Molecules and Compounds

Molecule

A molecule is formed when two or more atoms join together chemically.

Ex: O₂, NaCl, H₂O

Compounds

A compound is a molecule that contains at least two different elements.

Ex: NaCl, H₂O, O₂

NOTE: All compounds are molecules but not all molecules are compounds.

Compound or Molecule?

CO₂

H₂

H₂SO₄

N₂

Water (H2O), carbon dioxide (CO2) and methane (CH4) are compounds because each is made from more than one element.

For example, a single molecule of molecular hydrogen is made from two atoms of hydrogen (H₂)

A single molecule of water is made from two atoms of hydrogen and one atom of oxygen (H2O)

Chemical Equations

Chemical equations are used to show what happens in a chemical reaction.

This reaction shows the reaction of zinc with hydrochloric acid:

Zn(s) + 2HCl(aq) →  ZnCl2(aq) + H2(g)

Parts of a Chemical Equation:

(s) = solid

(l) = liquid

(g) = gas

(aq) = aqueous solution (the substance is dissolved in water)

Zn(s) + 2HCl(aq) →  ZnCl2(aq) + H2(g)

Physical States

Parts of a Chemical Equation:

Reactants are the chemicals used in the reaction.

Products are the chemicals produced as a result of the reaction.

The arrow separates the reactants from the products and is pronounced as “yields” or “produces”.

Zn(s) + 2HCl(aq) →  ZnCl2(aq) + H2(g)

Reactants ProductsArrow(Yields)

Parts of a Chemical Equation:

Coefficients are use to show how many molecules of that chemical is required for the reaction.

e.g. There are 2 molecules of HCl in this reaction.

Subscripts are used to show how many atoms of that element is in each molecule.

e.g. Each hydrogen gas molecule is made of two hydrogen atoms.

Zn(s) + 2HCl(aq) →  ZnCl2(aq) + H2(g)

Coefficients Subscripts

Parts of a Chemical Equation:

Additional symbols may be used depending on the nature of the reaction:

The reaction is reversible.

Heat is used in this reaction.

A catalyst is used in this reaction. In this case it is

platinum.

Chemical Bonds

For compounds or molecules to form, bonds must be created between them

A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms.

The bond is caused by the electrostatic force of attraction between opposite charges known as electrons 

Chemical Bonds

There are two different types of chemical bonds, ionic bonds and covalent bonds

Covalent bonds are formed when atoms share electrons and are generally weaker

Ionic bonds are formed when atoms give their electrons to other atoms and are generally stronger

Structural Formulas

When atoms bond together to form molecules or compounds, they take on a structure

We can represent this as a structural formula which shows us how many atoms make up the compound and the bonds between them

Structural Formulas

Example: Hydrogen Gas

H H

Structural Formulas

Example: Hydrogen Chloride

H Cl

Structural Formulas

Example: Sodium Chloride

Na Cl

Structural Formulas

Example: Water

O

H H

Structural Formulas

Example: Ammonia

H

N

H H

Structural Formula

Example: Rust

Structural Formula

Example: Sulfuric Acid

Condensed Formulas

We can write our structural formulas as molecular formulas that will still show the structure

We call these condensed formulas

Skeletal Formulas

Skeletal formulas are used in organic chemistry when learning about Carbon and Hydrogen bonding

When one Carbon atom bonds with Hydrogen, there must be four Hydrogen Atoms

Methane

This is the structural formula for Methane gasWe can represent this as a condensed formula CH₄

Butane

This is the structural formula for butane. We can represent this as a condensed formula CH₃(CH₂)₂CH₃

Hexane

This is the structural formula for hexane gas

We can represent this as a condensed formula CH3(CH2)4CH3

Propane

The is the structural formula for PropaneWe can represent this as a condensed formula

Skeletal Formulas

Skeletal formulas are used in organic chemistry when learning about Carbon and Hydrogen bonding

When one Carbon atom bonds with Hydrogen, there must be four Hydrogen Atoms

This is the structural formula for butane

Butane

This is the skeletal formula for butane

CH₃(CH₂)₂CH₃

Hexane

This is the skeletal formula for hexane

CH3(CH2)4CH3

Propane