SCH3U Chemical Bonding

Ionic vs. Covalent Compounds

Ms. Manning

Formation of BondsChemical Bonds are formed between two atoms using shared valence electrons; this is the force that holds atoms together in compounds.

Valence Electrons are the electrons that occupies the outermost energy level of an atom

Type of Bonds

Ionic vs. Covalent Ionic bonding occurs between metals and non-metals Covalent bonding occurs between non-metalsNote: Bonding between metals is called Metallic bonding

Video Clip: Chemical BondingBonding Animation

Octet RuleMost Noble gases have 8 electrons in their outer shell. This is a stable conformation. Thus, the noble gases do not react with other elements. In other words, noble gases are very stable.

Octet Rule (Continued)

When atoms form ions or combine in compounds they obtain electron configurations of the nearest noble gas (eight electrons in their valence shells)

Ionic BondingIonic Bonds are formed from the electrostatic attraction of positive and negative ions

An atom that can lose an electron to become a positively charged ion, called a cation

An atom can gain electrons to become a negatively charged ion, called an anion

•In each case, the atom attains a noble gas configuration with its valence electrons. •A noble gas configuration is defined as a completely filled outer shell, like the noble gases.

Ionic Bond: Example (Na and Cl)

Ionic Bond: Example (Al and O)

Al

Al

O

O

O

Covalent Bonding

Covalent bonding occurs between two non-metals. Covalent bonding is different from ionic bonding because electrons are shared instead of transferred.

Each atom has eight shared electrons

Covalent Bonding: Example (H and F)

H F HF

Electronegativity and Polarity

Nonpolar Covalent Bond

When electrons are shared between 2 atoms, a covalent bond is formed.

If the atoms are identical, e.g. Cl2, the electrons are shared equally (nonpolar)

Polar Covalent Bond If the electrons are shared between 2 different atoms, e.g. HBr, the sharing is unequal

The bonding electrons spend more time near the more electronegative atom

H Br

(Continued) This is not a complete transfer of an electron from hydrogen to

fluorine; it is merely a drifting of electrons toward fluorine

When a charge separation of this type is present, the molecule possesses an electric dipole, so called “dipole moment” and the bond is called a POLAR COVALENT BOND

H :F Cl

Cl

:

Polar Covalent Covalent

(Continued)

Example: Water (H2O)∆EN = 1.24 (which is between 0.5 and 1.7)

Polar Covalent BondOxygen = a slightly negative chargeHydrogen = a slightly positive chargeSince the hydrogen does not completely transfer its

electron to the oxygen, the their respective charges are indicated as + (the indication of partial positive charge) and - (indication of partial Negative charge).

O

H H

+ +

-

Electronegativity Differences The absolute value of the difference in electronegativities of two bonded atoms provides a measure of polarity of a bond.

The greater the difference, the more polar the bond.

Electronegativity Difference

0 to 0.4

Nonpolar covalent

0.41 to 1.69

Polar covalent

> 1.7

Ionic

∆EN3.3

Comparison of Ionic, Non-polar covalent and Polar covalent bonding

1.7 0.5 0

[Na]+[Cl]- H+ - Cl- Cl - Cl

Summary

Chemical Bonds are formed between the atoms in molecules

The bonds are formed from the valence electrons of the atoms and the resulting bond allows each atom to achieve a noble gas configuration (the most stable arrangement of electrons around the atom: Octet Rule)

The types of bonding can be classified as:l Covalent (non-metal bondin; 0<∆EN<0.5) l Polar covalent bonds (non-metal bonding with a dipole

moment; 0.5<∆EN<1.7)l Ionic (metals + non-metals; 1.7 <∆EN< 3.3)

Questions Complete the chart:

Identify each compounds as ionic or covalent: CCl4,HCI, MgF2,

H2O,NH3, NaCl, OH, H2

Show how the bond forms between Li+Cl, Mg +O (Follow the Octet Rule) as well as Li + O

Polar Molecules

Note: Not all molecules with polar bonds are polar molecules

Polarity

Polarity of Molecules

To predict the polarity of the molecule:

i. Determine if there are polar bonds using EN value

ii. If there are polar bonds, establish the direction of the dipoles.

(A dipole exists when two opposite charges are separated by a short distance)

( + -)

Polarity of Molecules

iii. Once you identify the shape of the molecule if the dipoles cancel due to the shape of the

molecule then the compound is non-polar if the dipoles do not cancel due to the shape of

the molecule then the compound is polar

Polarity of Molecules-Examples

Chemical formula

Lewis diagram

Stick diagram

Shape Polarity of molecule(polar/non-polar)

CH4

methane CO2

H2O

Shapes of Molecules

To illustrate the shape of the molecule using a Lewis diagram:

i. Determine the “central atom” , i.e. the atom requiring the most electrons.

ii. Determine how many electrons each atom needs to gain in order to become isoelectronic with its closest noble gas.

iii. Determine if multiple bonds are needed.iv. Determine the structural shape of the molecule based

on # atoms bonded to central atom # lone pairs of electrons (see handout of molecular shapes)

Shapes of Molecules-Examples

Chemical formula

Lewis diagram

Stick diagram

Shape

CH4

methane CO2

H2O