Evolution of Atomic Theory: From Democritus to JJ Thomson
Foundations of atomic theory trace back to Democritus and Aristotle, evolving through the discovery of laws in the 18th century by Dalton's Atomic Theory. Modern advancements and JJ Thomson's discoveries led to a deeper understanding of the structure of atoms.
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UNIT 3 ATOMS: THE BUILDING BLOCKS OF MATTER
Section 1 FROM PHILOSOPHICAL IDEA TO SCIENTIFIC THEORY
Foundations of Atomic Theory Particle Theory of Matter Democritus in 400 B.C. Stated that nature s basic particle was the atom ( indivisible in Greek). Aristotle Believed all matter was continuous (could be divided forever), and did not believe in atoms. Neither had experimental evidence to support their claims.
Fast Forward to the18thcentury Three laws discovered due to improved instrumentation and carefully observed chemical reactions 1. Law of conservation of mass Mass is neither created nor destroyed during an ordinary chemical reaction or physical change. 2. Law of definite proportions A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound. Example: Table salt will always consist of 39.34 % Na and 60.66% Cl. 3. Law of multiple proportions If 2 or more compounds are composed of the same two elements, the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers. Example: Carbon and Oxygen can combine to form CO2 or CO.
https://www.youtube.com/watch ?v=OUoV--CuLDA Dalton s Atomic Theory John Dalton proposed Atomic Theory Accounted for all three laws: conservation of mass, definite proportions, and multiple proportions. Dalton s Atomic Theory consists of the following statements: All matter is composed of atoms. Atoms of a given element are identical in size, mass, and all other properties. Atoms cannot be subdivided, created, or destroyed. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. In chemical reactions, atoms are separated, combined, or rearranged.
Modern Atomic Theory Advances in instrumentation have allowed some aspects of Dalton s theory to be proven incorrect. Example: The thoughts that atoms are not divisible into smaller particles and have the exact same mass are incorrect. The two most important aspects of Dalton s theory still hold true All matter is composed of atoms. Atoms of any one element differ in properties than atoms of another element.
JJ Thomson, 1900 Discovered the charge to mass ratio for the electron Concluded that the electron has a very large charge-to-mass ratio Conducted Cathode-ray experiments to prove that atoms are divisible and composed of electrons. Proposed plum pudding model for the atom (not a solid mass). http://www.youtube.com/watch?v=2xKZRpAs WL8
Discovery of the Electron Scientific advances allowed for the discovery of smaller particles that made up atoms. Electric current was passed through gases via a cathode ray tube.
Milikan and the oil drop experiment Measured mass of droplets Applied charge to droplets through x-rays. Applied electric voltage to top and bottom plates Measured the amount of voltage it took to keep the oil droplets suspended. Was able to measure the charge of a single electron! http://www.youtube.com/watch?v=EV1owO1H2dA
Ernest Rutherford Discovered the nucleus of an atom with his gold-foil experiment. http://www.youtube.com/watch?v=XBqHkraf8iE Concluded that the volume of the nucleus was very small compared to the total volume of the atom.
Section 2 COUNTING ATOMS
Atomic Number The number of protons in each atom of a given element Unique to each element and does not change Used to identify elements. # Protons = # Electrons for neutral atoms of an element
Practice 1. How many protons and electrons are in each neutral atom? Bromine Titanium, Ti 2. A neutral atom of an element contains 50 electrons. Which element is it? 3. An atom of an element contains 14 protons. Which element is it?
Isotopes The amount of protons and electrons are constant in all neutral atoms of an element, but the number of neutrons can vary Isotopes atoms of the same element that have different numbers of neutrons Example: Hydrogen atoms have three different isotopes. Protium 1 proton/0 neutrons (accounts for 99.985% of all hydrogen atoms). Deuterium 1 proton/1 neutrons (0.015%) Tritium 1 proton/2 neutrons (radioactive, and can be produced artificially) Most elements contain mixtures of isotopes. Tin (Sn) has 10 stable isotopes, the most of any element. Mass Number--The number of protons and neutrons that make up the nucleus of an isotope. Since electrons are so small, their mass is considered insignificant.
Designating Isotopes Isotopes are usually identified by their mass number. Two Methods: Hyphen notation has hyphen after name of element, followed by the mass number. Example: Tritium is hydrogen-3 since it has 2 neutrons and 1 proton. Example: Uranium-235 Nuclear symbol Mass number written as superscript Atomic number written as subscript Followed by element symbol
In both methods The number of neutrons is found by subtracting the atomic number from the mass number. Example: Uranium-235 or 23592U (Mass #) (Atomic #) = (# of neutrons) 235 (protons + neutrons) 92 protons= 143 neutrons
Notation Practice 1. Give the number of protons, electrons, and neutrons in each of the following atoms. a) 19979 Au b) 5124 Cr 2. Name each isotope and write it in nuclear symbol and hyphen notation. a) atomic number 16; mass number 33 b) atomic number 35; mass number 80 3. How many protons, electrons, and neutrons are in each of the following isotopes? a) Nickel-60 b) Calcium-40
Practice. 1. Give the number of protons, electrons, and neutrons in each of the following atoms. a) Neon-22 b) Iron-57 c) 6430Zn 2. Write the hyphen notation for the following: a) p= 12, e=12, n=14 b) p = 28, e=28, n=29 3. Write the nuclear symbol for the following: a) p= 47, e=47, n=60 b) p= 4, e=4, n=4
Review Atomic Number = # of protons In a NEUTRAL atom: # of protons = # of electrons MASS NUMBER = # of protons + # of neutrons ISOTOPE: same elements with a different number of neutrons Hyphen Notation = Name of element mass number Nuclear Symbol = Mass Number Element Symbol Atomic Number
In your Notes Atomic Number, Mass Number, Protons, Electrons, Isotope, Neutron 1. The atomic number of a stable atom tells you how many ___and ____are in an atom. 2. The whole number found on the periodic table is called the _______________. It is unique for each element. 3. The total number of protons and neutrons in an atom is called the ______. 4. An atom with the same number of protons but different number of neutrons is called an _____ . 5. The subatomic particle that has no charge is called a _________________. Atomic Number Mass Number Number of Protons Number of Neutrons Number of Electrons Symbol of Element Hyphen Notation Nuclear Symbol 6 13 25 12 13 15
Relative Atomic Masses The masses of subatomic particles are extremely small and hard to work with, scientists created a new unit Atomic mass units (amu) based on a standard of carbon-12 that has a mass of 12 amu Neutron 1.008665 amu Proton 1.007276 amu Electron 0.0005486 amu Important to note that the masses are slightly different between protons and neutrons.
https://www.youtube.com/watch?v=ZtKu HxJXH6I Average Atomic Mass Average atomic mass the weighted average of the masses of the isotopes of that element Average atomic mass = (mass x abundance)isotope1 + (mass x abundance)isotope2+ 100 To find abundance divide the percentage by 100 (all abundances must be decimals) Average atomic mass can help you determine which isotope of that element is the most abundant 100
Atomic Mass Practice Boron has two naturally occurring isotopes: boron-10 (abundance 19.8%, mass 10.013 amu) and boron-11 (abundance 80.2%, mass 11.009 amu). Calculate the atomic mass of boron. 1. Calculate the average atomic mass of lithium, which occurs as two isotopes that have the following atomic masses and abundances in nature: 6.017 amu, 7.30% and 7.018 amu, 92.70%. 2. Nitrogen has two naturally occurring isotopes, N-14 and N-15. Its atomic mass is 14.007. Which isotope is more abundant? Explain. 3.
Recall What is the atomic mass of hafnium if, out of every 300 atoms, 15 have a mass of 176, 57 have a mass of 177, 81 have a mass of 178, 42 have a mass of 179, and 105 have a mass of 180.0? Iodine is 80% 127I, 17% 126I, and 3% 128I. Calculate the average atomic mass of iodine.
Relating Mass to Number of Atoms The Mole (mol) Since atoms are so tiny, we count them in groups Same as 1 dozen = 12 donuts Mole = SI unit for amount of substance. Avogadro s number The number of particles in exactly 1 mol of a pure substance. 6.022 1023 atoms = 1 mol of any substance. 6.022 1023 molecules = 1 mol of any substance.
Practice How many atoms are in 5.76 moles of a pure substance? How many moles are in 8.56 1026 atoms?
1.) How many moles of AlCl3 contain 1.00x1019 molecules? 2.) How many atoms are contained in 5.23 mmol of Mn? 3.) How many moles of Na contain 4.32x1022 atoms? 4.) How many molecules are contained in 0.3120 mol of BH3? 5.) How many moles of HCl contain 8.030x1023 molecules?
Molar Mass The mass (in grams) of 1 mol of a pure substance. Units are g/mol Equal to the sum of the atomic masses of each element of a compound. Example: K2CO3 2(39.10 g/mol) + 1(12.01 g/mol) + 3(16.00 g/mol) = 138.21 g/mol K C O Cu(NO3)2
Examples Find the molar masses of the following compounds: SrCO3 C13H18O2 Sn2(SO4)4
Gram/Mole Conversions - HONORS Use molar mass to convert moles to grams and vice versa. Practice: Convert 7.00 g of He to moles. Convert 5.21 mol of NaCl to milligrams. Convert 5.67 mol of Xe to grams. Convert 65.4 kg of K2CO3 to mol.
Gram/Mole Conversions - GENERAL Use molar mass to convert moles to grams and vice versa. Practice: Convert 7.00 g of He to moles. Convert 5.21 mol of NaCl to grams. Convert 5.67 mol of Xe to grams. Convert 65.4 g of K2CO3 to mol.
BELL RINGER - HONORS 1) How many milligrams are in 4.5 moles of sodium fluoride, NaF? 2) How many moles are in 98.3 grams of aluminum hydroxide, Al(OH)3? 3) How many moles are in 3.4 x 10-7 grams of silicon dioxide, SiO2? 4) How many micrograms are in 1.11 moles of manganese sulfate, Mn3(SO4)7?
BELL RINGER - General 1) How many grams are in 4.5 moles of sodium fluoride, NaF? 2) How many moles are in 98.3 grams of aluminum hydroxide, Al(OH)3? 3) How many moles are in 3.4 x 10-7 grams of silicon dioxide, SiO2? 4) How many grams are in 1.11 moles of manganese sulfate, Mn3(SO4)7?
Extra Practice Molar Mass: 1) Co(C2H3O2)2 2) NH3 3) Fe2(CO3)3 Compute 6.59 moles of NH3 to atoms Compute 5.28x1030 molecules Fe2(CO3)3 to moles Compute 9.27x103 g Co(C2H3O2)2 to moles
Bell Ringer Compute 6.59 moles of NH3 to atoms Compute 5.28x1030 molecules Fe2(CO3)3 to moles Compute 9.27x103 g Co(C2H3O2)2 to moles
REMEMBER! Molar Mass Avogadro s Number Molecules Atoms Moles Grams Mass must always be converted to moles before being converted to atoms (NEED 2 CONVERSION FACTORS)! Atoms must always be converted to moles before converting to mass (NEED 2 CONVERSION FACTORS)!
More Practice - General How many atoms are in 4.56 g of Na2CO3? How many atoms are contained in 4.33 g of Br2 What is the mass of 5.67 1032 atoms of NaOH?
More Practice - Honors How many atoms are in 4.56 kg of Na2CO3? How many atoms are contained in 4.33 g of Br2 What is the mass, in mg, of 5.67 1032 atoms of NaOH?
WhiteBoarding Practice 1. Determine the mass of 5.26 103 mol of potassium nitrate (KNO3). 2. Calculate the number of molecules in 2.31 moles of hydrogen. 3. Determine the mass of 2.90 1024 molecules of calcium fluoride (CaF2). 4. How many atoms are in a 71.2 g sample of iron (II) oxide (Fe2O3)? 5. Calculate the number of molecules in 5.34x105 mol of gallium. 6. How many moles are in 98.3 grams of aluminum cyanide, Al(CN)3?
WhiteBoarding Practice - Honors 1. Determine the mass of 5.26 103 mol of potassium nitrate (KNO3). 2. Calculate the number of molecules in 2.31 mmol of hydrogen. 3. Determine the mass of 2.90 1024 molecules of calcium fluoride (CaF2). 4. How many atoms are in a 71.2 g sample of iron (II) oxide (Fe2O3)? 5. Calculate the number of molecules in 5.34x105 mmol of gallium. 6. How many moles are in 98.3 kilograms of aluminum cyanide, Al(CN)3?
