Chemical Bonding sec. 1

Chemical Bonding Unit 2

Section 1:

● Octet Rule

● Ionic Bonding

● Covalent Bonding

The Octet rule

Elements tend to gain, lose, or share electrons

to acquire a full octet

● In

n

=1 a full octet consists of 2 electrons.

○ There is only one 1s orbital.

From

n

=2 to

n

=7, a full octet consists of 8 electrons.

A maximum of two electrons can be held in a single

orbital. n=1 only has one orbital so a full octet in n=1

consists of only 2 electrons.

There are some exceptions to the octet rule such as:

Expanded and Reduced Octets

➔ Where the elements from n=2 to n=6 acquires

fewer or more than 8 valence electron.

The Formation of Ions

Metals lose electrons to become positively charged cations

Metals lose electrons to form positive ions

(called cations) and non-metals gain

Electron Configurations of Ions

Na

Incomplete Octet

1s² 2s² 2p⁶ 3s¹

= [Ne] 3s¹

Na⁺

Complete Octet

Lost the electron from n=3 to acquire a full octet

1s² 2s² 2p⁶

Electron Configurations of Ions

CI

Incomplete Octet

1s² 2s² 2p⁶ 3s² 3p⁵

= [Ne] 3s² 3p⁵

CI⁻

Complete Octet

Gained an electron in n=3 to acquire a full octet

1s² 2s² 2p⁶ 3s² 3p⁶

The charges on ions are usually related to the group they’re in.

Group 1A +1 (H can also be -1) Group 2A +2

d-block +1 to +4 (it’s hard to predict) Group 3A +3 (TI can also be +1)

Group 4A +4 (Sn and Pb can also be +2)

Group 5A -3 non-metals, +3 and +5 metals Group 6A -2

You can predict the charge of most ions from group

they belong to in the periodic table.

Group 1A Elements

form ions with a 1+ charge.

Hydrogen can act as an anion with a 1- charge as well

(e.g. MgH₂).

Group 2A Elements

form ions with a 2+ charge.

Group 3A Metals

form ions with a 3+ charge.

TI is in group 3A, but it produces ions with a 3+ or

a 1+ charge.

Group 4A Metals produce ions with a 4+ charge.

Sn and Pb also produce ions with 2+ charges, as they may only lose the 2 electrons

from their outer p-subshells.

All Group 5A Non-Metals produce ions with a 3- charge

All Group 5A Metals form ions with 3+ and 5+ charges.

Group 6A Non-Metals produce ions with a 2- charge.

Polonium (Group 6A) produces ions with 4+ or 2+ charges.

D-block cations

These elements lose electrons from their highest

s-sublevel first before losing from their d-s-sublevel

● Ex) Iron may form 2 types of ions.

○ Fe [Ar] 4s² 3d⁶

○ Fe²⁺ [Ar] 3d⁶

Lost electrons from 4s

Here we can see that these ions do not always follow

the octet rule.

Ionic Compounds are created when a metallic

cations forms an electrostatic bond with a

The ions may also be polyatomic.

Ionic Bonding Defined

1) Metals transfer electrons to non-metals, and the

two form bonds due to to electrostatic attractions

between them.

Or

2) Cations (metal ions) and anions (non-metal ions)

form electrostatic bonds based on

opposite charges.

You can form solid NaCl crystals by mixing solid sodium metal with chlorine gas.

-This is an exothermic reaction where electrons are transferred from Na(s) to the Cl₂(g).

-They then stick together due to their opposite electromagnetic charges.

You can also form solid NaCl by distilling (boiling off the water from) salt water.

Here the ions have already lost or gained electrons. As the water evaporates the ions become more attracted to one another and they stick together.

As we will learn about later in this course, individual Na⁺ and Cl⁻ ions are more attracted to polar water molecules than they are to each other.

Ionic Bonding

Na● + Cl Na⁺ + [

Cl ]⁻

An electron is transferred from the Na to the Cl.

This produces a positive and a

negative ion - both with full octets.

The oppositely charged ions will then stick together due to electrostatic attraction.

Properties of Ionic Solids

High Lattice Energy

High melting point.

Cleaves along planes

Brittle 3D structure

Ions line up in a repetitive pattern

Ions

Most are soluble in polar solvents

Conduct electricity when molten or dissolved in a

polar solvent

The higher the concentration of ions in a

solution, the higher the electrical

A 1.0

M

solution of Na

CO

is a better conductor of

electricity than a 1.0

M

solution of NaCl.

Covalent Bonds

One atom shares electrons with

another atom so that both acquire full

octets.

In covalent bonding, electrons are not transferred

from one atom to another.

Electrons are shared

The electron density from the valence electrons of

both atoms increases between the two nuclei.

Electronegativity and Bond Polarity

0 .5 1.9 _3.5

Covalent Bond

Polar Covalent Bond

Ionic Bond

What is electronegativity? An element's ability to attract electrons in a chemical bond.

The greater the electronegativity value associated with a particular atom in a molecule, the more strongly it attracts electrons to itself.

Electrons spend more time around the atom with the higher electronegativity.

Electronegativity difference is obtained by subtracting the electronegativity values of two atoms that are bonded together.

If this difference is zero the bond is completely covalent.

Most chemists will still consider a bond to be covalent up to an electronegativity difference of 0.5

Polar covalent means that the electron spends more time around the more electronegative atom.

It is polar as the bond has a net positive and a net negative end ( like the poles of a magnet).

Not all chemists agree on the upper value were a bond goes from being polar covalent to ionic. It generally ranges from 1.7 to 2.0

A bond is considered to be completely ionic at 3.5

Electrons are not considered to be shared in ionic bonds

Some ionic compounds of Al3+_{, such as Al}

2S3+ have polar-covalent

bonds.

The balloons represent the electron density distribution.

F_2, is perfectly covalent. The electrons are evenly distributed around the two fluorine atoms.

The electronegativity difference here is zero.

In the HF molecule, the blue region represents low electron density, and the green region represents high electron density.

The electrons spend most of their time around the more electronegative fluorine.

The electronegativity difference is 2.1 - 4.0 = 1.9 (polar covalent, almost ionic)

In NaF the electron that Na donated to F spends all of its time around the fluorine atom (or it was perfectly ionic)

Determining Bond Type

Electronegativity difference = 0.0 Completely Covalent

H-H

Determining Bond Type

Electronegativity difference = 0.7 Polar Covalent

H - Br

2.1

Determining Bond Type

Electronegativity difference = 1.0 Polar Covalent

O - C - O

Determining Bond Type

Electronegativity difference = 3.0 Ionic

LiF

Molecular Compounds Defined

Molecules:

Two or more non-metals bonded together to form a compound.

Metallic Bonding

Bonding in metals in neither ionic or covalent.

- There are not enough electrons in bonding metal atoms to fill octets.

Bonding between metals occurs from delocalized valence electrons throughout the structure. This is called the Sea of Electrons.

Bond strength increases as the number of bonding electrons increases.

For example:

- Na_(s) has 1 valence e-; its melting point = 97.5 ℃

Metallic Bonding

Nuclei are stationary, while the electrons are free to move throughout the solid. Sea of Electrons.

This creates

unique properties in metallic solids:

- Conduct Electricity

- Conduct Heat

- Malleable

Sea of Electrons

The Free movement of electrons allows metals to conduct electricity and heat very well.

If you heat one end of a metal bar, it will transfer the heat to the electrons in that area. These newly energized electrons will move to the other end of the bar carrying the heat with them.