionic Bonds

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 When sodium and chlorine atoms are placed together, there is a
transfer of electrons from the sodium to the chlorine atoms,
resulting in a strong electrostatic attraction between the
positive sodium ions and the negative chlorine ions.
 This explains the strong attraction between paired ions typical of
the gas or liquid state. Ionic bond is non-directional bond
because of ions attracted by cations in any direction.
Cont…
Figure 4b. Na+ and Cl- ions
formed by ionic mechanism.
Figure 4a. Formation of ionic
bond in NaCl.

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B. Covalent Bonds:
 Sharing of electrons to complete their outer shells.
 Each pair of shared atoms is called a covalent bond.
 Are called directional because the atoms tend to remain in fixed
positions with respect to each other.
 Are very strong. Examples, Many nonmetallic elemental molecules (H2,
Cl2, F2, etc.) as well as molecules containing dissimilar atoms, such as
CH4, H2O, HNO3, and HF, elemental solids such as diamond (carbon),
silicon, and germanium are covalently bonded.

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C. Metallic Bonds
 The metal atoms lose their outer electrons to form metal cations.
 The electrons form a "sea" of electrons that can flow around the
metal cations.
 These electrons are often described as delocalized electrons -
delocalized means "not fixed" or "free to move".
Figure 1. Metallic bond.

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 As the metals cations and the electrons are oppositely charged
,they will be attracted to each other, and also to other metal
cations. These electrotatic forces are called metallic bonds, these
hold the particles together in metals
Cont…
 A result of the sharing of electrons the cations arrange themselves
in a regular pattern.
 This pattern of atoms is the crystalline structure of metals. In the
crystal lattice, atoms are packed closely together to maximize the
strength of the bonds.
 The metallic bond yields three physical characteristics typical of
solid metals:

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Cont…
 Metals are good conductors of electricity.
 Metals are good conductors of heat.
 Metals have a lustrous appearance.
 In addition, most metals are malleable, ductile, and
generally denser than other elemental substance.
The metallic bond yields three physical characteristics typical
of solid metals:

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 Are much weaker than primary bonds.
 They provide a "weak link" for deformation or fracture.
 Examples for secondary bonds are: Hydrogen Bonds Van der
Waals Bonds
II. Secondary Bonds:
Hydrogen Bonds
 Are common in covalently bonded molecules which contain
hydrogen, such as water (H2O).
 The electrons are shared between the hydrogen and oxygen atoms.
 However, the electrons tend to spend more time around the oxygen
atom.
 This leads to a small positive charge around the hydrogen atoms,
and a negative charge around the oxygen atom.
 Therefor the negatively charged end of one molecule will be weakly
attracted to the positively charged end of the other molecule. The
attraction is weak because the charge transfer is small.

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Bonding Energies and Melting Temperatures for Various
Substances
Cont…

22. End
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