Bonding

Chemical Bond

• Mutual electrical attraction between the nuclei and valence electrons of different atoms that binds the atoms together.

Valence Electrons

• The electrons on the outside edge of the atom • This is where the action is- where bonding takes place• Atoms have no more than 8 valence electrons

Neon

Argon

Radon

1s22s22p6

1s22s22p6 3s23p6

[Xe]6s24f 145d106p6

Energy Level (Shell)

Maximum Number of Electrons

Max number of Valence Electrons

1 2 2

2 8 8

3 18 8

4 32 8

5 50 8

6 72 8

7 98 8

                                                                  

Electron-dot diagrams can be used to give the number of valence electrons

• The number of valence electrons is equal to the element’ group number or # -10

Group 1 Group 2 Group 13 Group 14 Group 15 Group 17

• Write the electron-dot symbols for the following elements:

iodine

phosphorus

gallium

argon

The Octet Rule:

• Atoms will combine to form compounds in order to reach eight electrons in their outer energy level.

• Atoms with less than 4 electrons tend to lose electrons.

• Atoms with more than 4 electrons tend to gain electrons.

• Some atoms share electrons

Types of Chemical Bonds

• Ionic bond - a transfer of one or more electrons from one atom to another– Forms attractions due to the opposite charges

of the atoms

• Covalent bond - attractive force due to the sharing of electrons between atoms

• Some bonds have characteristics of both types and not easily identified as one or the other

Why do compounds form?• Atoms are trying to get 8 valence

electrons

How do compounds form?• By ionic or covalent bonding

How can you tell if a compound is ionic or covalent?

• By the types of elements in the compound

• Ionic compounds form between metals and nonmetals

• Covalent compounds form between 2 nonmetals

Indicate whether a bond between the following would be 1) Ionic 2) covalent

____ A. sodium and oxygen

____ B. nitrogen and oxygen

____ C. phosphorus and chlorine

____ D. calcium and sulfur

____ E. chlorine and bromine

Note Question 4:

Ions

• Atoms with extra electrons or missing electrons–Extra electrons give an ion a

negative charge–Missing electrons give an ion a

positive charge

If they have to choose, atoms would rather be stable than neutral.

How Does This Happen?Some atoms have a few too many electrons

Some atoms only need a few electrons

What do you do if you are a sodium

(Na) atom?

Go look for an atom that wants the extra electron!

Now you are a sodium ion

• With 10 electrons• Just like neon, with 8

valence electrons• But you’re not neon• You’re not a plain sodium

atom• But you’re happy!

• You are now quite attractive... Electrically.

Ionic Bonding

• Attraction that holds oppositely charged ions together.

• Formed by the transferring of electrons

Ions from Metal Ions

In ionic compounds, metals in 1, 2, 13 and 3-12

lose electrons to nonmetals

Metals lose electrons to achieve the octet

arrangement in the next lowest energy level

The names of metal ions are the same as the

elements

Metal ionic charge: +1, +2, +3, or +4

Ions from Nonmetals

In ionic compounds, nonmetals in 15, 16,

and 17 gain electrons from metals

Nonmetal add electrons to achieve the octet

arrangement

Nonmetal ionic charge: -3, -2, or -1

The names of nonmetal ions end in -ide

Formation of Sodium Ion

Sodium atom Sodium ion

Na Na + + e

2-8-1 2-8 ( = Ne)

11 p+ 11 p+ 11 e- 10 e-

0 1+

Formation of Magnesium Ion Magnesium atom Magnesium

ion

Mg Mg2+ + 2e 2-8-2 2-8 (=Ne)

12 p+ 12 p+

12 e- 10 e-

0 2+

Fluoride Ion

unpaired electron octet 1 -

F + e F

2-7 2-8 (= Ne)

9 p+ 9 p+

9 e- 10 e- 0 1 -

ionic charge

Charges on Ions in Group 1-17

+1 +2 +3 -3 -2 -1

Ions

A. Number of valence electrons in aluminum1) 1 e- 2) 2 e- 3) 3 e-

B. Change in electrons for octet1) lose 3e- 2) gain 3 e- 3) gain 5 e-

C. Ionic charge of aluminum 1) 3- 2) 5- 3) 3+

Learning Check B 3

Give the ionic charge for each of the following:A. 12 p+ and 10 e-

1) 0 2) 2+ 3) 2-

B. 50p+ and 46 e-

1) 2+ 2) 4+ 3) 4-

C. 15 p+ and 18e-

2) 3+ 2) 3- 3) 5-

What If No One Will Give Up An Electron?

• Atoms with less than 8 valence electrons can move close to each other and share their electrons

• The electrons spend their time around both atoms.

• And they lived happily ever after!

Ionic Bonding• Ionic bond – attraction between a cation and an

anion

• Attraction of Na+ with Cl- forms NaCl ion pairs that aggregate into a crystal

Features of Ionic Bonding

• Ion formation takes place by electron transfer

• The ions are held together by the electrostatic force of the opposite charges

• Reactions between metals and nonmetals

(representative elements) tend to be ionic

Covalent Bonding

• Let’s look at the formation of H2:

H + H H2

• Each hydrogen has one electron in its valance shell

• Both hydrogen atoms have an equal tendency to gain or lose electrons

• Electron transfer from one H to another usually will not occur under normal conditions (No one will let go!)

• Instead, each atom attains a noble gas configuration by sharing electrons

• Covalent bonds form between atoms with similar tendencies to gain or lose electrons

• Compounds containing covalent bonds are called covalent compounds or molecules

• The diatomic elements have pure covalent bonds (totally equal sharing)

• The Polar Covalent Bond– Ionic bonding involves the transfer of

electrons– Covalent bonding involves the sharing of

electrons

• Polar covalent bonding - bonds made up of unequally shared electron pairs

Polar Covalent Bonding

• The electrons spend more time with fluorine

• This sets up a polar covalent bond• A pure covalent bond can only occur

when both atoms are identical

Electronegativity

• Electronegativity - a measure of the ability of an atom to attract electrons in a chemical bond

• Elements with high electronegativity have a greater ability to attract electrons than do elements with low electronegativity

• The difference in electronegativity determines the extent of bond polarity

Electronegativity and Bond Polarity

• The most electronegative elements are found in the upper right corner of the periodic table

• The least electronegative elements are found in the lower left corner of the periodic table

Know the trend!

• Which is more electronegative• Boron or gallium?• Calcium or zinc?

Table of Electronegativities(on your worksheet)

Electronegativity and Bond Polarity

• The greater the difference in electronegativity between two atoms, the greater the polarity of their bond

• Which would be more polar, a H-F bond or H-Cl bond?

• H-F … 4.0 - 2.1 = 1.9• H-Cl … 3.0 - 2.1 = 0.9

• The HF bond is more polar than the HCl bond

Look at your handout

• There is a chart in the lower right hand corner of the back

• This gives the break-off points for deciding what type of bond you have between two atoms

• If the difference is 0: Pure covalent• If the difference is ≥.4: Nonpolar covalent• If the difference is .5-1.6: Polar covalent• If the difference is >1.7: Ionic

• Using the table of electronegativites, classify the following bonds as ionic, polar covalent or nonpolar covalent.

• BCl3

• CS2

• There is no sharp distinction between bonding types.

• The positive end (or pole) in a polar bond is represented δ+ and the negative pole δ-.

Dipole Moments

• Consider HF:• – The difference in electronegativity

leads to a polar bond.• – There is more electron density on

F than on H.• – Since there are two different

“ends” of the molecule, we• call HF a dipole.

• We can show this in two ways:

• Some covalent molecules will have polar bonds, but the molecules themselves will be nonpolar

• The polarity of the bonds cancel each other out

Water

• Water is a very polar molecule• This accounts for some of water’s

unusual properties.

• Water dissolves ionic and polar substances

• Water molecules hold on tightly to each other

• This accounts for properties like surface tension

• Water has a high heat capacity • Water is a liquid at room

temperature, even though other compounds in its size range are gases