The model that scientists use to describe the structure of matter is the particle model, which is also known as the atomic model of matter. This model has evolved over thousands of years and is used because it is successful in describing the behavior of matter. The atomic model accurately describes such behavior as phase change from solids to liquids to gases, and physical and chemical change. It also accurately describes elements, compounds and mixtures. In the evolution of the atomic model, whenever this model did not accurately describe the behavior of matter, it was changed so that it did.

Although the atomic model of matter had its beginnings in ancient Greece, the atomic model in use today was largely formed by a British scientist named John Dalton (1766-1844). In this atomic model, the atom was considered to be the smallest unit of matter. However, the atom has been discovered to be composed of even smaller particles called protons, neutrons and electrons. Recently, protons and neutrons were discovered to be composed of even smaller particles called quirks.

Protons are particles found in the center or the nucleus of an atom. Protons have a positive electrical charge. Neutrons are also found in the nucleus of an atom. However, neutrons have no electrical charge, so they are described as being electrically neutral. The number of protons and neutrons in the nucleus of an atom are equal to each other.

In the atomic model of matter, electrons are much smaller than protons and neutrons. Electrons orbit the nucleus (protons and neutrons) of an atom in a three dimensional orbital called an electron cloud. In an atom, the number of electrons is equal to the number of protons. Therefore, in an atom the number of protons equals the number of neutrons--equals the number of electrons.

The number of protons in the nucleus of an atom is called the atomic number. Experiments have shown that protons and neutrons weigh much more than do electrons. The nucleus of an atom (protons and neutrons) contain 99.99% of the mass or weight of an atom. The weight of an atom is expressed in Atomic Mass Units (AMU).

a. protons have a mass of about 1 AMU. b. neutrons have a mass of about 1 AMU. c. electrons have a mass of .00055 AMU's.

Look at the periodic table. #6, carbon, has an atomic number of 6. Therefore, carbon has 6 protons and 6 neutrons. The atomic mass is about 12. #7, nitrogen, has an atomic number of 7. Therefore, nitrogen has 7 protons and 7 neutrons, and an atomic mass of about 14. #8, oxygen, has an atomic number of 8. Therefore, oxygen has 8 protons and 8 neutrons, and an atomic mass of about 16. Therefore the atomic mass (atomic weight) is approximately equal to the number of protons plus the number of neutrons.

Matter is classified as elements, compounds and mixtures.

1. elements are matter composed of only one kind of atom. The elements are listed in the periodic table. Some elements were named for their properties, such as hydrogen. Hydro means water and genesis means beginning. So hydrogen is the water forming element. Others were name for the location where they were discovered: germanium in Germany, californium in California, europium in Europe and indium in India. Helium was discovered while studying sunlight, so it was named for Helios, the Greek word for sun. Other elements were named after famous scientists: einsteinium for Albert Einstein, mendelevium for Gregor Mendel, nobelium for Alfred B. Nobel, and curium for Marie Curie. Other elements were name for Roman gods such as mercury, neptunium and plutonium. While the name applied to the elements vary slightly from language to language and from country to country, the symbols for the elements are universal. So while scientists in different countries may use a different name for an element, the symbols are the same everywhere. The symbols are usually the first letter or first two letters in the common or Latin name for an element:

1. Cu=cuprium--Latin for copper

2. Au=aurum--Latin for gold

3. Fe-ferrum--Latin for iron

4. Pb=plumbum--Latin for lead

5. Hg=hydrargyrum--mercury

6. K=kalium--potassium

7. Ag=argentum--silver

8. Na=natrium--sodium

Sometimes the common name world-wide is different from the common name in the U.S.A. Tungsten is known in most of the world as wolfram, so the symbol W represents tungsten. The names of elements are not capitalized, but the first letter of the symbols are capitalized. If two letters are used in a symbol, the first is capitalized and the second is in lower case: Fe, He, Pb, Mg. Even though the symbols are abbreviations, no periods come behind them.

2. compounds--matter composed of two or more elements which are chemically bonded together. The smallest unit of a compound is called a molecule. Example: H2O=water. So two hydrogen atoms bonded to one oxygen atom is equal to one molecule of water.

3. mixture--matter composed of two or more elements which are not chemically bonded together. Example: salt water and brass are mixtures.

Structure of Matter Study Sheet
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