Molecular Structure and

Properties of Compounds

Chemistry 30Unit 2

Covalent Bonding

• Is the ____________________• It occurs between a:

• nonmetal and nonmetal • semimetal and nonmetal• semimetal and semimetal

• We already know how to name covalent compounds(By adding prefixes and ending the last element in

“ide”.)

How do Covalent Bonds Form?• Atoms share electrons with other atoms in order to

complete their shell.• __________________: Atoms can have a maximum of

8 valence electrons in their outer shell.

• _________ – The attraction btwn atoms (2 shared electrons)

• _______________- a pair of electrons that are left on their own around a central atom.

• ___________________– the one that has the most atoms attached to it (usually the one with the lowest electronegativity; exception is hydrogen, it is never the central atom).

How to Draw Lewis Dot Structures with Molecules• Count the total number of valence electrons for the molecule• Choose a central atom (usually the one with the lowest

electronegativity – will always be carbon if it’s there)• Place electrons around atoms so that the octet rule is satisfied• If the molecule is an ______________, place brackets around the

entire structure and write the charge on the outside of the bracket.

Example: CH4

H C HHH

H H

H

C

HC

H

H H

H

A bond is indicated by a dash/line

Example: OH-

O H

First, count up the total number of valence electrons of all the atoms.

H

Oxygen has 6 valence electrons

Hydrogen has 1 valence electron

That equals 7 valence electrons. However, we must take a look at the charge. In this case, it is -1, which means that there is one extra electrons, giving us a grand total of 8 valence electrons.

ONow put brackets around the molecule and add the

charge on the outside.

Double and Triple Bonds

• Double bonds occur when there are 4 shared electrons in one spot. Ex: CO2

• Triple bonds occur when there are 6 shared electrons in one spot. Ex: CO

Ionic vs. Covalent Bonding• Ionic: Transfer of

electron(s) from one atom to another. Example is Sodium Chloride, NaCl

ClNa+ -

H C HHH

Covalent: The atoms share electrons. Example is Methane, CH4

H HHCH

CH

H HH

ClNa

MOLECULAR GEOMETRYMOLECULAR GEOMETRY

VSEPR Theory

• ____________________

____________________• Most important factor in determining geometry is relative repulsion between electron pairs.

A Molecule adopts the shape that minimizes

the electron pair repulsions.

A Molecule adopts the shape that minimizes

the electron pair repulsions.

VSEPR charts

• Use the Lewis structure to determine the geometry of the molecule

• Electron arrangement establishes the bond angles• Geometry of the molecule can depend on either the

regions of electrons (Electron Pair Geometry) or on the number of atoms (Molecular Geometry).

• Charts look at the CENTRAL atom for all data!• Think REGIONS OF ELECTRON DENSITY rather than bonds

(for instance, a double bond would only be 1 region)

Electron Pair Geometry

• Count up the total number of regions of bonds and # of lone pairs around the central atom

• (If double bond or triple bond, it counts as 1)

# of Regions of Electrons Electron Pair Geometry Bond Angle

2 Linear 180

3 Trigonal Planar 120

4 Tetrahedral 109.5, 107, 104.5

Electron Pair Geometry Examples• BeH2 has two regions of electrons, therefore it is

linear.

• CO32- has three regions of electrons, therefore it is trigonal planar.

• H2O has four regions of electrons, therefore it is tetrahedral.

Molecular Geometry

• Depends on Electron Pair Geometry as well as the number of atoms around the central atom

• Count up the number of atoms that are connected to the central atom

Electron Pair Geometry Bond Angle # of Atoms around

CentralMolecular Geometry

Linear 180 2 Linear

Trigonal Planar 120 2 Bent

120 3 Trigonal Planar

Tetrahedral 104.5 2 Bent

107 3 Trigonal Pyramidal

109.5 4 Tetrahedral

Molecular Geometry ExamplesCO2 has two atoms around C, therefore it is linear.

NO2- has two atoms around N, therefore it is bent.

H2O has two atoms around O, therefore it is bent.

Bond Angles

• The angle between atoms• Depends on Electron Pair Geometry and Molecular Geometry

Linear Electron Pair Geometry

180° Bond Angle

______________Electron Pair Geometry

120°

Tetrahedral Electron Pair Geometry

___________ ___________________ _________

Molecular Geometries

Exceptions to the Octet Rule

• Sometimes there are exceptions, and an atom doesn’t need to satisfy the octet rule (there aren’t enough electrons)….

• Sometimes an atom exceeds the octet rule…

But don’t worry about these!

Bonding between molecules or atoms in solids or liquids

• Recall that molecules are farthest apart in gases, but closest together in solids.

Physical and chemical properties depend on the type of bonds involved

• Ionic compounds typically have ___________boiling points and melting points

than molecular compounds, due to the strength of the ionic attraction.

• Recall that sodium chloride is a solid at room temperature, while carbon

dioxide is a gas.

• vs

Properties of Ionic Compounds

• ___________ melting/boiling point• Dissolve in water• Form crystals when solid• Conduct electrical current

Why do Ionic Compounds have High Melting Points?• Recall that ionic compounds form from oppositely charged ions.

• This creates strong bonds!

• Thus a lot of energy is needed to separate the atoms.

Properties of Covalent (Molecular) Compounds

• Due to weak intermolecular forces, are generally liquids and gases.• Conduct little to no electricity• Generally have low melting points and boiling points

Properties of Molecular Compounds Vary

• Covalent bonds differ in terms of how the bonded atoms share the electrons. The

number and type of atoms joined together determine the molecular properties.

• The electrons which make up the

covalent bond are being pulled, like

a tug-of-war, toward each nucleus.

Nonpolar Covalent Bonds

• Recall that a magnet has a north and south pole. When the atoms in the

bond pull equally, the bonding electrons are shared equally and there are no

‘north or south poles’ formed in the bond. We call this bond

_________________.

• Polarity increases _________________________and therefore increases

boiling and melting points.

• The diatomic elements are nonpolar covalently bonded (e.g. hydrogen) and

thus are gases at room temperature (except bromine).

Polar Covalent Bonds• Formed when the electrons are shared unequally between atoms

• Is a result of ________________________

• Electronegativity: the ability of an atom to attract electrons when the atom is in a compound (aka how hard it pulls in the tug-of-war)

• The more electronegative atom attracts electrons more strongly and gains a slightly negative charge. The less electronegative atom has a slightly positive charge

Describing Polar Bonds• In hydrochloric acid (HCl), hydrogen has an electronegativity of 2.1 and

chlorine has 3.0. These values are significantly different, so the covalent bond is polar.

• Chlorine pulls the electrons closer towards itself and becomes slightly negative, leaving hydrogen slightly positive as shown:

Describing Polar Bonds

• Water is also a polar molecule (elecronegativities H: 2.1, O: 3.5)

• This explains why most ionic compounds are soluble (can dissolve) in water:

Determining Bond Type• Using the electronegativity chart, we can determine which bond type will

occur:

Attractions Between Molecules• How do the strengths of intermolecular attractions compare with ionic and

covelent bonds?

• Intermolecular (attraction between molecules) are _____________ than either ionic or covalent bonds. However, these interactions still impact physical properties.

• They include:• Van der Waals forces:

• Dipole interactions• Dispersion forces

• Hydrogen bonding

van der Waals Forces• The two weakest interactions between molecules• Named after Dutch chemist Johannes van der Waals • Includes:

• _________________ forces:• Weakest of all forces; occurs in all molecules• Caused by the motion of electrons• Very weak, very temporary attraction between slightly charged regions of a molecule and its

neighbours• _________________ interaction:

• Attraction between the slightly charged regions of polar molecules:

Hydrogen Bonding

• Attractive forces in which a __________________ atom covalently bonded to a very

electronegative atom is also weakly bonded to an unshared electron pair of another

electronegative atom

• In other words, it is a dipole interaction that involves hydrogen and an

electronegative atom (N, O, F, Cl)

• This is a relatively strong attraction which serves to increase the melting and boiling

point of the substances affected by it

Hydrogen Bonding is Responsible for:• Surface Tension• Ice floating• Helical structure of DNA

Intermolecular Attractions and Molecular Properties

• Recall that the physical properties of a compound depend on the type of bonding it displays – in particular, whether it is ionic or covalent.

• A great range of physical properties occurs among covalent compounds.

• The diversity of physical properties among covalent compounds is mainly because of widely varying intermolecular attractions.

• A few solids that consist of molecules break our rules – they will not melt unless at extremely high temperatures or will not melt at all

Network Solids• Aka network crystals• Solids in which all of the atoms are covalently bonded to each other• Melting a network solid would require breaking covalent bonds

throughout the solid• E.g. diamond

Physical Properties

• The greater the strength ___________ the bonds (INTRAMOLECULAR

FORCES) and _________ the molecules (INTERMOLECULAR FORCES) of

a substance, the more energy you need to break those bonds (i.e. to

change state by melting or vaporization)

That explains why you see such a variety in physical properties:

Lesson Check!

• Intramolecular Forces Worksheet

Melting and Boiling Points

• Dependent on intermolecular forces

• The stronger the intermolecular force is, the more energy is required to melt or boil a solid or liquid

• Therefore, intermolecular forces raise the melting and boiling points

Properties of Covalent (Network) Compounds• Network = connected in many ways to molecules around it• Have high melting and boiling points• Cannot conduct electricity

Properties of Metal Compounds

• High melting points• Very good electrical conductors• Crystal arrangement• Malleable (ability to

keep shape withoutbreaking)

• Dense• Shiny