Do Now: Solve the following using the correct number of significant figures. 1) 7.76 m + 2.1 m = 2) 5.750 cg - 1.1 cg = 3) 5555 kg + 444 kg = 4) 1.23 m x 3.2 m = 5) 6.9 mm / 3 mm = YWBAT • Explain how Democritus and John Dalton described atoms History of Atomic Theory http://ed.ted.com/lessons/the-2-400-yearsearch-for-the-atom-theresa-doud Do Now: 1. What models have you used? 2. What other models are you aware of? 3. Why do people use models? YWBAT • Explain how Democritus and John Dalton described atoms • Identify instruments used to observe atoms. • Identify three types of subatomic particles. • Describe the structure of atoms according to the Rutherford atomic model. Democritus • Greek philosopher (460– 370BC) • Among the first to suggest the existence of atoms. atomos – Greek word for indivisible • Reasoned that atoms were indivisible and indestructible. Dalton • English chemist & school teacher • 1766 – 1864 • Used experimental methods to transform Democritus’ ideas into scientific theory • Studied ratios in which elements combine in chemical reactions. Dalton’s Atomic Theory 1. All elements are composed of tiny indivisible particles called atoms. Atoms of element A Dalton’s Atomic Theory 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Atoms of element A Atoms of element B Dalton’s Atomic Theory 3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. Mixture of atoms of elements A and B Dalton’s Atomic Theory 4. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction. Compound made by chemically combining atoms of elements A and B Dalton’s Atomic Theory • Do you think all parts of Dalton’s Atomic Theory are still believed to be true today? Sizing Up The Atom • A pure copper coin the size of a penny contains about 2.4 x 1022 atoms 24,000,000,000,000,000,000,000 atoms * Note – Earth’s population is ~ 7 x 109 people Sizing Up The Atom • Individual atoms are observable with a Scanning Electron Microscope (SEM). SEM Pollen grains Structure of the Atom What is in the box? Subatomic Particles 1. Protons 2. Neutrons 3. Electrons Electrons • In 1897, physicist J.J. Thomson discovered the electron. • Electrons are negatively charged subatomic particles https://www.youtube.com/watch?v=Rb6MguN0 Uj4 Protons • In 1886, Eugene Goldstein observed a cathode ray tube and found rays traveling in the direction opposite to that of the cathode ray tubes. – He concluded they were composed of positive particles. – These positively charged subatomic particles are called protons Neutrons • In 1932, James Chadwick confirmed the existence of the neutron. – Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton. Subatomic Particles How are these three subatomic particles (protons, neutrons, and electrons) put together in an atom? Plum Pudding Model • Thompson’s atomic model • Electron’s stuck in a lump of positive charge, similar to raisins stuck in dough. Rutherford’s Gold Foil Experiment • In 1911, Ernest Rutherford, a former student of Thomson’s, tested the plumpudding model. • A narrow beam of alpha particles was directed at a very thin sheet of gold Rutherford’s Gold Foil Experiment • Most alpha particles went straight through, or were slightly deflected. • A small fraction of the alpha particles bounced off the gold foil at very large angles. Rutherford’s Atomic Model The Nuclear Atom • Rutherford suggested a new theory of the atom based on the experimental results. – The atom is mostly empty space. – All the positive charge and amost all of the mass are concentrated in a small positively charged region (nucleus) – Protons & neutrons are in the positively charged nucleus – Electrons are distributed around the nucleus and occupy almost all the volume of the atom. Rutherford’s Atomic Model The Nuclear Atom If an atom were the size of a football stadium, the nucleus would be the size of a marble.