The ancient Greeks started the atomic ball rolling. Democritus was the first to theorize that matter was made of small pieces. Leucippus was the first to use the term atom (atomon) which meant "indivisible" in Greek. We now know that the atom is divisible and is made of even smaller pieces — the puzzling subatomic particles. Because the Greeks had no way to test and verify their theories, we had to wait almost 2000 years to confirm that atoms do exist, though not quite the way the Greeks imagined.

In the 16th century, Robert Boyle came up with the notion that there were elements that could not be broken down any further, but it was not until the 18th century that John Dalton reasoned that elements might be made of atoms.

The basic facts to know about the atom are that it is made up of three basic subatomic particles: 1) electrons (negative charge) that spin in shells around a nucleus that consists of 2) protons (positive charge) and 3) neutrons (neutral charge). Generally, the number of protons and electrons balance out to make the atom have an electrically neutral charge. Electrons that are farthest away from the nucleus of an atom (valence electrons) are the ones that are most easily shared with or transferred to other atoms. The atoms that are missing an electron or share an additional electron are called ions and combine easily with other ions to make molecules.

The number of protons in an atom is called the atomic number. This number determines the element of the atom. Within an element, the number of neutrons may vary, creating the different isotopes or nuclides. For the most part, this does not have much affect on the electrical and chemical behavior of the atom. (There is some exception with the mass of the isotope, as heavier isotopes tend to react more slowly than lighter ones.) There are some things that affect the number of protons and neutrons in the nucleus of an atom, including nuclear fission, nuclear fusion and radioactive decay. Normally, though, the number of electrons is the particle that is most easily changed, because of its lower bonding energy.

Traditionally, the atom was represented as a kind of miniature solar system. Now, scientists understand that if we could see an atom, it would look more like a fuzzy little cloud. In fact, scientists can only predict where an electron might be in its shell using the probability theory: the exact position and momentum of an electron can not be determined simultaneously.

Figure 1. Atomic Scale click for copyright

Protons and neutrons are about the same mass; however, electrons are over 1000 times lighter. How small are we talking? Well, as shown in Figure 1, we're talking very, very tiny.

• atom = 1 x 10^-10 meters
• nucleus = 1 x 10^-15 to 1 x 10^-14meters
• neutron or proton = 1 x 10^-15 meters
• electron - not known exactly, but thought to be on the order of 1 x 10^-18 meters

Fun Fact: If we drew the atom to scale and made protons and neutrons a centimeter in diameter, then the electrons and quarks would be less than the diameter of a hair and the entire atom's diameter would be greater than the length of thirty football fields! In fact, 99.9% of an atom's volume is just empty space! (Source: http://particleadventure.org/particleadventure/.)

Discussion Question: Solicit, integrate and summarize student responses.

• Ask students to look around at the items in the classroom, and then ask them what they think the "stuff" is that makes up the items in the classroom. Include technology items, such as computers, telephones and intercoms. (Possible answers: atoms, matter, solid stuff, etc.)

Voting: Ask a true/false question and have students vote by holding thumbs up for true and thumbs down for false. Count the votes and write the totals on the board. Give the right answer.

• True or False: An atom is the smallest building block of matter (Answer: True.)
• True or False: Molecules are made up of two or more atoms. (Answer: True; a molecule is also the smallest part of a substance that still has all the properties of that substance.)
• True or False: Electrons are found in the nucleus of an atom. (Answer: False; electrons are found in shells around the outside of the nucleus.)
• True or False: Engineers use knowledge of atoms and molecules to develop new technologies. (Answer: true)
• True or False: Lasers are only used in science laboratories. (Answer: False; lasers are used in many things, including industry, dental and medicine, military and consumer products, such as computers and DVD players.)

Flashcards: Each student on a team creates a flashcard with a question on one side and the answer on the other. If the team cannot agree on the answers, they should consult the teacher. Pass the flashcards to the next team. Each member of the team reads a flashcard, and everyone attempts to answer it. If they are right, they can pass on the card to the next team. If they feel they have another correct answer, they should write their answer on the back of the flashcard as an alternative. Once all teams have done all the flashcards, clarify any questions. Sample questions follow:

• Is the charge of a proton positive, negative, or neutral? (Answer: positive)
• What atomic particles exist in the nucleus? (Answer: protons and neutrons)