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The term matter describes all of the physical substances around us: your computer, your body, a pencil, water, etc. Thousands of years ago people believed that different types of matter were made of different combinations of 4 things: earth, fire, air, and water. Around 400 B.C., the Greek philosopher Democritus suggested that matter was actually composed of tiny particles. He called these particles atomos.

In 1803, the British scientist John Dalton improved upon Democritus' idea of atomos and developed the first 'atomic' theory. This theory stated that all matter was composed of tiny particles called atoms. Dalton hypothesized that different types of matter were made of different types of atoms, a simple but revolutionary theory. Dalton knew that there were a limited number of chemically pure substances that were called elements - he hypothesized that these different elements were made of different atoms. Dalton's atomic theory had 4 basic concepts:

1. All matter is composed of indivisible particles called atoms.
2. Atoms are indestructible and unchangeable.  Chemical reactions involve the combination of atoms, not the destruction of atoms.
3. All atoms of a given element are identical.  Atoms of different elements have different properties.  (Dalton characterized elements according to their atomic weight, however this changed when isotopes of elements were discovered.)  
4. Compounds are formed when atoms of different elements combine chemically.  When elements react to form compounds, they react in defined, whole number ratios.

Elements: We now know of approximately 118 different elements.  Elements are pure substances that cannot be broken down by chemical means.  For example, gold cannot be chemically changed into another substance.  Each of the elements has been given a one or two letter symbol to make them easy to write.  For example, the element hydrogen can be abbreviated using the symbol H.  The element helium is given the symbol He.  When writing the symbol of an element, the first letter is always capitalized and the second letter (if there is one) is always lowercase. 

Water(H2O)

Compounds:   Clearly there are more than 118 substances in the universe.  Dalton knew that in addition to the 'pure' elements, there were many substances that were formed by the chemical combination of two or more elements, these substances were called compounds. Most of the substances we come into contact with in everyday life are compounds.  Water is a good example.  If we can imagine a single particle of water, it might look like the picture to the right.  Each of the colored circles represents an atom, and the entire picture represents a single molecule of water.  Compounds have chemical formulas. For example, water is always H₂O (two hydrogen atoms and one oxygen atom), never HO or H₂O₂.

The idea that compounds have defined chemical formulas led to the law of definite proportions.  This law states that when elements react to form specific compounds, the reaction always takes place in a defined whole number ratio.  For example, hydrogen reacts with oxygen to form water.  When this reaction occurs, 2 parts of hydrogen are consumed for every one part of oxygen consumed, yielding one part water.  Let's imagine that we have balloons filled with hydrogen and oxygen (which are gases at room temperature), the law of definite proportions can be illustrated:

The law also applies to multiples of the fundamental proportion, for example:

When reactants are present in excess of the fundamental proportions of the chemical reaction, then some reactants will remain unchanged after the chemical reaction has occurred:

A note on chemical formulas: Chemical formulas are written by listing the symbols of the elements together, without any spaces between them.  If a molecule contains more than one atom of a single element, a number is written as a subscript after the symbol to show how many atoms are in the molecule.  For example, the formula H₂O means that water is composed of 2 hydrogen atoms and 1 oxygen atom.

States of Matter

We already know that matter comes in different forms. For example, water can be in the form of ice, liquid or steam. Are these different chemical compounds? No, these different forms are called states. There are 5 fundamental states of matter. Different states of matter differ in the amount of energy they possess.  

Solids are formed when the attraction between individual molecules is greater than the energy causing them to move apart. Because the molecules are locked in position near each other, solids have a defined shape and volume. The atoms or molecules of solids are still in motion, however they remain fixed in place and vibrate back and forth.

Liquids are formed when the energy (usually in the form of heat) in a system is increased and the rigid structure of the solid state is broken down. In liquids, molecules can move past one another and bump into other molecules, but they remain relatively close to each other. As a result, liquids can 'flow' to take the shape of the container they are in but they cannot be easily compressed. Thus liquids have an undefined shape, but a defined volume.

Gases are formed when the energy in the system exceeds the attraction between molecules. In the gaseous state, molecules move quickly and are free to move in any direction spreading out anywhere within their container. Gases expand to fill their containers and have low density. Because individual molecules are widely separated and can move around easily in the gaseous state, gases can be compressed easily and they have an undefined shape.  

Use the animation below to compare these three states of matter.

Plasmas are hot ionized gases. Plasmas have so much energy that electrons are stripped from individual atoms forming a gas of charged ions. The most common place that plasmas occur is in the center of stars such as the sun.

Bose-Einstein Condensates are super cooled materials that exist only when their temperature remains within a fraction of absolute zero.  This weird state is one in which all the atoms attain the same quantum-mechanical state. 

Related Lessons

• Atomic Theory I