Covalent Bonding - BYU Physics and Astronomy · • Forces between atoms in molecules. –...

• What are non-metal bonds like?– covalent, polar,

hydrogen bonds, dispersion

• How strong are they?

• Can this help explain trends in melting and boiling temperatures and in conductivity?

Covalent Bonding

“To find the truth you have to try and you have to persist in trying. Sometimes it’s fun. Sometimes it’s hard or boring. But it’s always worth it…

“The Creator of the Universe has implanted a message in every created thing. Geology, astronomy, physics – all science is really nothing more than an effort to read those messages.”

Henry Eyring

Today’s class

• Forces between atoms in molecules.– “Intramolecular”

• Forces between molecules in a liquid or solid.– “Intermolecular”

• What are non-metal bonds like?– covalent, polar,

hydrogen bonds, dispersion

• How strong are they?

• Can this help explain trends in melting and boiling temperatures and in conductivity?

Covalent Bonding

Today’s class

• Forces between atoms in molecules.– “Intramolecular”

• Forces between molecules in a liquid or solid.– “Intermolecular”

• Metallic bonding:– Metals and

semiconductors

– A lot of atoms share a lot of electrons

• Ionic bonding– Electrons are transferred

from the metal to the nonmetal

A short review:intramolecular

Which of the fundamental interactions is important for forces between molecules?

A. The weak nuclear interaction

B. The strong nuclear interaction

C. The electromagnetic interaction

D. The gravitational interaction

Which of the following are important for determining the strength of electromagnetic forces?

A. The distance between the charges

B. The magnitude of the charges

C. The mass of the particles

D. Both A & B

Comparing the distances between two bound atoms in a molecule and the distances between molecules, which is greater (on average)?

A. The distance between bound atoms

B. The distance between molecules

C. Neither

Which forces do you expect to be greatest?

A. Bonding forces between atoms within a molecule

B. Attractive forces between different molecules

C. There is no way to know

Covalent bonding

• Electrons are shared by nonmetal atoms

• “Real” molecules

• Many of life’s important compounds are covalently bonded:– H2O, CO2, SO4, NH3,

sugars, fats, carbohydrates, etc.

• Metallic bonding:– Metals and

semiconductors

– A lot of atoms share a lot of electrons

• Ionic bonding– Electrons are transferred

from the metal to the nonmetal

A short review:intramolecular

Covalent bonding

• Electrons are shared by nonmetal atoms

• “Real” molecules

• Many of life’s important compounds are covalently bonded:– H2O, CO2, SO4, NH3,

sugars, fats, carbohydrates, etc.

• Atoms share pairs of electrons

Multiple bonds

Bond Strength

• More shared pairs= more bond energy

• Determines – Molecule stability

– Activation energy

– Energy extracted in reactions

• (demo)

• Atoms share pairs of electrons

Multiple bonds

Bond Strength

• More shared pairs= more bond energy

• Determines – Molecule stability

– Activation energy

– Energy extracted in reactions

• (demo)

• Covalent/ionic bonding depends on electronegativity– how strongly atoms

attract electrons

Intramolecular:inside the molecule

High

Low

Polarity

• Polarity: from unequal electronegativity AND geometry

• Polarity determines forces between molecules

• Covalent/ionic bonding depends on electronegativity– how strongly atoms

attract electrons

Intramolecular:inside the molecule

High

Low

Polarity

• Polarity: from unequal electronegativity AND geometry

• Polarity determines forces between molecules

• NON-POLAR: H2

O2

N2

• Low melting and boiling temperatures

Intermolecular:outside the molecule

Electron density largest between atoms.

Also non-polar

• CO2 combines atoms with unequal electronegativity.

• Non-polar by geometry

• NON-POLAR: H2

O2

N2

• Low melting and boiling temperatures

Intermolecular:outside the molecule

Electron density largest between atoms.

Also non-polar

• CO2 combines atoms with unequal electronegativity.

• Non-polar by geometry

• Water is polar.

• The oxygen side of the molecule is more negative (greater electronegativity)

• The hydrogen side is more positive (smaller electronegativity).

Polar Molecules

The extreme:an ionic bond

• In covalent bonds, electronegativity is nearly the same

• For metal/non-metal bonds, electronegativityis strongly unequal: IONIC BONDS

• Water is polar.• The oxygen side of the

molecule is more negative (greater electronegativity)

• The hydrogen side is more positive (smaller electronegativity).

• Hydrogen bondinga free proton

Polar Molecules

The extreme:an ionic bond

• In covalent bonds, electronegativity is nearly the same

• For metal/non-metal bonds, electronegativityis strongly unequal: IONIC BONDS

• Electron distribution not static

• Formation of temporary dipoles

• +/- combination is favored

• Very weak

• “dispersion” or Van derWaals interactions

Nonpolarmolecules attract

Forces between molecules: how do they compare?

weaker stronger

Forces between molecules in a liquidForce between

atoms in a molecule

Some examples: Nitrogen

• Strong covalent bonds.

• No dipole.

• Only weak dispersion forces for attraction between molecules.

• Good electrical insulator.

• Low melting and boiling temperatures.

Some examples: Water

• Large dipole.

• Hydrogen bonding between molecules

• Higher melting and boiling temperatures.

• Can lose H+ to form an acid

• Excellent solvent

(polarity demo)

Melting and boiling temeratures: how do they compare?

Low melting temperature Higher melting temperature

nitrogenoxygenhelium

water

Some examples: Carbon Dioxide

• Strong covalent bonds.

• No dipole.

• Only weak dispersion forces for attraction between molecules.

• Good electrical insulator.

• Low melting and boiling temperatures.

Some examples: Glucose

• Large dipole.

• Hydrogen bonding between molecules

• “Sticky” likes to form crystals

• Make your own: 2Cup sugar, 1Cup water, string, patience

• Higher melting and boiling temperatures.

• A carbohydrate (“carbon water”)

Some examples: Carbon

• DIAMOND– Each carbon is attached

to 4 other carbons

– Strong bonds between atoms: network

– Poor electrical conductor

• GRAPHITE– Strong bonds to 3 other

carbons in a plane

– Weak bonds between planes

– Excellent lubricant (for your pinewood derby)

Molecular ions

• Think of a happy family of atoms that is missing a few electrons.

• They can’t share any more, so they go steal them.

Summary

• Covalent bonds occur in non-metals, when atoms share electrons and form molecules.

• Usually strong forces between atoms in molecules.

• Usually weak forces between molecules in a liquid or solid.

• Bond characteristics help determine properties of more complex systems (stay tuned…).