1. Elements and atoms

Elements are made of one type of atom, e.g. diamond consists only of carbon atoms; aluminum foil contains only aluminum atoms.

The periodic table arranges elements in order of their atomic number (number of protons in the nucleus), so that elements with similar properties are arranged in columns.

Compound - contains 2 or more elements with a fixed ratio of the number of atoms. Every compound has its own characteristic physical and chemical properties.

More than one compound may be formed by the same two elements, for example water and hydrogen peroxide. However, the atomic ratios are different.

H₂O is water; the water molecule contains two hydrogen atoms and one oxygen atom
H₂O₂ is hydrogen peroxide; each molecule contains two hydrogen atoms and two oxygen atoms

A solution of salt in water is not a compound, because the proportions of salt and water may be varied indefinitely. Such a solution is a mixture. Mixtures may be homogeneous or heterogeneous (see Lecture 1).

Molecular structures: at the microscopic level, individual molecules can be distinguished. Distances between the atoms that make up the molecule are small compared to the distances between molecules. Iodine is an example of a compound with distinguishable diatomic molecules.

Distance "a" is the distance between the centers of I atoms in the same molecule. Distance "b" is the distance between I atoms in two adjacent molecules.

Network structures: at the microscopic level, there are no distinguishable molecules. The atoms or ions form a regular lattice. Salt (sodium chloride, NaCl) is an example of an ionic network structure.

In the picture below, you can see how the sodium ions (green) and chloride ions (red) alternate regularly so that there is no distinguishable "molecule" of NaCl.

4. Physical and chemical changes

A physical change does not involve the creation or destruction of a chemical compound. Examples of physical changes:

melting of ice to form liquid water
dissolving sugar in water to form a solution
breaking a stick of wood into smaller pieces

A chemical change DOES involve the creation of new chemical substances. Examples of chemical changes:

burning gasoline in your car's engine to form carbon dioxide and water
the reaction of hydrogen and oxygen to form water
the electrolysis of NaCl to form metallic sodium and chlorine gas

a. By a formula (symbolic representation). Methane is the chief component of natural gas. It is a compound of carbon and hydrogen, and its formula is CH₄.

b. By a drawing to show how the atoms are bonded.

c. By models (or pictures of models, as shown here) to represent not only the correct bonding, but also the shape of the molecule. For instance, methane is not a flat molecule, it has a three-dimensional shape called tetrahedral . Models may be stick models, ball-and-stick models, or space-filling models (shown below). Ball-and-stick is most common in chemistry unless there is a specific reason to use one of the other two.

Almost everyone knows that carbon exists in more than a single form. Diamond and graphite are two well-known forms of carbon (both are network structures), and many students know about "buckyballs" that are spherical C₆₀ molecules. However, in the last few years carbon has also been found to form tubular structures based on graphite. These tubes are called "nanotubes" because their diameter is on the order of a few nanometers (1 nm = 1 x 10^-9 m). They have exciting properties and are the object of intensive research.

Let's look at how nanotubes are formed from graphite. Graphite consists of layers of carbon atoms with each carbon joined to three others to make a planar network of hexagonal rings. Below is a ball-and-stick model for part of the graphite structure.

Now if we imagine that we take this planar structure and hook together the carbons on the right- and left-hand sides, we get a tube, shown both as ball-and-stick and as space-filling models. The "hole" in the center of the tube is clearly visible. The hole size varies with the number of carbons in the graphite sheet. The holes can be filled with other atoms or molecules, which makes possible endless ideas for how nanotubes might be used in the future.

Let's look at the density of water as a function of temperature:

Above zero degrees C, we notice that the density of water decreases steadily. We interpret this in terms of the kinetic-molecular theory, that molecules move more rapidly with increasing temperature, thus increasing the distance between them and the volume that the liquid water occupies. Since density is the mass divided by the volume, the density falls.

But how about ice? The molecules in ice aren't moving very much, so why is it that a given mass of ice occupies more volume than the same mass of liquid water? (I.e., ice has a lower density than liquid water).

To find the answer we need to look at the structures of ice and consider what happens when ice melts.

Ice is made up of water molecules linked by hydrogen bonds (weak interactions between water molecules) to form a very open structure made of linked rings containing 6 water molecules:

However, at zero degrees C, some of the water molecules move enough to start breaking up these 6-membered rings. Then water molecules can twist and turn to start filling the "empty" space that is found in the rigid ice structure. Thus the volume occupied by the liquid water is less than the volume occupied by solid ice, and the density (=mass/volume) is higher for liquid water than for ice.

This page is http://chemiris.labs.brocku.ca/~chemweb/courses/chem180/CHEM1P80_Lecture_2.html
Created September 12, 2000 by M. F. Richardson
© Brock University, 2000