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Chemical Bonding

• Intramolecular Bonding: refers to the attraction ions or atoms have for one another within a molecule

• Intermolecular Bonding: refers to the attraction between molecules

• Note: Intramolecular is much stronger than Intermolecular

Intramolecular Bonding

Ionic

Polar Covalent

Non Polar Covalent

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Electronegativity

• the ability to attract and/or gain electrons

• some elements are more electronegative than

others

• Do you think metals or non-metals are more

electronegative? (think about full electron orbitals)

• NON-METALS, of course…as they want to gain

electrons to become isoelectronic with a noble gas

whereas metals want to lose electrons

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• Would smaller or larger atoms be more electronegative?

• Smaller, as their positively charged nucleus is not shielded by many electrons and can pull the electrons in question very close due to their small size

• Distance ‘over-rides’ amount of protons…

• Thus…the electronegativity trend in the periodic table goes like this…

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Periodic Trends in

Electronegativity

Electronegativity Increases

Noble gases don’t count as they don’t want electrons!!

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Electronegativity Values

(0.7 to 4.0)

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Here’s another look…

The smaller atom often has fewer occupied energy

levels and consequently has a greater attraction for

electrons – electronegativity increases up a group.

• Conversely, a larger atom with more energy

levels occupied has less attraction for the

bonding electrons.

Non-metals have a greater tendency to gain electronsin order to become isoelectronic with a noble gas,therefore electronegativity increases as you go right.

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Summary

• Elements on the right (nonmetals) like to

gain electrons to become isoelectronic with

a noble gas and therefore have a large

electronegativity

• Elements on the left (metals) like to lose

electrons to become isoelectronic with a

noble gas and therefore have a small

electronegativity (large electropositivity)

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Type of Intra Bonding Depends

on Electronegativity of Atoms

• What happens when Na bonds with Cl to make table salt (NaCl)?

• The bond that forms between Na and Cl depends on the electronegativity difference between the two

• Cl is very electronegative (3.0) as it wants one more electron to become isoelectronic with Argon

• Na is not very electronegative (0.9) as it actually wants to lose an electron to become isoelectronic with Neon

• The difference: 3.0 – 0.9 = 2.1 predicts that an IONICbond results

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•Na + Cl [Na]+ + [ Cl ]-

•This is an example of an ionic bond

•Na gives its valence electron to chlorine and ionic charges result

•The bond is the + / -(electrostatic) attraction

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Electronegativity Difference

Ranges

• If the difference in electronegativity of two

atoms is:

• 0.0 – 0.2 : non polar covalent bond results

• 0.3 – 1.6 : polar covalent bond results

• Greater than/equal to 1.7 : ionic bond

results

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Some Examples

• Take a look at Hydrogen and Chlorine

• The value for H is 2.1

• The value for Cl is 3.0

• The electronegativity difference is .9

• the bond type is polar covalent

• Look at Lithium and

Fluorine

• Li is 1.0

• F is 4.0

4.0 – 1.0 = 3.0

The bond type is ionic

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For you to try

• Lithium and Bromine

• Cesium and Fluorine

• Fluorine and Fluorine

• Sodium and Chlorine

• Potassium Chlorine

• Oxygen and Oxygen

• Silicon and Carbon

• Phosphorus and Bromine

• = 1.8 is Ionic

• = 3.3 Ionic

• = 0 Non Polar

• = 2.1 Ionic

• = 2.2 Ionic

• = 0 Non Polar

• = .7 Polar

• = .7 Polar

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Ionic Bonding

• What happens when potassium comes in

contact with fluorine gas (F2)? Write an

explanation!

• 4.0 – 0.8 = 3.2 therefore IONIC bond

• A fluorine atom will take a valence electron

from a potassium atom

• Results in K+ and F – ions

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• The K+ and F- ions are attracted due to

electrostatic forces, forming a crystal lattice

made up of many K+ and F- ions.

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Crystal Lattice

• the ions are fixed in their position within the crystal lattice and have difficulty moving from these positions due to the electrostatic attractions

• ionic solids are brittle but have very high melting points

• each ionic solid has a unique crystal lattice arrangement

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Ionic Bonding Summary

Complete transfer of electron(s) !!Bond due to electrostatic forces!

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Practice

• Any compound with a polyatomic ion

creates an ionic bond i.e. Na2CO3 has ionic

bonding between the sodiums and the

carbonate

• Predict what happens when magnesium

reacts with chlorine gas

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Melting Points and Electronegativity

• The higher the electronegativity difference

between two atoms, the stronger their bond will

be.

• The stronger the bond between two atoms, the

higher the melting point of the resulting

compound.

• If, however, you are comparing melting points of

two or more substances, more info must be taken

into consideration.

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• Eg. Which has a higher melting point, NaCl or KF?

NaCl = 2.1 KF = 3.2 So, KF has a higher melting pt.

Eg. Which has a higher melting point, BaO or MgS?

BaO = 2.6 MgS = 1.3 So, BaO has a higher melt. pt.

Eg. Which has a higher melting point, BaO or KF?

Even though KF’s electronegativity difference is higher than that of BaO, BaO has a higher melting point. Why?

Because two electrons were transferred so the ionic bond is stronger due to the 2+/2- attraction.

So, when the number of electrons transferred between the compared species is the same, look at the electronegativity difference to determine the higher melting point.

If the number of electrons transferred are unequal, the species with the MOST transferred has a higher melting point.

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Non Polar Covalent Bonds

• Occurs when the electronegativity

difference is between 0.0 and 0.2

• Most common when two identical atoms are

bonded together (e.g. Cl2, O2, N2, etc.)

• Electrons are not transferred as in ionic

bonding, but instead are shared equally

between the two atoms

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Cl2

• How many valence electrons does chlorine

have?

• 7

• How many electrons does chlorine need to

be isoelectronic with argon?

• 8

• So each chlorine wants to gain an electron

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Cl2

• If each of the two chlorines shares an

electron with the other, each will then have

8 (or zero) valence electrons (and be happy)

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• When two electrons are shared between two

atoms, the result is a covalent (shared) bond

• Cl - Cl

This ‘bond’ is two shared electrons,giving each chlorine a full orbital!

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Practice

• How does the bonding work for…

• H2 ?

• F2 ?

• O2 ?

• N2 ?

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Polar Covalent Bonding

• Occurs when electronegativity difference is

between 0.3 and 1.7

• Electrons are shared to create bonds, but the

sharing is unequal as one atom in the bond

is more electronegative than the other

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A tug of war between atoms

• Picture two atoms, each one is represented by a person below. Both pull on the other’s valence electrons. Neither is strong enough to pull the electrons totally away, but one can pull a bit harder and the electrons move toward the harder pull.

e- e-

The electrons are shared, but not equally, the

resulting molecule has a partial negative end

and a partial positive end.

d - d +

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Unequal Sharing

• Let’s look at the bond formed between Hydrogen and

Chlorine

• Their electroneg difference is .9 which puts them in the

polar covalent range.

• Both the H and the Cl want to gain one electron so each

will donate one to share to create a single bond

• Since Cl is more electronegative than H, it will have a

stronger attraction for the electrons and the Cl end of the

molecule will have a somewhat negative charge while the

H end will have a somewhat positive charge.

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Hydrogen Chloride

H Cl

d +d -

The electron from H is pulled toward the Cl.

δ means partial charge

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Water is a polar molecule

• Water is H2O, but let’s look at each H-O

bond separately.

• The electroneg diff for H-O is 1.4, which

makes it polar covalent.

• The Oxygen is most electronegative and the

electrons will be pulled in that direction

• Let’s look at a model like the one for HCl

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The Hydrogen – Oxygen bond

O Hd -

d +The electrons are pulled harder by the oxygen than the hydrogen atom.

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Summary

0.2 1.7