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Unit 3 Atoms and their structure How we started to think about atoms Original idea came from Ancient Greece (400 B.C.) – Democritus and Leucippus were Greek philosophers How we started to think about atoms Democritus looked at the beach – Made of sand – If you cut sand particles, you get smaller sand particles How we started to think about atoms There must be a smallest possible piece – Called those pieces “Atomos” – not able to be cut Another Greek Aristotle - Famous philosopher – All substances are made of 4 elements »Fire - Hot »Air - light »Earth - cool, heavy »Water - wet »Blend these in different proportions to get all substances The Hellenic Market Fire ~ Water Earth Air Who Was Right? None of the philosophers experimented to determine who was right – Greeks settled disagreements by argument – Aristotle was a better debater - He won – His ideas carried through middle ages »Later on, alchemists tried to change lead to gold (they did not understand atoms) Who’s Next? England in the late 1700’s - John Dalton – Teacher who summarized results of his experiments and those of others – Elements are substances that can’t be broken down – In Dalton’s Atomic Theory, he combined the idea of elements with that of atoms Dalton’s Atomic Theory All matter is made of tiny indivisible particles called atoms. Atoms of the same element are identical, those of different atoms are different. Atoms of different elements combine in whole number ratios to form compounds. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed. Law of Definite Proportions (part 3 in Dalton’s Theory) Each compound has a specific ratio of elements – It is a ratio by mass – Water is always 8 grams of oxygen for each gram of hydrogen Law of Multiple Proportions If two elements form more than one compound, the ratio elements in each compound, is a simple whole number – The ratio of the ratios is also a whole number What? Water is 8 grams of oxygen per gram of hydrogen Hydrogen peroxide is 16 grams of oxygen per gram of hydrogen 16 to 8 is a 2 to 1 ratio This happens because you have to add a whole atom, you can’t add a piece of an atom Parts of Atoms J. J. Thomson - English physicist, 1897 – Made a piece of equipment called a cathode ray tube »It is a vacuum tube - all the air has been pumped out »A limited amount of other gases are put in and an electric current is applied to the tube Thomson’s Cathode Ray Tube Voltage source - + Metal Disks Thomson’s Experiment Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source + By adding an electric field he found that the moving pieces were negatively charged Thomson & his atomic model Discovered the electron He did not know where positive charges were Said the atom was like plum pudding – A bunch of positive stuff, with the electrons able to be removed Rutherford’s Experiment Ernest Rutherford - English physicist, 1910 – Believed the plum pudding model of the atom was correct – Wanted to see how big atoms are – Used radioactivity »Alpha particles - positively charged pieces given off by uranium »Shot them at gold foil which can be made a few atoms thick Rutherford’s experiment When the alpha particles hit a fluorescent screen, it glows Here’s what it looked like Lead block Fluorescent Screen Uranium Gold Foil Lead block Uranium Fluorescent Screen Gold Foil He expected that… The alpha particles would pass through without changing direction very much Because… – The positive charges were spread out evenly - alone they would not be enough to stop the alpha particles What he expected Because, he thought the mass was evenly distributed in the atom What he got How he explained it Atom is mostly empty space – There is a small dense, positive piece at the center – Alpha particles are deflected by it, if they get close enough + + Homework Questions 1. 2. Make a table that lists the different philosophers and scientists (mentioned so far), identify the experiment they conducted AND describe what they discovered. What is Dalton’s atomic theory? Which parts are still hold true today? Modern View The atom is mostly empty space – Two regions »Nucleus protons and neutrons »Electron cloud region where you might find an electron Density and the Atom Since most of the particles went straight through the gold foil, the atom was mostly empty space Because the alpha particles turned so much, the positive particles must have been heavy Small volume and big mass = big density. This small dense positive area is the nucleus Subatomic particles Electron – located outside of nucleus, has negative charge Proton - positively charged particles inside of nucleus that are many times heavier than the electron Neutron - no charge but about the same mass as a proton, located inside of nucleus Subatomic particles Relative Particle mass Actual Name Symbol Charge (amu) mass (g) Electron e-1 1/1840 9.11 x 10-28 Proton p+ +1 1 1.67 x 10-24 Neutron n0 0 1 1.67 x 10-24 Structure of the Atom There are two regions – The nucleus »With protons and neutrons »Positive charge »Almost all the mass – Electron cloud - most of the volume of an atom »The region where the electron can be found Size of an atom Atoms are small – Measured in picometers, 10-12 meters – Hydrogen atom, 32 pm radius – Nucleus is very tiny compared to atom »If the atom was the size of a stadium, the nucleus would be the size of a marble »Radius of the nucleus is near 10-15 m »Density near 1014 g/cm3 Counting the Pieces Atomic Number = number of protons – # of protons determines kind of atom – the same as the number of electrons in the neutral atom Mass Number = the number of protons + neutrons – Includes all the particles with mass – NOT found on the periodic table What about when Electrons ≠ Protons Electrons may be gained or lost (IONS) – Gaining electrons gives a negatively charged ion called an anion – Losing electrons gives a positively charged ion called a cation What about when Electrons ≠ Protons Practice with ions… – Magnesium makes ions with a 2+ charge. Are electrons lost or gained? How many electrons are moved? – Fluorine makes ions with a 1- charge. Are electrons lost or gained? How many electrons are moved? – An ion has 13 p+ and 10 e-. Give the symbol and charge for the ion. – An ion has 34 p+ and 36 e-. Give the symbol and charge for the ion. Isotopes Dalton was wrong –Atoms of the same element can have different numbers of neutrons »different mass numbers (will have the same atomic number »called isotopes Symbols for Isotopes Nuclear Notation – Contains the symbol of the element, the mass number and the atomic number Mass number Atomic number X Symbols for Isotopes Hyphen Notation – Contains the symbol (or name) of the element and the mass number. »carbon- 12 »carbon -14 »uranium-235 Symbols for Isotopes Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass Number –Name 24 11 Na Symbols for Isotopes Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass Number –Name 80 35 Br Symbols for Isotopes if an element has an atomic number of 34 and a mass number of 78 what is the –number of protons –number of neutrons –number of electrons –Symbol – Nuclear & Hyphen notation –Name Symbols for Isotopes if an element has 91 protons and 140 neutrons what is the –Atomic number –Mass number –number of electrons –Symbol – Nuclear & Hyphen notation –Name Symbols for Isotopes if an element has 78 electrons and 117 neutrons what is the –Atomic number –Mass number –number of electrons –Symbol – Nuclear & Hyphen notation –Name Atomic Mass How heavy is an atom of oxygen? – There are different kinds of oxygen atoms – More concerned with average atomic mass »Based on abundance of each element in nature »Don’t use grams because the numbers would be too small Atomic Mass Is not a whole number because it is an average are the decimal numbers on the periodic table Measuring Atomic Mass Unit is the Atomic Mass Unit (amu) –One twelfth the mass of a carbon-12 atom »6 p+ and 6 n0 –Each isotope of an element has its own atomic mass »we get the average atomic mass of an element using weighted averages (need mass & percent abundance) Calculating averages You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks? Total mass = (4 x 50) + (1 x 60) = 260 g Average mass = (4 x 50) + (1 x 60) = 260 g 5 5 Calculating averages If 80% of the rocks were 50 grams and 20% of the rocks were 60 grams what is the weighted average mass of the rocks? Weighted Average = (% as decimal x mass) + (% as decimal x mass) + … Weighted Average = (0.8 x 50 g) + (0.2 x 60 g) = 52 g Homework Questions 1. 2. 3. 4. Describe the structure of an atom. Compare the size of the nucleus to the atom. What is the difference between an ion and an isotope? Write nuclear notation and hyphen notation for an element that has 24 protons, 24 electrons, and 26 neutrons.