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Models of the Atom a Historical Perspective Early Greek Theories Democritus • 400 B.C. - Democritus thought matter could not be divided indefinitely. • This led to the idea of atoms in a void. fire earth air water • 350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. Aristotle Atomic Theory The Beginning Ancient Greece – 440 BCE Who was right?? • Democritus – atomos • “not able to be divided” – Thought that atoms were different shapes and sizes of a single substance. • Aristotle – Disagreed with Democritus – Believed matter can be infinitely divided – Had great influence •Aristotle was wrong. However, his theory persisted for 2000 years. Atomic Theory Late 1700’s - England • John Dalton – Wanted to know WHY! – 1803 – Dalton’s Atomic Theory suggested that elements are made up of single atoms. Dalton’s Atomic Theory 1803 - England • Three main ideas of Dalton’s Atomic Theory – All substances are made of atoms which cannot be created, divided, or destroyed. – Atoms of the same element are exactly alike and atoms of different elements are different. – Atoms join with other atoms to make new substances. One of the main ideas of Dalton’s Atomic Theory stated that atoms join with other atoms to create new substances. What do we now call these substances? A. B. C. D. Elements Mixtures Compounds Solutions His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios). Atomic Theory Science never rests! • While most scientists agreed that Dalton’s theory was mostly correct, new information was being discovered which forced atomic theory to change. – Four major discoveries • • • • J.J. Thomson Ernest Rutherford Niels Bohr Erwin Schrodinger and Werner Heisenberg Atomic Theory • What’s missing??? Adding Electrons to the Model Materials, when rubbed, can develop a charge difference. This electricity is called “cathode rays” when passed through an evacuated tube (demos). These rays have a small mass and are negative. Thompson noted that these negative subatomic particles were a fundamental part of all atoms. 1) Dalton’s “Billiard ball” model (1800-1900) Atoms are solid and indivisible. 2) Thompson “Plum pudding” model (1900) Negative electrons in a positive framework. 3) The Rutherford model (around 1910) Atoms are mostly empty space. Negative electrons orbit a positive nucleus. Atomic Theory 1897 - England • J.J. Thomson – Discovered that there were small particles inside the atom. • Importance: Meant that atoms were divisible – How did he discover this? Atomic Theory J.J. Thomson’s Discovery of the Electron • Cathode-ray tube experiment Atomic Theory J.J. Thomson’s Discovery of the Electron • The Plum Pudding Model Atomic Theory • J.J. Thomson’s discoveries caused a change in the view of the atom proposed by Dalton. – What was the change? • All substances are made of atoms which cannot be created, divided, or destroyed. • Atoms of the same element are exactly alike and atoms of different elements are different. • Atoms join with other atoms to make new substances. J.J. Thomson’s experiment caused one of the main ideas of Dalton’s Atomic Theory to be changed. Which idea was no longer valid? A. All substances are made of atoms which could not be created, divided, or destroyed. B. Atoms of the same element are exactly alike and atoms of different elements are different. C. Atoms join with other atoms to make new substances. Atomic Theory Ernest Rutherford – 1909 England • Student of Thomson’s – Designed an experiment to test Thomson’s Plum Pudding Model – Made a surprising discovery. Ernest Rutherford (movie: 10 min.) • Rutherford shot alpha () particles at gold foil. Zinc sulfide screen Thin gold foil Lead block Radioactive substance path of invisible -particles Most particles passed through. So, atoms are mostly empty. Some positive -particles deflected or bounced back! Thus, a “nucleus” is positive & holds most of an atom’s mass. Rutherford’s experiment changed the view of the atom once again. Which change to the atom did Rutherford’s experiment force? A. Most of the mass of an atom is contained in the electrons. B. Atoms contain very little empty space. C. Most of the mass of the atom is contained in the dense nucleus. Atomic Theory Niels Bohr – 1913 Denmark • Associate of Rutherford • Developed new model of the atom by studying how atoms react to light. Atomic Theory Niels Bohr – 1913 Denmark • Like rungs of a ladder. • Electrons can only exist at specific energy levels and not in between. • Electrons can jump from one level to another. Bohr’s model • Electrons orbit the nucleus in “shells” • Electrons can be bumped up to a higher shell if hit by an electron or a photon of light. There are 2 types of spectra: continuous spectra & line spectra. It’s when electrons fall back down that they release a photon. These jumps down from “shell” to “shell” account for the line spectra seen in gas discharge tubes (through spectroscopes). Atomic numbers, Mass numbers • There are 3 types of subatomic particles. We already know about electrons (e–) & protons (p+). • Neutrons (n0) were also shown to exist (1930s). • They have: no charge, a mass similar to protons • Elements are often symbolized with their mass number and atomic number E.g. Oxygen: O 8 16 These values are given on the periodic table. For now, round the mass # to a whole number. These numbers tell you a lot about atoms. # of protons = # of electrons = atomic number # of neutrons = mass number – atomic number Calculate # of e–, n0, p+ for Ca, Ar, and Br. Atomic Mass p+ n0 e– Ca 20 40 20 20 20 Ar 18 40 18 22 18 Br 35 80 35 45 35 Modern Atomic Theory • Erwin Schrödinger – Austrian physicist • Werner Heisenberg – German physicist Modern Atomic Theory • Electron clouds: regions in an atom where electrons are likely to be found. Atomic Theory Summary • 440 BCE: Democritus – first describes atoms • 1803: Dalton – proposes first model of the atom and three principles • 1897: Thomson – cathode ray tube experiment discovers electrons • 1909: Rutherford – gold foil experiment discovers positively charged, dense nucleus of atom • 1911: Rutherford – offers revised model of the atom • 1913: Bohr – proposed the idea of energy levels • 1927: Schrödinger/Heisenberg – modern cloud model