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History of the Atom - Timeline 1766 – 1844 1887 – 1961 James Chadwick discovered the neutron in 1932 460 – 370 BC 0 Scientists and Their Contribution to the Model of an Atom Niels Bohr proposes the Bohr Model in 1913 1700s 1800s 1900s History of the Atom Antoine Lavoisier Thomson makesJ.J. a substantial the number discovers of contributions electron and to the field of proposes the Chemistry Plum Pudding Model 1871 in 1897 – 1937 Democritus proposes the 1st atomic theory 1743 – 1794 Erwin John Dalton Ernest Rutherford Schrodinger proposesperforms his the Gold Foil describes 1891 – 1974 atomic theory Experiment in in 1909 the electron 1803 cloud in 1926 1885 – 1962 Click on picture for more information 1856 – 1940 • Believed that all matter was composed of indivisible particles he called “ATOMS” Democritus (460 BC – 370 BC) • Proposed an Atomic Theory which states that all atoms are small, hard, indivisible and indestructible particles made of a single material formed into different shapes and sizes. • Aristotle did not support his Atomic Theory • Derived from the Greek word “Atomos” – meaning indivisible • He also believed that different atoms: Image taken from: https://reichchemistry.wikispaces.com/T.+Glenn +Time+Line+Project – Are different sizes – Have different properties 1 Antoine Lavoisier (1743 – 1794) Image taken from: www.ldeo.columbia.edu/.../v1001/ geotime2.html • Known as the “Father of Modern Chemistry” • Was the first person to generate a list of thirty-three elements in his textbook • Devised the metric system • Was married to a 13-year old Marie-Anne Pierette Paulze; she assisted him with much of his work • Was a tax-collector that was consequently guillotined during the French Revolution • Discovered/proposed that combustion occurs when oxygen combines with other elements • Discovered/proposed the Law of Conservation of Mass (or Matter) which states, in a chemical reaction, matter is neither created nor destroyed “Father of Atomic Theory” Dalton’s ideas were so brilliant that they have remained essentially intact up to the present time and has only been slightly corrected. John Dalton (1766 – 1844) • In 1803, proposed an Atomic Theory which states: o All substances are made of atoms; atoms are small particles that cannot be created, divided, or destroyed. o Atoms of the same element are exactly alike, and atoms of different elements are different o Atoms join with other atoms to make new substances • Calculated the atomic weights of many various elements • Was a teacher at a very young age • Was color blind Image taken from: chemistry.about.com/.../JohnDalton.htm Dalton’s Atomic Theory (1803) aka: 5 Postulates 1. All matter is composed of extremely small particles called atoms. (I agree with Democritus!) 2. All atoms of a given element are identical, having the same: - size - mass - chemical properties. 3. All atoms of different elements are different. 2 Dalton’s Atomic Model Dalton’s Atomic Theory (1803) aka: 5 Postulates 4. Atoms cannot be created, divided into smaller particles, or destroyed. **In a chemical reaction, atoms of different elements are separated, joined or rearranged. They are never changed into the atoms of another element.** 5. Atoms combine in definite whole number ratios to make compounds (you can’t have a ½ of a Carbon bonding with Oxygen; it’s a whole atom or no atom) • Based on Dalton’s Atomic Theory (5 postulates), most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts. • Dalton was credited for the three Atomic Laws that were proven after his time. Dalton’s Atomic Laws 1. Law of Conservation of Mass Matter cannot be created or destroyed in any physical or chemical process, just transferred. 2. Dalton’s Atomic Laws 3. Law of Multiple Proportions – if two elements can combine to form more than one compound, then the ratio of the second element combined with a certain mass of the first element is always a ratio of small whole numbers. Law of Constant Composition When atoms combine to form molecules, the ratio of atoms is constant. Example – H2O will always have 2 times as many Hydrogen atoms as Oxygen. Example: CO vs. CO2 Formula Ratio of N:O 3 J.J. Thomson (1856 – 1940) Image taken from: www.wired.com/.../news/2008/04/ dayintech_0430 • Proved that an atom can be divided into smaller parts • While experimenting with cathoderay tubes, discovered corpuscles, which were later called electrons • Stated that the atom is neutral • In 1897, proposed the Plum Pudding Model which states that atoms mostly consist of positively charged material with negatively charged particles (electrons) located throughout the positive material • Won a Nobel Prize Cathode Ray Tubes • Cathode rays had been used for some time before Thompson’s experiments. • A cathode ray is a tube that has a piece of metal, called an electrode, at each end. Each electrode is connected to a power source (battery). • When the power is turned on, the electrodes become charged and produce a stream of charged particles. They travel from cathode, across the tube to the anode. • Used cathode rays to prove that Dalton’s Solid-ball model could be broken into smaller particles • Thomson is credited with discovering electrons Cathode Ray Tubes • Thomson put the tube in a magnetic field. He predicted that the stream would travel in a straight path. • Instead, he found that the path curved away from a negatively charged plate and toward a positively charged plate • Why? ----------------------------- ++++++++++++++ • Like charges repel each other, and objects with unlike charges attract each other, Thomson concluded that the stream of charged particles had electrons in them. 4 Cathode Ray Tube Experiment • Thomson’s Plum Pudding model is a + charged sphere that has (-) charged electrons scattered inside, like “raisins” in “plum pudding.” • Overall, the atom is neutral because the atom had the same number of positive and negative charges. • Thompson Concluded: • Cathode rays are made up of invisible, negatively charged particles called Electrons. • These electrons had to come from the matter (atoms) of the negative electrode. • Since the electrodes could be made from a variety of metals, then all atoms must contain electrons! Ernest Rutherford (1871 – 1937) • In 1909, performed the Gold Foil Experiment and suggested the following characteristics of the atom: o It consists of a small core, or nucleus, that contains most of the mass of the atom o This nucleus is made up of particles called protons, which have a positive charge o The protons are surrounded by negatively charged electrons, but most of the atom is actually empty space • Did extensive work on radioactivity (alpha & beta particles, gamma rays/waves) and was referred to as the “Father of Nuclear Physics” • Won a Nobel Prize • Was a student of J.J. Thomson • Was on the New Zealand $100 bill Thomson’s Plum Pudding Model • From Thomson’s experiments, scientists concluded that atoms were not just neutral spheres, but somehow were composed of electrically charged particles. • The balance of positive and negative charge supports the neutral atom. Rutherford (1871-1937) • Used the “Gold Foil Experiment” to discover the existence of: – An atomic Nucleus – Protons (in later experiments) Image taken from: http://www.scientific-web.com/en/ Physics/Biographies/ ErnestRutherford.html 5 Gold Foil Experiment • Rutherford directed a narrow beam of alpha particles (+ charges) at a thin piece of gold foil. • Based on observations from other experiments involving alpha particles, he predicted that the (+) charges would go through the foil The Gold Foil Experiment Results from Gold Foil Experiment • Rutherford found that every once and a while, a + particle was deflected bounced back. (about 1% of the time) • Why? • Because the + charge hit a central mass of positive charge and was repelled. Conclusions from Rutherford’s Gold Foil Experiment • The atom contains a positively charged “nucleus” • This nucleus contains almost all of the mass of the atom, but occupies a very small volume of the atom. • The negatively charged electrons occupied most of the volume of the atom. • The atom is mostly empty space. 6 Niels Bohr Rutherford’s Planetary Model (1885 – 1962) Image taken from: commons.wikimedia.org/wiki/ File:Niels_Bohr.jpg The electrons orbit the nucleus like the planets revolve around the sun. Bohr (1885-1962) • Worked in Rutherford’s lab • Wondered why – electrons are not attracted to the + nucleus and cluster around it • Disproved Rutherford’s Planetary Model • Experimented with light and its interaction with matter to develop a new model. • In 1913, proposed the Bohr Model, which suggests that electrons travel around the nucleus of an atom in orbits or definite paths. Additionally, the electrons can jump from a path in one level to a path in another level (depending on their energy) • Won a Nobel Prize • Worked with Ernest Rutherford Bohr’s Energy Level Model Energy Level Model: Electrons are arranged in circles around the nucleus. Each circle has a different energy. • Electrons are in constant motion, traveling around the circle at the speed of light. n • Electrons can “jump” from one circle to the next • But they can’t go to the nucleus they are traveling too fast to be fully attracted. 7 Bohr’s Energy Level Model He proposed the following: 1. Protons and neutrons are in the nucleus 2. Electrons can only be certain distances from the nucleus. 3. The electrons orbit the nucleus at fixed energy levels. 4. The electrons must absorb or emit a fixed amount of energy to travel between these energy levels James Chadwick (1891 – 1974) Erwin Schrodinger (1887-1961) • In 1926, he further explained the nature of electrons in an atom by stating that the exact location of an electron cannot be stated; therefore, it is more accurate to view the electrons in regions called electron clouds; electron clouds are places where the electrons are likely to be found • Did extensive work on the Wave formula à Schrodinger equation • Won a Nobel Prize Image taken from: nobelprize.org/.../1933/ schrodinger-bio.html Wave Model • Realized that the atomic mass of most elements was double the number of protons à discovery of the neutron in 1932 • Worked on the Manhattan Project • Worked with Ernest Rutherford • Won a Nobel Prize Image taken from: www.wired.com/.../news/2009/02/ dayintech_0227 8 The Wave Model • Today’s atomic model is based on the principles of wave mechanics. • According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun. Electron Cloud: • A space in which electrons are likely to be found. • Electrons whirl about the nucleus billions of times in one second • They are not moving around in random patterns. • Location of electrons depends upon how much energy the electron has. The Wave Model • In fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has. • According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral. Electron Cloud: • Depending on their energy they are locked into a certain area in the cloud. • Electrons with the lowest energy are found in the energy level closest to the nucleus • Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus. 9 Indivisible Electron Greek X Dalton X Thomson Nucleus Orbit Electron Cloud Progression of the Atomic Model - - X Rutherford X X Bohr X X Wave X X - X - --+- - + - - Electron Cloud X The structure of an atom, according to: Democritus James Ernest Erwin Neils Schrodinger Chadwick Rutherford Bohr& J.J. Thomson John Dalton 10