Gilbert Newton Lewis

(surrounded by pairs of

electrons)

Lewis diagrams Lewis diagrams

• Lewis suggested a means of keeping track of outer (or valence) electrons

Why and how do atoms combine ?Lewis diagrams When atoms combine, only electrons in the outer (valence) shell are involved. We can represent these valence electrons with Lewis diagrams. Lewis diagrams for the first 20 elements are as follows:

IA IIA IIIA IVA VA VIA VIIA 0

H He

Li Be B C N O F Ne

Na Mg A Si P S Cl Ar

K Ca

To write Lewis diagrams:1. Write the element symbol. Around this

draw dots – one for each valence electron.

2. The dots should be spread over four sides. Dots are not paired until all sides have at least one dot.

3. It does not matter on which side the dots are placed. For example, hydrogen can be drawn four ways:

H = H = H = H

4. The number of valence electrons is equal to the group number. For example, hydrogen is in group IA (group 1) and it has one valence electron. Neon is in 0 (group 8) and it has 8 valence electrons. The only exception is He which is in group 8 but has 2 valence electrons.

• Q1 – Write Lewis diagrams for a) Ne, b) Sb (Z=51), c) Rb, d) a neutral atom with 9 total electrons

• Q2 – Write all possible variations of the Lewis diagram for phosphorus.

Lewis diagrams (Q1, Q2)Lewis diagrams (Q1, Q2)

Ne Sb Rb F

P P P P

Q1

Q2

The Octet Rule • The noble gasses do not react with other elements. In other words, noble gasses are very stable.

Let’s look at the electron configuration of noble gasses (note: K is the name given to shell 1, L to 2, etc.)

• Element Shell

K L M N O P • He 2 • Ne 2 8 • Ar 2 8 8 • Kr 2 8 18 8 • Xe 2 8 18 18 8 • Rn 2 8 18 32 18 8

• Most Noble gasses have 8 electrons in their outer shell as shown in the above stable conformation

The Octet Rule • When atoms form ions or combine in

compounds they obtain electron configurations of the nearest noble gas (usually this means that there will be 8 outer electrons).

The Octet rule and ion formation • As stated, when atoms form ions they seek an electron

configuration like that of the nearest noble gas. • Nearest refers to the number of representative elements

between an element and a noble gas. • For example Br is 1 space removed from Kr, but 7

spaces removed from Ar (see your periodic table). So, Br would gain 1 electron to form Br–. Similarly, Mg would lose 2 electrons to become Mg2+.

• Q3–Complete the chart:

• Group IVA elements are equal distances from two noble gasses. These elements tend not to form ions

Br P Ne Al Ca

Nearest Noble gas (spaces removed)

Kr (1)

Resulting ion Br–

Ions and the octet rule (Q3)Ions and the octet rule (Q3)

• Read the remainder of the handout. Do Q3.

Ions and the octet rule (Q3)Ions and the octet rule (Q3)Answers

Br P Ne Al CaNoble gas(spaces)

Kr(1)

Ion Br–

Ar (3)

P 3–

Ne (0)

none

Ne (3)

Al 3+

Ar (2)

Ca 2+

Overview: Types of Bonds (Q4)Overview: Types of Bonds (Q4)

• There are 2 major types of bonds : ionic and covalent

• In ionic bonding one atom has a stronger attraction for electrons than the other, and “steals” an electron from a second atom

• In covalent bonding the attraction for electrons is similar for two atoms. They share their electrons to obtain an octet.

Cl– Na+ Cl–

Ionic bondingIonic bondingIonic bonding involves 3 steps • 1) loss of an electron(s) by one element • 2) gain of electron(s) by a second element• 3) attraction between positive and negative

Na Cl

e–1) 2)

3)

Na+

Ionic bonding (Q5 - Li + Cl)Ionic bonding (Q5 - Li + Cl)• Ionic bonding (stealing/transfer of electrons)

can be represented in three different ways1. Li + Cl [Li]+[Cl]–

3p+

4n02e-1e- 17p+

18n07e- 8e- 2e-

1e-

3p+

4n02e- 17p+

18n08e-8e-2e

Li Cl [ Cl ]–[Li]+

2.

3.

Ionic bonding: Mg + OIonic bonding: Mg + OMg + O [Mg]2+[O]2–

12p+

12n02e- 8e- 2e-

1e-

[ O ]2–[Mg]2+

6e- 2e-

8n0 8p+

1e-

8e- 2e-

8n0 8p+ 12p+

12n02e- 8e-

OMg

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1.

2.

3.

Ionic bonding: Li + OIonic bonding: Li + O

1e-

[ O ]2–[Li]2+

6e- 2e-

8n0 8p+

1e-

3p+

4n02e-1e-

3p+

4n02e-1e-

8e- 2e-

8n0 8p+

3p+

4n02e-

3p+

4n02e-

2Li + O [Li]2+[O]2– or 4Li + O2 2[Li]2

+[ O]2–

OLi

Li

2.

1.

3.

1e-

17p+

18n07e- 8e- 2e-

13p+

14n0 2e- 8e-3e-

17p+

18n07e- 8e- 2e-

17p+

18n07e- 8e- 2e-

Ionic bonding: Al + ClIonic bonding: Al + Cl

1e-

1e-17p+

18n08e- 8e- 2e-

17p+

18n08e- 8e- 2e-

17p+

18n08e- 8e- 2e-13p+

14n0 2e- 8e-

Ionic bonding: Al + ClIonic bonding: Al + Cl

[ Cl ]3–[Al]3+

Al + 3Cl [Al]3+[Cl]3–

Cl

Al Cl

Cl

Covalent bondingCovalent bonding

• Thus far we have looked at when atoms bond due to the transfer of electrons

• An ionic bond forms when an atom has a greater attraction for e–s than a second atom

• However, if two atoms have approximately the same pull on electrons, they share the electrons (forming a “covalent” bond)

• Read “Covalent bonding” and “Covalent bonding continued”. Do Q7.

Covalent bondingCovalent bondingQ7

CCl4 - Covalent

C

Cl

Cl

Cl

Cl

HCl - Covalent

H Cl

MgF2 - Ionic

[ F ]2– [Mg]2+

H2O - Covalent

H O H

NH3 - Covalent

H N H

H

NaCl - Ionic

[ Cl ]– [Na] +

OH– - Covalent

O H

H2 - Covalent

H H

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Multiple Multiple bondsbonds

Do Q. 8. Build models of molecules, O3 as challenge (can’t build it or ionic)

HCl - Covalent

H Cl H Cl

CO2 - Covalent

C OO

Na2O - Ionic

[ O ]2– [Na]2+

H N H

H

H N H

H

OO

OO

O2 - Covalent

OO C

II

II

I2 - Covalent

[ O ]32– [Al]2

3+

Al2O3 - Ionic

NH3 - Covalent

OO O

O OO

O3 - Covalent

H C H

H

H

H C H

H

H

Assignment:Question 4 Answers

• Read “types of bonds” on handout. Do Q4.

• MgO (ionic), CaCl2 (ionic),

SO2 (covalent), PbCl2 (ionic),

CCl4 (covalent),CH4 (covalent)