BONDING

Electronegativity• Atomic property : how tightly an atom holds onto it electrons• An atom with higher electronegativity has stronger pull on electrons

than an atom with lower electronegativity

Electronegativity• If two atoms come together to form a bond, the atom with higher

electronegativity will attract the electrons away from the atom with lower electronegativity

• The degree to which the electrons are attracted to the more electronegativity atoms depends on the difference in electronegativities.

Electronegativity• Periodic table (the Pauling electronegativity of the elements)

How electronegativity affects the two atoms interact?

• Periodic table (the Pauling electronegativity of the elements)

Primary Bonds• Material are not made of individual atoms, they are made of groups of

atoms.• Groups of all kind of atom (a piece of iron)• Groups of different kinds of atoms (polyethylene, made of carbon and

hydrogen)

• The bonds that form between the atoms have significant influence on the material’s properties.

• Eg. Melting point, stiffness, thermal expansion : directly related to the strength of bonds

• relationship between structure & properties : understadnig types of bonds is very important

Primary Bonds• One way to define chemical compounds is that they are groups of

atoms held together by primary bonds. • Three basic types of primary bonds: each of them formed by different

ways of sharing the electrons to satisfy the octet rule (atoms are most stable when they have filled valence shell, which for many atoms means 8 electrons, exception is hydrogen)

• Three types : ionic, covalent and metallic bonds.

Primary Bonds

Metallic bonding

Primary Bonds

Metallic bonding

Primary Bonds• Ionic bond: Bond form when electron is transferred from one atom to

another to satisfy the octet rule for each of them, resulting in positive and negative ions.

• These ions are then attracted to each other through electrostatic interactions

• Ionic bonds are the strongest bonds, and melting temperatures of ionic compounds are the highest of any materials

Primary Bonds• Covalent bond: Bond formed when two atoms have share electron

equally. • These atoms are then bound together because the octet rule is

satisfied only while share electrons are considered to be in the valence shell of both atoms.

• Covalent bonds are considered strong, but generally weaker than ionic bonds

Primary Bonds• Metallic bond: Bond formed when a group of atoms contribute their

valence electrons to form “sea of electrons” around the atoms.• The octet rule is satisfied on average for all the atoms.• Metallic bonds are considered to be the weakest of the primary

bonds, although some metals can form bonds that are stronger than covalent bonds.

Primary Bonds• Use you knowledge of electronegativity to predict the types of bonds

that will form between atoms.• Use below table to help you.

Primary Bonds• What kind of primary bonds in the compounds CaF2 and GaN?

• “How big of a difference in electronegativity is needed for a bond to be ionic?”• In reality, electron are not shared exactly equally between two atoms because

one will have a higher electronegativity than the other (except diatomic molecules with the same atoms, eg. N2)

• This means always some ionic character to a covalent bond. • The % ionic character of a bond: • %ionic character = 100 X (1-exp(-0.25(ENA – ENB)2)) , where ENA and ENB :

electronegativity of the two atoms ; limitation: only compares the ionic and covalent characters of the bonds.

Primary Bonds• Another way to look at the type of bonding is through the bond-type

triangle. • Considers both the electronegativity difference between the two

atoms and the average value of the electronegativities of the two atoms to determine the bonding type.

Primary Bonds

Bond-type triangle, showing the regions for metallic (M), semimetallic (SM), covalent (C), and ionic (I) bonding as a function of the average electronegativity and electronegativity difference for the two atoms that make up the bond.

How to use this triangle?Bond character

Nonbonding Interaction

• Nonbonding interactions = “secondary bonds”• Secondary bonds not actually bond, confusing. term

“nonbonding interactions”• Unlike primary bond, nonbonding interactions do not result from

sharing of electrons.• They occurs because of attraction between partial charges that are

present in the molecules , and much weaker than primary bonds

Nonbonding Interaction

• Types of nonbonding interactions and strength of the different types of nonbonding interactions.

Nonbonding Interaction

• Hydrogen bond: Occurs between oxygen, nitrogen, or fluorine on one molecule, and hydrogen atoms on another molecule that are bound to oxygen, nitrogen, or fluorine.

• This is the strongest nonbonding interaction.• Remember , commonly called “bond”, it is not a primary bond; it is a

nonbonding interaction.

Schematic diagram of hydrogen bonds formed in water.Partial positive charge

Partial negative charge

Nonbonding Interaction

• Permanent dipole: Occurs when a very electronegative atoms forms a covalent bond with a less electronegative atom, resulting in partial positive and negative charges in the molecule.

• The partial charges on the this molecule can then interact with the partial charges on another molecule, forming a permanent dipole interaction.

•

Schematic diagram of permanent dipole interactions formed in hydrogen chloride.

Nonbonding Interaction

• Induced dipole: This type of interaction is formed by random fluctuations in the electron distribution in atoms.

• A random fluctuation results in partial positive and negative regions on an atom.

• The charges that are randomly crated on this atom can attract or repel electrons on a nearby atom, resulting in a weak attraction between the atom.

• Also called Van der Walls bonds.

Schematic diagram of induced dipole interactions.

Nonbonding Interaction

• The difference between primary bonds and nonbonding interaction?• Bonds vs. Nonbonding Interactions.• What kinds of primary and nonbonding interactions are present in water?

Nonbonding Interaction

• How primary bonds and nonbonding interactions affect properties?• How their structure affect their boiling points? • Bonding and Properties.

Summary• Fundamental atomic properties of electronegativity has an important

influence on materials properties.• The way in which atoms interact, as defined by electronegativity, affects both

primary bonds and nonbonding interactions, which in turn affect the properties.

• Here we discuss about only boiling point as a properties.• Next we will discuss how bonding can also be used to explain engineering

properties such as stiffness and thermal expansion.• Connection among electronegativity, bonding, and properties that we

discussed, illustrates an important aspect of materials science and engineering properties.

• Understanding this connection allows us to predict and control the properties of materials.

Crystalline defects• Q1: List and describe in your own words three types of point defect in

crystals.• Q2 : List and describe in your own words three types of line defect in

crystals.• Q3: List and describe in your own words three types of planar defects in

crystals.

• Individual Assignment ( Material Science Group 1 : Submit on 29 Sept during class, Material Science Group 2: Submit on 30 Sept during class)