Chapter 9  Chemical Bonding

Bondng of Atoms

How can electronegativity be used to predict the character of a bond?

What similarities and differences exist between ionic, covalent, and polar covalent bonds?

What is the electron sea model, and how does it describe metallic bonding?

A model of bonding

• Why do atoms form bonds?

• What kinds of bonds exist?

• What physical properties do compounds of similar bond character share?

Degrees of sharing

• There is no clear-cut division between ionic and covalent compounds

• A more realistic view is to consider that all chemical bonds involve some level of sharing 

  • Equal sharing

  • Not equal sharing

  • Almost no sharing at all

• Properties of compounds are related to how equally the electrons are shared

Bond Character

Electronegativity:  A measure of the ability of an atom in a bond to attract electrons

Electronegativity and Bond Character

• Think of a bond between atoms as a tug-of-war over the shared valence electrons

• How each atom fares in the tug-of-war is determined by the difference in electronegativity

Electronegativity Values

Periodicity: the tendency to recur at regular intervals

• Modern Periodic Law: states that physical and chemical properties of elements repeat in a regular pattern when they are arranged in order of increasing atomic number 

Periodic Trends

Trends in Electronegativity

• Values _increase across a period and up a group on the Periodic Table

  • Noble gases?

• As you move down a group, the size of the atom _increases

  • Orbitals layer as the number of electrons increases

• Valence electrons are further  from the nucleus as the size increases

  • Less attraction  between the nucleus and the valence electrons

Shielding Effect- Electrons in the inner energy levels BLOCK  the attraction between the nucleus and the valence electrons

Trends in Electronegativity

• Nuclear charge is related to the number of ___________ in the nucleus

  • The more protons, the __________ the charge

• Increasing nuclear charge without increasing the _____________ of energy levels results in a greater pull on the electrons

• As you move across a period, the number of protons _____________ (though # of energy levels stays the same)

  • Electronegativity values increase across because the nucleus has __________ control over its valence electrons

Bond Character and Electronegativity Differences

• The greater the difference in electronegativity between two atoms, the more unequal the sharing of electrons will be

•            ∆EN    = difference in electronegativity

  • Big- small 

• Ex. Fluorine and cesium

  • 4.0 – 0.7 = 3.3 = ΔEN

Ionic character

• If the ΔEN is > than 2.0 , the sharing of electrons is so unequal that you can assume there was a complete _transfer

  • The more electronegative atom will gain the electron and become an anion

  • The less electronegative atom will lose the electron and become a cation

• The ions will be held together by the attraction of opposite charges (electrostatic attraction)

  • Ionic bond

Covalent Character: nonpolar

• If ΔEN is < 2.0

• Equal sharing (nonpolar covalent)

  • ΔEN= 0 

• If there is a very small ΔEN (< 0.5), the sharing may not be equal but it isn’t significant enough to have an effect on the properties of the molecule

• Nonpolar covalent = ΔEN= 0-0.5

Covalent Character: Polar

• Unequal sharing (polar covalent)

  • ΔEN is between 0.5 and 2.0

• Partial transfer of electrons results in a partial charge on each atom

  • More electronegative atom will be _partially negative (δ-)

  • Less electronegative atom will be _partially positive  (δ+)

• Some degree of ionic character in the bond

  • Reflected in the physical properties

<0.5 is NONPOLAR COVALENT

0.5-2.00 POLAR COVALENT

>2.0 IONIC 

Classify

• Using a table of electronegativity, classify the bond between the following pairs of atoms:

  • Na-S

  • N-H

  • P-Cl

  • N-N

  • Al-O

Bonding in Metals

Properties of metallic bonds

• Metals bond, but not to form a compound

• A metallic bond  is the interaction that holds metal atoms together (pure substance or alloy)

• Properties are a result of the interaction between electrons

  • malleable

  • ductile

  • Electrically conductive 

Sea of electrons

• Valence electrons are held loosely by the positive nuclei of the atoms

• In a metallic bond, electrons are released by their atoms into a sea of electrons shared by all the metal atoms

  • Bonds that result from this shared pool of electrons is called a metallic bond

  • Electrons are delocalized

Metallic properties and bonding

• Malleability and ductility both involve the movement of atoms

• Atoms are bonded in a network of atoms instead of to a single atom

  • Atoms can slide past one another  easily through the sea of electrons to their new position 

• Conductivity involves the movement of electric charges 

• Valence electrons are not attached to any one metal atom

  • Electrons can move through the metal when an external force is applied

Chapter 9 Notes part 2

Molecular Shape and Polarity

How are Lewis dot diagrams for molecules constructed?

How can a Lewis dot diagram be used to formulate the three-dimensional geometry of a molecule?

How can a molecules polarity be determined?

Keeping Track of Electrons

The electrons responsible for the chemical properties of atoms are those in the outer energy level.

Valence electrons - The s and p electrons are at the outer energy level.

Core electrons -those in the energy levels below. 

Atoms in the same column

• Have the same outer electron configuration.

• Have the same valence electrons.

• Easily found by looking up the group number on the periodic table.

Group 2A - Be, Mg, Ca, etc.

 2 valence electrons

The valence electrons are the electrons that participate in chemical bonding.

Electron Dot diagrams (Lewis)

A way of keeping track of valence electrons.

How to write them:

• Write the symbol.

• Put one dot for each valence electron

Don’t pair up until they have to (*hybridization)

Nitrogen has 5 valence electrons.

First we write the symbol.

Then add 1 electron at a time to each side*

Until they are forced to pair up*.

Covalent bonds

Nonmetals hold onto their valence electrons.

They can’t give away electrons to bond.

Still want _noble gas configuration.

Get it by sharing valence electrons with each other.

By sharing, both atoms get to count the electrons toward noble gas configuration.

Fluorine has seven  valence electrons

A second atom also has seven

By sharing electrons, both end with full orbitals

Single Covalent Bond

A sharing of two valence electrons.

Only nonmetals and Hydrogen.

Different from an ionic bond because they actually form molecules

Two specific atoms are joined.

How to show how they formed

• It’s like a jigsaw puzzle.

• You will be given the final formula

• You put the pieces together to end up with the correct bonding.

For example- show how water is formed with covalent bonds.

Water

* all elements are stable with 8 expect for HYDROGEN AND HELIUM. They are stable with 2

Multiple Bonds

Sometimes atoms share more than one  pair of valence electrons.

A double bond - when two pair (4) of electrons are shared between two atoms

A triple bond - when three pair (6) of electrons are shared between two atoms

Carbon dioxide

CO₂ - Carbon is central atom 

Examples

NH₃ 

HCN  C is the central atom

Another way of indicating bonds

Often use a line  to indicate a bond

Called a structural formula

Each line represents 2 valence electrons

Structural Examples

Lengths of Covalent Bonds

VSEPR

Valence Shell Electron Pair Repulsion 

• Predicts three-dimensional geometry of molecules using Lewis Dot diagrams

• The name describes the theory.

• Valence shell – refers to the outer electrons. (dots)

• Electron Pair Repulsion - electron pairs try to get as far away as possible from each other while still bonding to the central atom. (360 is the amount of space we have)

• Can predict the predict the angles of bonds.

• Based on the number of pairs of valence electrons both bonded (shared) and unbonded, around the central atom.____

  • Central atom = any atom with TWO OR MORE bonds 

  • Unbonded pair are called lone pairs. (just two dots, unbonded) 

  • Bonded pairs are called SHARED Pair (the line) 

CH₄- draw the structural formula

Single bonds fill all atoms.

There are four pairs of electrons pushing away (from central atom).

The furthest they can get away is 109.5*

• 4 atoms bonded

• Basic shape is TETRAHEDRAL SHAPES HAVE A BONDING DEGREE OF 109.5

• A pyramid with a triangular base.

• Same shape for everything with 4 pairs.

BASED ON NUMBER OF CENTRAL ATOMS MARK ONE SHAPE

3 bonded - 1 lone pair (NH3)

• Still basic tetrahedral but you can’t see the electron pair.

• Shape is called trigonal pyramidal

• 107* is the ANGEL FOR ALL TRIGONAL PYRAMIDAL 

2_ bonded -  2 lone pair

• Still basic tetrahedral but you can’t see the 2 lone pair.

• Shape is called BENT

• FOR BENT IT IS 104.5*

3 atoms bonded,  no lone pair

• The farthest you can get the electron pair apart is 120*

• Shape is flat and called trigonal planar

2 atoms bonded, no lone pair

• With three atoms the farthest they can get apart is 180*

• Shape called linear

•  Molecules with only 2 atoms are ALWAYS linear 

Bond Angles

• Nonbonding (lone) pairs exhibit greater repulsive forces on adjacent electron pairs

• Results in slight as the number of angles between bonding pairs 

• Bond angle decreases  slightly as the number of nonbonding (lone) pairs increases

• Multiple bonds also affect bond angles

• Greater repulsive force than single bonds, so slight compression of angles between single bonding pairs

Molecular Shapes