Molecular CompoundsSection 8.1

Objectives When you complete this

presentation, you will be able to … distinguish between the melting

points and boiling points of molecular compounds and ionic compounds.

describe the information provided by a molecular formula.

Introduction Molecular compounds are like the combinations we

can make from a Tinker-Toy set: There are a certain number of hub pieces that are

different colors. There are a certain number of connecting pieces.

Likewise, we have limits on the number and kinds of … atoms that we can use to make molecular compounds bonds that we can use to join the atoms together

Molecules and Compounds

Matter in nature takes on many forms. Noble gases exist as individual atoms.

They are monoatomic. They have very low melting points and boiling points.

We just learned about ionic compounds and metallic compounds. They exist as ions in a crystalline structure. They have high melting points and boiling points.

We will now learn about a different kind of matter that is much more varied in its properties.

This kind of matter is called a molecular compound.

Molecules and Compounds

Molecular compounds are bound together by sharing electrons between atoms. The bond formed is called a covalent bond.

The neutral compounds formed with covalent bonds are called molecules. Molecules formed from two atoms are called diatomic

molecules. HCl H2

O2

Molecules and Compounds

All of the molecules of a given molecular compound are all the same. All of the molecules in water are H2O.

All of the molecules in carbon dioxide are CO2.

Ionic compounds are not the same as molecular compounds. For example, there is no such thing as a CaCl2 molecule.

In general, molecular compounds tend to have relatively lower melting points and boiling points that ionic compounds. This mean that covalent bonds are weaker than ionic bonds.

Molecular Formulas A molecular formula is the chemical formula of a

molecular compound.

The molecular formula tells us exactly how many of each type of atom a molecule contains. Each molecule of water, H2O, contains two hydrogen

atoms and one oxygen atom. Each molecule of glucose, C6H12O6, contains six carbon

atoms, twelve hydrogen atoms, and 6 oxygen atoms.

A molecular formula has the total number of atoms in a molecule – it is not a lowest whole number ratio.

Molecular Formulas A molecular formula does not tell us anything about

the actual three-dimensional structure of the compound.

Molecular Formulas A molecular formula does not tell us anything about

the actual three-dimensional structure of the compound. For example: the molecular formula for ammonia, NH3,

does not show the distribution of atoms in space.

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. For example: the molecular formula for ammonia, NH3,

does not show the distribution of atoms in space.

Molecular Formulas

NH3

Molecular formula

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. For example: the molecular formula for ammonia, NH3,

does not show the distribution of atoms in space.

Molecular Formulas

H−N−H

H

. .−

NH3

Molecular formula Structural

formula

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. For example: the molecular formula for ammonia, NH3,

does not show the distribution of atoms in space.

Molecular Formulas

H−N−H

H

. .−

NH3

Molecular formula Structural

formula Perspective drawing

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. For example: the molecular formula for ammonia, NH3,

does not show the distribution of atoms in space.

Molecular Formulas

H−N−H

H

. .−

NH3

Molecular formula Structural

formula Perspective drawing

Space-filling molecular model

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. For example: the molecular formula for ammonia, NH3,

does not show the distribution of atoms in space.

Molecular Formulas

H−N−H

H

. .−

NH3

Molecular formula Structural

formula Perspective drawing

Space-filling molecular model

Ball-and-stick molecular model

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. We can look at other molecules to see the three

dimensional arrangement of atoms vs. the molecular formulas.

Molecular Formulas

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. We can look at other molecules to see the three

dimensional arrangement of atoms vs. the molecular formulas.

Molecular Formulas

Water (H2O)

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. We can look at other molecules to see the three

dimensional arrangement of atoms vs. the molecular formulas.

Molecular Formulas

Water (H2O)

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. We can look at other molecules to see the three

dimensional arrangement of atoms vs. the molecular formulas.

Molecular Formulas

Water (H2O)

H H−

−O

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. We can look at other molecules to see the three

dimensional arrangement of atoms vs. the molecular formulas.

Molecular Formulas

Water (H2O) Carbon dioxide (CO2)

H H−

−O

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. We can look at other molecules to see the three

dimensional arrangement of atoms vs. the molecular formulas.

Molecular Formulas

Water (H2O) Carbon dioxide (CO2)

H H−

−O

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. We can look at other molecules to see the three

dimensional arrangement of atoms vs. the molecular formulas.

Molecular Formulas

Water (H2O) Carbon dioxide (CO2)

H H−

−O O=C=O

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. We can look at other molecules to see the three

dimensional arrangement of atoms vs. the molecular formulas.

Molecular Formulas

Water (H2O) Carbon dioxide (CO2) Ethanol (C2H6O)

H H−

−O O=C=O

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. We can look at other molecules to see the three

dimensional arrangement of atoms vs. the molecular formulas.

Molecular Formulas

Water (H2O) Carbon dioxide (CO2) Ethanol (C2H6O)

H H−

−O O=C=O

A molecular formula does not tell us anything about the actual three-dimensional structure of the compound. We can look at other molecules to see the three

dimensional arrangement of atoms vs. the molecular formulas.

Molecular Formulas

Water (H2O) Carbon dioxide (CO2) Ethanol (C2H6O)

H H−

−O O=C=O CH3 OH−

−CH2