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Atomic Theory Atomic Theory • Democritus- 440 B.C. • He proposed that if you kept cutting something in half you would eventually end up with an “uncutable” particle. Which he called an atom • Greek: Atomos - indivisible • Aristotle- 338-322 B.C • He disagreed with Democritus. He believed you would never end up with an indivisible particle. Atomic Theory • Dalton- 1808 • In search of understanding why elements combine in specific ratios he discovered through his experiments that this happens because they are made of individual atoms. • His theory states 3 things 1- Atoms are small and cannot be created, divided or destroyed. 2- Atoms of the same element are all alike 3-Atoms join with other atoms to make new substances Atoms Atomic Structure The Atom • Atoms are extremely small. • Atoms contain a nucleus. • Atoms are made of subatomic particles • Protons • Neutrons • Electrons How small is small? • If you could enlarge a penny until it was as wide as the continental United States(about 3000miles), each of its atoms would be only about 3 cm in diameter- about the size of a ping pong ball. Protons • • • • • Positively charged particles (+). Found in the nucleus. All protons are identical Each proton= 1 amu (atomic mass unit) Every atom of an element has the same number of protons. Neutrons • • • • Particles that have no charge Found in nucleus All neutrons are identical Protons & Neutrons are the most massive particles in the atom but located in a very small area (nucleus) Electrons • Negatively charged particles (e-) • Located outside of the nucleus in rings or levels called atomic clouds • Their mass is so small that it is usually considered zero. • It takes more than 1,800 electrons to equal the mass of one proton. • However electrons occupy most of an atoms volume. Some Things to Remember! • In an atom, the number of protons equals the number of electrons. As a result, the positive charge from protons equals the number of electrons which makes the atom neutral. • All atoms of an element have the same atomic number. Periodic Table Key Atomic Number • Atomic # = the number of protons in an atom. Atomic Mass • Atoms are too small to be measures in everyday units of mass (grams). • Scientists use units knows as atomic mass units (amu). • A proton or a neutron has a mass equal to about 1 amu. • Atomic Mass is equal to the sum of protons and neutrons in an atom. Isotopes • The number of neutrons in an atom can vary. • Atoms with the same number of protons and a different number of neutrons are called isotopes. • We identify an isotopes by the atoms mass. Isotopes Arrangement of Electrons • The first ring can only hold 2 electrons Arrangement of Electrons • The 2nd and 3rd rings can both hold 8 electrons