Elements and Compounds

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A Classification Scheme for Matter

I would like to lead into discussing elements and compounds by first considering a general classification scheme for all matter.

The word "matter" describes everything that has physical existence. If something has physical existence, then it occupies space and has mass. That leads to the most common definition of matter:

Matter is that which occupies space and has mass.

We can successively separate matter into categories by asking a sequence of "yes/no" questions.

Question #1: "Is only one chemical substance present in the sample being considered?"

YES - Pure Substance
NO - Mixture

We can represent this question graphically:

Keep in mind that terms like "chemical substance" or "mixture" haven't really been defined yet. Hopefully, their definitions will be clearer as we go on.

All pure substances can be separated into two categories by asking the following question.

Question #2: "Can the sample be further decomposed by chemical means?" (Notice we are ignoring mixtures for the moment.)

YES - Compound
NO - Element

We can represent this question graphically:

The classification is developed in more detail in Mixtures and Pure Substances. The portion above is repeated with two additional questions concerning mixtures. The next question in that tutorial will concern the difference between heterogeneous and homogeneous mixtures.

I. Elements

An element is a pure substance which cannot be broken down by further chemical techniques. These include heating, cooling, electrolysis and reacting with other chemicals. (By the way, it is correct that an atom can be destroyed, but NOT by chemical means. You must use a more powerful reaction, called a nuclear reaction, to destroy or change atoms. That is a topic for a lesson in a different unit.)

A sample of an element contains only one kind of atom in the sample. Suppose you had a lump of copper in your hand. The ONLY type of atom in the lump is copper. In the lump there are trillions and trillions and trillions of copper atoms. NOTHING else. (I am ignoring impurities such as a tiny piece of rock occluded within the lump, some zinc atoms randomly trapped among the copper atoms, grease from your skin sticking to the surface of the lump or oxygen atoms from the atmosphere absorbed onto the surface of the copper.)

If you were to heat the lump of copper, it would melt and eventually vaporize. The smallest unit of the copper, called the atom, would remain unaffected by this. The atoms of copper would be in the solid state, the liquid state or the gaseous state, but they would be EXACTLY the same in each state.

Here is a definition of the word 'atom:'

An atom is the smallest subdivision of an element which still retains the properties of that element.

Here is another definition of the word 'atom:'

An atom is the smallest unit of an element that can enter into a chemical combination.

Still another way to phrase the definition is this:

An atom is the smallest unit of an element that can still be tested and shown to be that element.

Keep in mind that the above is not three different definitions of the word 'atom.' It is three different ways to say the same thing.

There are around 118 elements known to man, of which 20-30 are really, really important. Almost every element that exists has some form of use. There are some which are so unstable they only last for seconds or even tiny fractions of a second and no use has yet been found for them. However, ya never know!

Elements have names and symbols. For example hydrogen has the symbol H and iron has the symbol Fe. Please note that Fe is one symbol, not two. Also, make sure to use lower case for the second letter. Writing BR for bromine is incorrect, writing it as Br is correct.

II. Compounds

A compound is a pure substance composed of two or more different atoms chemically bonded to one another. A compound can be destroyed by chemical means. It might be broken down into simpler compounds, into its elements or a combination of the two. The key distinction is that compounds break down whereas the SAME techniques do not cause an element to break down.

The molecule is smallest subdivision of a compound that still retains the properties of that compound. The parallel definition (to the element one above) for the molecule is:

the smallest part of a compound that can enter into a chemical combination

Another definition, equally good, is that a molecule is the smallest stable part of a compound.

Water is a typical example of a compound. One molecule of water is composed of two hydrogen atoms and one oxygen atom, chemically bonded together. It is identified with its formula: H2O.

If you were to heat water (let's start with ice), it would eventually melt, then vaporize. Each water molecule (each H2O) would act as an independent unit and zoom around in the gas sample. The three atoms making the water molecule would stay attached to each other. In addition, water would enter into a chemical reaction acting like a water molecule, NOT little separate atoms of hydrogen and oxygen.

An important point to remember is that the compound is going to have distinctly different properties than its elements. Hydrogen has a set of properties, as does oxygen. However, the set of properties that water has is in no way like the two elements. For example, at room temperature (about 20-25 °C) water is a liquid while hydrogen and oxygen are gases.

Another classic example is sodium chloride (formula = NaCl). Sodium metal (Na) and chlorine gas (formula = Cl2) have very, very different properties from each other and neither one of the two is like sodium chloride at all.

Compounds have names and formulas. The formula is made from the symbols of the elements in the molecule and how many of each element there are. For example, glucose's formula is C6H12O6.

There are something over 12 million known chemical compounds. Well over 75% of them are mentioned in only one scientific article. Of the remaining bunch, there are several thousand of great interest and usefulness to science.

III. An Important Point about Compounds and Molecules

The difference between compound and molecule causes distress among students. I will try and explain it more.

The word compound is meant when you are making general reference to a chemical substance, as in "Go get a bottle of glucose from the storeroom" or "Glucose is the one of the end products of photosynthesis."

Inside the bottle is 500 grams of the compound glucose. Making up the 500 grams of glucose are trillions and trillions of individual glucose molecules, the formula of which is C6H12O6.

The plant makes the chemical compound called glucose. A sample of the chemical compound glucose is made up of many glucose molecules, all having the formula C6H12O6.

I hope that helps a little bit.

IV. An Important Point about Elements and Molecules

At room temperature and pressure, there are nine elements which exist as molecules.

(1) These are the seven diatomic molecules: H2, N2, O2, F2, Cl2, Br2, I2
(2) P4 and S8 also exist.

At elevated temperatures, these molecules will break down into single atoms, but even when in the molecular state, they are considered to be elements. The atoms are chemically bonded to each other, so they are considered to be molecules, but they are not considered to be compounds. They are elements.

All the other elements are considered to be mono-atomic.

These are the six mono-atomic gases: He, Ne, Ar, Kr, Xe, Rn.

There is one mono-atomic liquid: Hg (by the way, bromine is also a liquid at room temperature and pressure).

At room temperature and room pressure, all other elements exist as solid aggregates of atoms.

I wish to stress that these aggregrates (as well as liquid mercury) are not thought of as molecules. The elements in question are considered to exist as single atoms.

With the diatomics, an area of confusion sometimes arises: how do I know if you are discussing a molecule (for example: a molecule of oxygen) or individual atoms of the element? Good question.

If I am discussing the element oxygen with no context, I mean O2. If I am discussing the element sulfur with no context, I mean S8. If I wished to discuss oxygen atoms or sulfur atoms, I would have to say it explicitly or the context would clearly have to demand that individual atoms of the element are being discussed.

This area of confusion is often dealt with by saying 'dioxygen' or 'dichlorine.' When you say this, you clearly mean O2 or Cl2.

Having said the above, I think that the problem arises because the speaker (often the teacher) is more well-versed in the chemistry that the listener (you, the student). What happens is that the contect is clear to the speaker, but not clear to the student. In that case, you really have only one recourse: ask the person if they mean molecules or atoms.

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