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International University of Sarajevo

Lecture 6:Ionic Vs Covalent bonding

Course lecturer : Jasmin Šutković1th April 2014

Contents International University of Sarajevo

1. Ionic bonding 2. Lewis electron structures 3. Lewis acids and bases4. Covalent bonds 5. Models of covalent bonding 6. Valence Bond Theory 7. Hybridization of s and p orbital's

Overview of Chemical Bonding

Chemical bond — the force that holds atoms together in a chemical compound Covalent bonding — electrons are shared between atoms in a

molecules (nonmetals+nonmetals) Ionic bonding — positively and negatively charged ions are

held together by electrostatic forces (metal + nonmetals) Ions : charged atoms, cations (atoms that loses electrons) and

anions (atoms that gains electrons)

Ionic compounds — dissolve in water to form aqueous solutions that conduct electricity!

Covalent compounds — dissolve to form solutions that do not conduct electricity!

Chemical bonds share 3 features :

1. Interaction between ATOMS creates aggregates, such as compounds and crystals, which lower the total energy of the system (because they are byproducts of the chemicals equations)

2. Energy is required to separate bonded atoms or ions into isolated atoms or ions.

a. In ionic solids– ions form a three-dimensional array called a lattice;–energy is called lattice energy, the enthalpy change that occurs when a

solid ionic compound is transformed into gaseous ions.b. In covalent solids

– energy is called the bond energy, the enthalpy change that occurs when a given bond in a gaseous molecule is broken.

3. Each chemical bond is characterized by a particular optimal internucleardistance called the bond distance.

1.Ionic Bonding

Ionic bonds are formed when positively and negatively charged ions are held together by electrostatic forces (lecture 2).

Energy of the electrostatic attraction (E) is a measure of its strength and is inversely proportional to the distance between the charged particles ( r ) and directly proportional to the magnitude of the charges on the ions.

Ionic Bonding cont …

Energy is released when the ionic bond is formed !

If Q1 and Q2 have opposite signs ( NaCl) then E is negative – energy is released

Q is the charge 1 and 2, r is the distance and k is the proportionality constant , equal to : 2.31x10 -28 J*m

NaCl

Ionic compounds characteristics

1. Usually rigid, brittle, crystalline substances with flat surfaces

2. Not easily deformed

3. Melt at relatively high temperatures

These properties are the results of regular ion arrangement in the crystalline lattice and due to the strong electrostatic attractive forces between ions with opposite charges

Ionic Bonding cont …

Lattice Energies in Ionic Solids

Lattice energy

1. Formation of ion pairs from isolated ions releases large amounts of energy

2. More energy is released when these ion pairs condense to form an ordered three-dimensional array

Calculating Lattice Energy

Lattice energy, U, of an ionic solid can be calculated by the equation

U > 0.

• U, a positive number, represents the amount of energy required to dissociate a mole of an ionic solid into the gaseous ions:

AB (s) → A + (g) + B - (g) ∆H = U

Q1 and Q2 are the charges on the ions;ro is the internuclear distance.

1. Melting point

a. Temperature at which the individual ions have enough kinetic energy to overcome the attractive forces that hold them in place

b. Temperature at which the ions can move freely and substance becomes a liquid

c. Varies with lattice energies for ionic substances that have similar structures

The Relationship between Lattice Energies and Physical Properties

2. Hardness

a. Resistance to scratching or abrasion

b. Directly related to how tightly the ions are held together electrostatically

3. Solubility of ionic substances in water:

a. The higher the lattice energy, the less soluble the compound in water

The Relationship between Lattice Energies and Physical Properties

2. Lewis Electron Structures

Lewis dot symbols

1. Used for predicting the number of bonds formed by most elements in their compounds

2. Dots represent the atoms valenceelectrons

3. A single electron is represented as a single dot

4. Number of dots in the Lewis dot symbol is the same as the number of valence electrons, which is the same as the last digit of the element’s group number in the periodic table.

5. Unpaired dots are used to predict the number of bonds that an element will form in a compound.

The Octet Rule

Atoms tend to lose, gain or share electrons to reach a total of eight valence electrons, called an octet.

The octet rule explains the stoichiometry of most compounds in the s and p blocks of the periodic table.

Number eight corresponds to one ns and three npvalence orbital's, which together can accommodate a total of eight electrons.

Creating a Lewis Dot Symbol

Example Fluorine….has 7 valence electrons

LETS DO AN EXAMPLE WITH N and F ….

How do we proceed …?

Example of covalent compound – covalent bond

It is NF3…why ? First find the valence electrons of each

element… N has 5 and F has 7 …. Let arrange them around the elements …. First lets look at the N…

What about F ?To satisfy the octate rule we need to have 3 x F

Each Fluorine atom has only one gap in its outer shell, so it can bond in only one place. Nitrogen has three gaps in its outer shell so it makes sense to place the Nitrogen in the middle of the molecule and attach the Fluorine atoms to it. Note that the eight electrons in the outer shell can be moved around to suit the build of the molecule, though they must be evenly spread across the four quarters of the atom as outlined in the electron dot structure page.

From Lewis Dots To Lines

Ionic bond example …NaF

Covalent Bonding example

The H2 molecule

Two identical neutral atoms

Contains a purely covalent bond with each hydrogen atom containing one electron and one proton and with the electron attracted to the proton by electrostatic forces

Covalent Bonding in H2

H2 molecule

Lewis electron structures construction1. Arrange the atoms to show specific connections – place the

central atom The central atom is usually the least electronegative element in the

molecule or ion; hydrogen and the halogens are usually terminal.2. Determine the total number of valence electrons in the

molecule or ion.3. Place a bonding pair of electrons between each pair of

adjacent atoms to give a single bond.4. Beginning with the terminal atoms, add enough electrons to

each atom to give each atom an octet (two for hydrogen).5. If any electrons are left over, place them on the central atom.6. If the central atom has fewer electrons than an octet, use lone

pairs from terminal atoms to multiple (double or triple) bonds to the central atom to achieve an octet.

What is electronegativity?

Represented with the symbol χ,

is a chemical property that describes the tendency of an atom or a functional group to attract electrons .

How t determine electro negativity ….

H2O example

Example with CH2O

Book page 534 ..

Formal Charges

It is Possible to write more than one Lewis structure for a substance, and it does not violate the octet rule, but not all of the Lewis structures may be equally reasonable and stable.

To determine the most stable Lewis structure we consider the formal charge on the atoms.

Formal charge represents the difference between the number of valence electrons in the free atom and the number assigned to it in the Lewis electron structure

Facts regarding the formal charge:

a) the sum of the formal charges on the atoms within a molecule or ion must equal the overall charge on the molecule or ion

b) the formal charge is NOT the true charge on an atom; simply used to predict the most likely structure when a compound has more than one Lewis structure

Formal Charges cont ..

Calculation of Formal Charge

Assign electrons in the molecule to individual atoms according to the following rules:

1. Nonbonding electrons are assigned to the atom on which they are located2. Bonding electrons are divided equally between the bonded atoms3. For each atom, the formal charge is computed by the following equation:

formal charge = valence e- - (nonbonding e- + bonding e-)2

free atom Atom in Lewis structure

Formal charge example (page 538)

NH3Picture :

Formula :

formal charge = valence e- - (nonbonding e- + bonding e-)2

formal charge of NH3 = 5 v.e – (2 nonb. e- + 6 bond. e- / 2 ) = 0

Properties of Covalent Bonds

In the Lewis bonding model, the number of electron pairs that hold two atoms together is called the Bond order

Single bonds have a bond order of one Double bonds have a bond order of two Triple bonds have a bond order of three

Bond length decreases as bond order increases !

Relationship between bond length and bond order is not linear!

Bond energy is the energy required to break a particular bondFor covalent bonds, bond energies and bond lengths depend on many factors:

electron affinities, sizes of atoms involved in the bond, differences in their electronegativity overall structure of the molecule.

There is a general trend in that the shorter the bond length, the higher the bond energy!The larger the bond energy, the stronger the bond.

Bond Energy

The Relationship Between Bond Order and Bond Energy

The Relationship Between Bond Order and Bond Energy

Bonds of different order between like atomsa. Triple bonds between like atoms are stronger and shorter than double bonds

b. Double bonds between like atoms are stronger and shorter than single bonds

Page 560

Polar Covalent Bonds

One of the four types of Chemical bonds

1. Ionic

2. Covalent

3. Polar covalent — electrons are shared unequally between the bonded atoms

4. Polar bond — bond between two atoms that possess a partial positive charge (õ+) and a partial negative charge (õ-)

Polar Covalent Bonds

Bond polarity is the extent to which it is polar !

Determined largely by the relative electronegativities of the bonded atoms!Electronegativity (χ) — ability of an atom in a molecule or ion to attract electrons to itself

Bond polarity

Rules of bond polarity

A bond is nonpolar if the bonded atoms have equal electronegativities!

If electronegativities of the bonded atoms are not equal, bond is polarizedtoward the more electronegative atom!

A bond in which the electronegativity of B (χB) is greater than the electronegativity of A (χA) and is indicated with the partial negative charge on the more electronegative atom

õ+ õ-

(less electronegative) A --- B (more electronegative)

To estimate the ionic character of a bond (the magnitude of the charge separation in a polar covalent bond), calculate the difference in electronegativity between the two atoms

Net electronegativity = χB - χA

Electronegativity table

Example for bond polarity

Estimate the polarity of bond in Cl2 and NaCl ? Cl has χ = 3.16 , so according the formula

χB - χA = 3.16-3.16 = 0 and it is NONPOLAR

NaCl , Na χ =0.9 and Cl χ = 3.16 = 3.16-0.90 = 2.26 (The electronegativity difference)

Any difference higher than 2.0 proves that the electronegativity of one of the elements is high enough to remove an electron from the other atom. This proves that the atoms fulfill the octet rule by transferring electrons and creating an IONIC bond.

1. Produced by the asymmetrical charge distribution in a polar substance ( like in HCl)

2. Abbreviated by µ3. Defined as the product of the partial charge Q on the bonded

atoms and the distance r between the partial chargesµ = Q x r

Q measured in coulombs (C)r measured in meters (m)

4. Unit for dipole moment is the debye (D)

1D = 3.3356 x 10-30 C x m

Dipole moments

Reading task ….

Pages : 511- 520, 528-575 (Book chapter 8)

Please read the book according the power point slides (according the titles and subtitles).

HW2 by 8th April

Homework 2 ( submit lately by 7th April 2014) – late homework's will not be accepted !!!1

IONIC BONDS:

1. How are negative ions formed? 2. When a metal atom loses one or more electrons to form a positive ion, energy is ________ (absorbed/released). -" how sure you are in your answer here?" 3.When a nonmetal atom gains one or more electrons to form a negative ion, energy is ________ (absorbed/released). -" how sure you are in your answer here?“

COVALENT BONDS

1. When atoms are bonded together covalently, what two kinds of structures may result? 2. What is a single covalent bond? 3. What is a triple covalent bond?

NOTE: PLEASE ANSWER ONLY IN ONE SENTENCE.

Quiz 1 make up

5 question – 55 minutes maximum 3 essay plus 2 solving problems questions

Midterm exam is 8th April , 15-17h

in the indoor basketball field in the basement floor of the new building