8.5A: Atoms Matter and Energy The History of Atomic Theory Lexile 950L 1 Since the ancient Greek civilization, people have wondered about the nature of the atom. The atom is the universe’s most basic building block. The word “atom” comes from the Greek language. “A…” is a prefix meaning “not,” and the word “tomos” means “cut.” Therefore, “atomos” or “atoms” means “cannot be cut or divided.” 2 Many people have discovered important aspects of the atom. In the 4th century BC, a Greek named Democritus thought that all matter was made of tiny particles. He thought that empty space filled the area between the solid atoms. That concept of the atom continued until 1803 when an English scientist, John Dalton, proposed an “atomic theory.” Atomic theory says that solid atoms have an atomic mass and can react because of forces. In 1873, James Clerk Maxwell proposed that electric and magnetic fields filled the empty space in atoms. 3 In 1897, J. J. Thomson discovered subatomic particles called electrons. He did experiments and found that these electrons orbited the nucleus of each atom. He then calculated they had a much smaller mass than that of other particles, protons and neutrons. 1 8.5A: Atoms Matter and Energy 4 Building upon Thomson’s work, Ernest Rutherford was the first to show the nuclear model of the atom. This model shows that the center of an atom contains positively charged protons (p+) surrounded by the negatively charged electrons (e-). 5 In 1932, Sir James Chadwick found the neutrally charged neutron (n0). The atomic model now seemed complete after all these discoveries. However, scientists continue to study with much more accurate instruments now. They have found many more mysterious things about atoms. 6 Many people had a hand in developing atomic theory as we know it today. Scientists are still studying the tiniest building block of our universe as the nuclear model of the atom continues to change. 2 8.5A: Atoms Matter and Energy 1 The word “atom” is derived from two Greek words meaning ____ and _____. A “not”, “large” B “not”, “cut” C “tiny”, “world” D “unexplained”, “component” 2 The Greek _____ theorized that all matter is composed of tiny, indivisible particles called atoms. A Democritus B Aristotle C Epicurius D Demosthenes 3 8.5A: Atoms Matter and Energy 3 Who was the first to theorize that electrons orbited the nucleus? A Ernest Rutherford B James Maxwell C J. J. Thompson D Albert Einstein 4 Which scientist discovered the neutron and gave us the current basic model of the atom? A James Maxwell B John Dalton C James Chadwick D Sir Isaac Newton 5 Around the ________ is when the atom was first theorized. A 5th century AD B 1st century BC C 4th century BC D 12th century AD 4 .