BELLWORK- VSEPR

State the main assumption of the valence shell electron-pair repulsion

model (VSEPR).

What is a valence shell electron-pair?

Are they involved in bonding or not?

Lecture 8.4a – Polar bonds

Salt crystals are repeating patterns of positive+ cations and negative- anions held together by electrostatic attraction.

IONIC COMPOUNDS

COVALENT COMPOUNDS

Biological molecules are covalently bound

Most consist of the non-metals Carbon, Oxygen, Hydrogen, and Nitrogen.

SO…..

Ionic and covalent bonds are very different, but how do we predict if a bond will be ionic or covalent?

electronegativityThe ELECTRONEGATIVITY of an element helps us understand the difference between ionic and covalent bonding

• Electronegativity is the measure of the ability of an atom in a bond to attract electrons.

• With only a few exceptions, electronegativity values increase as you move from left to right in any period of the periodic table.

• Within any group, electronegativity values decrease as you go down the group.

That means that the most electronegative elements are in the upper-right corner of the table.

Every element has an electronegativity value

Francium has the lowest electronegativity 0.7 Fluorine has the highest 4.0

Elements with a HIGH electronegativity have a STRONG pull on electrons.

Elements with a LOW electronegativity have a WEAK pull on electrons.

difference in electronegativity

When two atoms bond their DIFFERENCE in electronegativity determines the bond type.

A large difference in electronegativity means one atom will win the “tug of war” and take the electrons completely. This is an ionic bond.

Which one has the high electronegativity?

When the electronegativity of two bonding atoms is very similar, neither

atom wins the “tug of war” and the electrons are shared equally.

This produces a covalent bond

In a true covalent bond electrons are shared equally

IONIC COVALENT

Transfer electrons Share electrons

Between an atom of high electronegativity and an atom of low electronegativity

Between two atoms of equal or very close electronegativities

NaCl N2

If the electronegativity difference between two bonded atoms is very high the bond is ionic.

If the electronegativity difference is very low the bond is covalent.

What if the difference in electronegativity between the two bonded atoms is in-between?

A POLAR COVALENT BOND occurs when two atoms share electrons unequally.

The atom with a high electronegativity value holds the bonding electrons more often, but it doesn’t remove the electrons completely.

Are the bonds polar covalent, non-polar covalent, or ionic?

1) H-C 2) K-Cl 3) O-F 4) Cl-Cl

5) C-N 6) S-O 7) B-S

A polar bond has a partial positive charge (+) and a partial negative charge (-)

The N-H bond is polar, with N being the most electronegative.

The N-H bond is polar, with N being the most

electronegative.

-

+

+

+

End of section 8.4a lecture

Bond polarity and 3D shape determine if a molecule is polar

Bond polarity --- When a bond has a partial negative charge on one atom and a partial positive charge on the other atom.

Molecule shape--- the arrangement of atoms in three dimensions (3-D)

A polar molecule has polar bonds and asymmetry

Polar bonds Non-polar molecule

Symmetry- all sides are the same

Polar bonds Polar molecule

Asymmetry- has different sides

δ-

δ-

δ-

δ-

δ-

δ+δ+

negative side

Positive side

If the electrons are not distributed equally, the molecule is said to be

polar.

The molecule has a negative end and

a positive end.

Polar molecules are affected by electric fields

It has two poles and is polar; it has a measurable dipole moment.

POLAR MOLECULES INTERACT!!

A partial positive charge (+) is attracted to

negative ions

and

negative partial charges (-) of other polar bonds.

A partial negative charge (-) is attracted to

positive ions

and

partial positive charges (+) of other polar bonds.

POLAR MOLECULES INTERACT!!

+

Water is a molecule that consists of two polar covalent O-H bonds.

The electrons are not distributed evenly so the water molecule is polar. The negative end of the

molecule is the oxygen end. O is more electronegative

than H and pulls the negative electrons toward itself. Also, there are two

lone pairs around oxygen.

negative end

positive end

Na+(aq)

A dissolved sodium ion

Practice- Draw the 3D structures for these molecules and label the bond polarity and the

molecule polarity.

1. H2O

2. CH4

3. CH3F

4. CH3CH3

5. CH3CH2OH

6. NH3

7. CO2

8. CH2CH2

9. HCN

10.

11.