Lewis Symbols

To help us to focus on the valence electrons – those that can participate in bonding - we use Lewis Symbols (in honor of scientist G.N. Lewis)

Lewis Dot Symbols Lewis Dot symbol (or Electron dot symbol)

Dots placed around an element’s symbol represent valence electrons

Pair electrons as needed Ions are placed in brackets with charge outside Easily shows “Octet rule”

Tendency of an atom to achieve an electron configuration having 8 valence electrons Same as the electron configuration of a noble

gas The 8 electrons shown as 4 pairs

Group Practice:

Write out dot diagrams showing the valence electrons of the following atoms. Use principles of electron configuration to predict and explain the ionic compounds each pair will react to form: Magnesium fluoride Aluminum oxide Nickel(II) chloride

7-3

Molecular Compounds

Write the orbital diagram for a hydrogen atom.

We have seen evidence from gaseous reactions that elemental hydrogen exists as a diatomic molecule. Use your orbital diagram to explain why hydrogen atoms would be more stable (lower chemical energy) as H2 molecules.

7-4

Electron Dot Diagrams for theDiatomic Elements

How does hydrogen obtain a noble-gas electron configuration?

Covalent Compounds and Bonding

When two nonmetals form a compound, the bond between atoms is covalent.

Both atoms are close to the noble-gas electron configuration, so sharing electrons will allow both to obtain it.

In a covalent bond, each shared electron is attracted simultaneously to two nuclei.

Covalent Bonds

Electrons not transferred in this case

Electrons typically shared in pairs

Carbon Dioxide Example The atoms of CO2 molecules are held together by

strong covalent bonds. No bonds connect the molecules, so CO2 molecules

separate easily from each other into the gas state at room temperature.

Figure 8.13 Figure from p. 28

Whiteboard Practice Use valence electron concepts and

electron dot diagrams to represent these compounds:HFCF4

O2

CO27-9

The Octet Rule

Just as in ionic bonding, covalent bonds are formed so that each atom can have the noble-gas electron configuration. Noble gases have 8 valence electrons, an octet.

The Halogens

Do the atoms in each of these molecules have an octet?

Why do the halogens exist as diatomic molecules?

Multiple Bonds How many valence electrons does an

oxygen atom have? How many does it need to obtain an

octet? O2 has a double bond, two pairs of

shared electrons How many valence electrons does a

nitrogen atom have? How many does it need to obtain an

octet? N2 has a triple bond, three pairs of

shared electrons

The Octet An unreactive or stable compound usually

has the maximum number of valence electrons per core (8) Same as the electron configuration of a

noble gas Covalently bonded atoms achieve 8

valence electrons by sharing electrons The 8 electrons exist in 4 pairs

H atoms bond with other atoms to obtain a total of 2 electrons like He (duet).

Practice: Valence Electrons and Number of Bonds

How many bonds do each of the following atoms tend to form?

a) Hb) Clc) Od) Ne) C

Carbon Compounds

Carbon has: Four valence electrons The ability to form four bonds The ability to bond to itself Very strong bonds when bonded to itself

Carbon molecules are ubiquitous in nature. Aside from what I’ve mentioned here, skip “Bonding in

Carbon Compounds”, pp. 307-310 (top).

Figure 8.21

Hydrocarbons Aromatic hydrocarbons

A class of hydrocarbons which has carbon atoms arranged in a six-atom ring with alternating single and double bonds

Delocalized structures

Figure 8.22

Figure 8.22

Functional Groups in Hydrocarbons

Ionic and Covalent In ionic compounds, ions are held

together by electrostatic forces – forces between oppositely charged ions.

In molecular compounds, atoms are held together by covalent bonds in which electrons are shared.

Figure 8.2

Activity: Identifying Types of Bonding

Identify the type of bonding in each of the following substances:

1. NaF2. ClO2

3. FeSO4

4. SO2

5. Ca(ClO2)2

Steps for Writing Lewis Structures

1. Write an atomic skeleton.2. Sum the valence electrons from each atom to get the total

number of valence electrons.3. Place two electrons, a single bond, between each pair of

bonded atoms (can also be drawn as a line)4. Place remaining valence electrons to complete the octet of

each outer atom. These are called non-bonding electrons or lone pairs. If there are “extra” electrons, place the pairs around the central atom.

5. If necessary to satisfy the octet rule, shift unshared electrons from non-bonded positions on atoms with completed octets to positions between atoms to make double or triple bonds.

Activity: Lewis Structures

Draw Lewis structures to show how electrons are shared in these molecules.

1. C2H6

2. C2H4

3. C2H2

4. HCN5. CO2

6. NH3