 As early as 400 BC, a few people believed that atoms are the building blocks of all matter. Yet, until recently, even scientists had never seen evidence.

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 As early as 400 BC, a few people believed that atoms are the building blocks of all matter. Yet, until recently, even scientists had never seen evidence of atoms.  More than 2000 years later, scientists found evidence that supported this idea. The Greek philospher Democritus proposed in 440 B.C. that matter was made of very tiny particles he called atomos. (Greek: “atomos”= not to be cut)

 Before there were chemists, there were alchemists— scientists who wanted to find methods to turn common metals into gold.  The alchemists practiced, often in secret, throughout the world during the Middle Ages.  Although they were never successful in their quest for gold, alchemists provided much information and helped establish chemistry as a science.

 In 1808, John Dalton, an English school teacher proposed the first Atomic Theory.  Dalton showed that elements are composed of only one kind of atom and compounds are made of two or more kinds of atoms.

 1. All matter is composed of extremely small particles called atoms, which cannot be subdivided, created, or destroyed. According to Dalton, atoms cannot be broken into smaller pieces.

 2. Atoms of a given element are identical in their physical and chemical properties. In any element, all atoms are exactly alike. Atoms of each element have the same mass. Carbon atom

 3. Atoms of different elements differ in their physical and chemical properties. Atoms of different elements are different, particularly in their mass. Hydrogen atom Oxygen atom Silicon atom

 4. Atoms of different elements combine in simple, whole-number ratios to form compounds.

 5. In chemical reactions, atoms are combined, separated, or rearranged but never created, destroyed, or changed. According to Dalton, atoms are indestructible. The combustion of methane and oxygen forms water and carbon dioxide. Note that the numbers and types of atoms are the same. All atoms are accounted for at the end.

 Law of Definite Proportions: A chemical compound always contains the same elements in exactly the same proportions by weight or mass.  For Example: Every sample of table salt has the same proportions of sodium and chlorine, NaCl. It is always a 1:1 ratio of Na:Cl.

 Law of Multiple Proportions: When two elements combine to form compounds, the mass of one element that combines with a given mass of the other is in the ratio of small whole numbers.

 Law of Conservation of Mass: Mass cannot be created or destroyed in ordinary chemical and physical changes.

 Today, scientists can divide an atom into even smaller particles and can destroy and create atoms.  For example, in 2006, at Russia's Joint Institute for Nuclear Research, scientists produced 3 atoms with 118 protons. All it took was smashing "bullets" of calcium at a target of Californium about 10,000,000,000,000,000,000 times.  Each atom lived only a fraction of a millisecond before decaying!

 In the mid-1800s scientists discovered that atoms can be broken into pieces after all. The smaller parts that make up atoms are called subatomic particles.  Electrons (- charged)  Protons (+ charged)  Neutrons (neutral)

 An English physicist, J. J. Thomson discovered electrons by using cathode rays. He was awarded the Nobel Prize in 1906 for his work.  Using a cathode ray, he was able to show that atoms have some distinct particles with a negative charge.

 Electrons are subatomic particles that have a negative charge. They orbit the nucleus of an atom and are extremely small, making up only a tiny fraction of the mass.

 J. J. Thomson proposed a model of an atom called the Plum-Pudding model. This model had negatively charged electrons embedded in a ball of positive charge.  Ernest Rutherford, a student of Thomson, disproved this when he discovered that atoms have a nucleus.

 Rutherford carried out an experiment where they shot a beam of small, positively charged particles (called alpha particles) at a thin gold foil. He measured the angles at which the particles were deflected.  He found that most of the particles went straight through the foil. Only a few were deflected.

 The nucleus is the dense, central portion of the atom.  The nucleus has all of the positive charge, nearly all of the mass but only a very small fraction of the volume of the atom. If an atom the size of a sewing pin was placed in the middle of the 50 yard line on a football field, the electrons would be orbiting around the goal posts! Most of an atom is empty space!

 Protons are subatomic particles that have a positive charge and are found in the nucleus. The number of protons in the nucleus is the atomic number, which identifies the element.  Neutrons are subatomic particles that have no charge and are located in the nucleus. Neutrons add mass to the atom.