Microscopic Classification of Substances

Pure Substances can be separated into four categories.

We will now look at each of these categories in turn. Focussing on differences at the microscopic level.

Atomic Gases

Group VIIIA of the periodic table contains the noble gases. Under standard conditions these elements have the simplest of all microscopic structures, free non-interacting atoms. They behave much like ping pong balls in a lottery drawing.

When cooled to very low temperatures the atoms develop a weak attraction to each other (they begin to stick together), which leads to the formation of liquids and solids at low temperatures. The boiling points of the noble gases range from 4.2 K for He to 165 K for Xe. The melting points for these same two elements are 0.95 K for He and 161 K for Xe.

Metallic Substances

Metallic substances can be classified as those elements and compounds which contain only metal atoms (from the left side of the periodic table), such as Fe, Cr, Nb3Sn, Cu3Au, etc.

Metals normally are solids at room temperature, but can melt to form liquids at elevated temperatures.

Molecular Substances

Structure and Properties of Molecules

A molecule is an assembly of two or more atoms tightly bound together. We shall later come to learn that atoms in a molecule are held together by covalent bonds.

The simplest molecule is a diatomic molecule, where two atoms are held together by a covalent bond. There are 7 elements which exist as diatomic gases ® H2, N2, O2 and the halogens.

A molecule behaves as an independent entity with properties that are often markedly different from other molecules or even from the constituent atoms contained within the molecule.

Example 1

Example 2 To understand why molecules have different properties from the atoms consider the analogy between water and a bicycle.

A bicycle is made up of a frame and two tires. In many ways this is similar to a water molecule, which is made of one oxygen and two hydrogen atoms.

In a bicycle, the tires are attached to the frame with nuts and bolts, in a water molecule the atoms are held together with chemical bonds. In each case you can take the bicycle/molecule apart providing you put in some energy (work).

In each case separating the entity into its component parts leads to a dramatic change in the physical properties. For example imagine the difference between drinking a glass of water and a mixture of hydrogen and oxygen, or the difference between riding a bicycle vs. a bicycle frame with the tires detached.

Formulas and Symbolism

We can describe a molecule at various levels of complexity. To illustrate this consider a molecule of benzene.

Empirical Formula – Provides the relative numbers of atoms contained in the molecule. For benzene there is a 1:1 ratio of carbon to hydrogen so the empirical formula is CH.

Molecular Formula – Provides the actual number of each type of atom in the molecule. The molecular formula for benzene is C6H6.

Structural Formula – Describes the connectivity of the molecule. For benzene the structural formula is:



More precise representations include the perspective drawing, the ball and stick model and the space filling model. These all provide information regarding the bond angles, and in some cases the relative sizes of the atoms and perhaps the bond distances (although these are often expressed numerically).

Ionic Compounds

Ions are formed when electrons are added or removed from atoms.

Removing an electron(s) ®

Adding an electron(s) ® When possible atoms prefer to either gain or lose electrons in order to attain the same number of electrons as the nearest noble gas. As a consequence metals tend to lose electrons to form cations, and non-metals tend to gain electrons to form anions.

Thus we can use the periodic table to predict the charges of ions.

Example 1

Cl has 17 electrons, if it gains one electron it will have the same number as Ar (18). Thus chlorine forms ions with a –1 charge.

Cl ® Cl-

Example 2

Ca has 20 electrons, if it gives up 2 it will have the same number as Ar (18). Thus calcium forms ions with +2 charge.

Ca ® Ca2+

Similar arguments can be used to predict the charges of several groups of the periodic table.

You will be expected to know these along with N3-, Al3+, Zn2+ and Ag+.

Atoms in the middle of the table (i.e. the transition metals and metalloids) sometimes have to either gain or lose more electrons than is feasible. Therefore, they do not always follow this rule.

Polyatomic Ions

Molecules can lose or gain electrons just like atoms do. When this occurs the result is known as a polyatomic ion. You will need to memorize several polyatomic ions:

Cations

Anions Coulombic forces dictate interactions between ions. Opposite charges (+ and -) attract each other, while like charges repel. As a consequence the total negative charge must be equal to the total positive charge. This allows us to accurately predict the ratios of each type of ion in a binary ionic compound (note an elemental sample cannot be ionic).

Examples

At room temperature and pressure pure ionic substances are typically found as crystalline solids. At high temperatures they can melt to form molten salts (i.e. SrCl2 melts at 880° C). An ionic compound may also dissolve when placed in a liquid to form a solution (i.e. NaCl in H2O). Substances which dissolve in the solvent are said to be soluble, while those which do not are said to be insoluble.

Upon either melting or dissolution in a solvent the discrete ionic species are preserved (i.e. Na+, Cl-, NH4+, SO42-) and the charge balance between cations and anions is maintained, but the individual ions are now able to freely move around and exchange neighbors, whereas they were pretty much fixed in place in a solid.

Isolated ions can exist in the gas phase, but such species are not very stable and usually have short lifetimes. Thus ionic substances do not exist as gases.

Comparing Molecular, Ionic and Metallic Substances

There are several ways we can differentiate and compare the various substances we have just discussed.

By Composition

By Physical State By Arrangement in a Solid

The difference between each category can be traced back to the forces which hold the atoms to each other. In turn, the origin of these interatomic forces is derived from the manner in which electrons are redistributed when individual atoms are brought together.
 
Classification
Interatomic Force
Electron Distribution
Atomic
Very weak attraction
No redistribution
Molecular
Chemical bond
Localized sharing
Ionic
Coulombic
Cations donate electrons to anions
Metallic
Metallic bond
Delocalized sharing

We will discuss these concepts further when we study chemical bonding in chapters 8 and 9.