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Development of the Atomic Theory READ pp. 1 – 10 YELLOW STEMcopedia book at home. ATOMIC MODELS • In ancient Greece Democritus argued: – Atoms were the smallest particles of matter. – “Atom”: (from Greek atomos, meaning “not able to be divided” Toga! Toga! John Dalton’s model: **An atom was a hard sphere that was the same throughout • Matter is made up of atoms • Atoms cannot be divided into smaller pieces • All atoms of an element are exactly alike • Different elements are made of different atoms William Crookes’ experiment • Cathode ray tube • Electrode = metal that conducts electricity • Anode (+) Cathode (-) • Battery connected, greenish glow appeared in tube • Shadow of cross fell at other end of tube • Proof that something was traveling in a straight line through the tube and was deflected by the cross JJ Thomson Model • Discovered electrons!! • Thomson thought the electrons were evenly mixed throughout the atom The CRT particles were negatively charged. These particles – electrons – are part of every kind of atom PLUM PUDDING MODEL Ernest Rutherford • Rutherford discovered that atoms contain a small, dense, positively (+) charged center called the NUCLEUS. **1920 – He identified protons!! Neils Bohr: Improvements on Rutherford’s model Electrons move around nucleus in different energy levels. Protons Neutrons + Electrons Bohr Model Electrons (negative charge) move around nucleus in energy levels in the electron cloud. Electrons Valence electrons are in outermost level. Give atom its chemical properties/reactivity Nucleus Electrons Current Atomic Theory • 1932 - neutrons were discovered because of inconsistencies in the mass of atoms (Chadwick) Matter What is Matter? • Matter is anything that has mass and takes up space. • The basic building blocks of all matter are atoms. • An atom is the smallest particle into which an element can be divided and still be the same substance. Parts of the Atom • Proton: a positively (+) charged particle of the nucleus. • Neutron: a neutral particle of the nucleus. • Electron: a negatively (-) charged particle of the nucleus. Electron (-) Nucleus (98% of the mass of atom) Neutron (neutral) (no charge) Electron Cloud (levels/orbits) Proton (+) • Atomic Mass Unit (amu): the SI unit for the masses of particles in atoms. • Atomic Number: the number of protons in the nucleus of an atom. (P+) • Mass Number: number of protons and neutrons in an atom. (P+ + No) • Atomic Mass: the weighted average of the masses of all the naturally occurring isotopes of an element. Molecules • A particle made of two or more atoms bonded together forms a molecule. • For example, two hydrogen atoms join with one oxygen atom to form a water molecule. Elements • An element is the simplest form of matter that cannot be changed into another simpler form by ordinary means. • Examples of elements: Oxygen, Helium, Gold, Silver…. Compounds • A substance made of two or more elements chemically combined form a compound. • Example: NaCl, H2O, CO2 What are atoms made of? • Protons 18e-e32 – Found in nucleus – Positive charge (+) 8e- e8 (18) • Neutrons – Found in nucleus – NO charge (0) – neutral • Electrons – Orbits around the nucleus – Negative charge (-) 8e8 e- P+ No 22e-e-