2Objective:You will explain why the model of the atom changed throughout history.
3What is a model?Models are used to help us understand things that cannot be seen directlyModels are used when something is too large, too small, or too dangerous to be studied directly.
4Examples of Scientific Models This model shows the alignment between the sun, moon, & earth. As it rotates, it shows the phases of the moon and how we measure a year.This model is a mathematical representation of a sound wave. You cannot see sound, but you can see how it affects other objects with its vibrations.
5Examples of Scientific Models What other examples can you think of?Are there other models present in this room?
6A good model…Must be based on observations and indirect experimentation.Must explain as many characteristics of the original object as possible.Should be as simple as possible.
7When do you change a model? All models have limitations —No model has ever been totally complete.A model changes when observations of a new situation do not agree with the current model.
8Creating a ModelThe “Think Tube” is also a model for something you cannot see directly.?THINK TUBE
9Creating a Model How many strings are on the inside? Make your own model showing howthe “Think Tube” works.
10AtomsThe atom has not changed over time, but our idea and model of the atom has.
11Definition of Atom:the smallest particle of an element that retains the chemical properties of that element.
12Democritus 400 BCBy convention there is color, By convention sweetness, By convention bitterness, But in reality there are atoms and space. -Democritus (c. 400 BCE)Convention means because we said so- doesn’t really exist
13Democritus 400 BCDemocritus was smashing up sea shells one day and thought that you can break down the shell to tiny pieces, but it can not be completely destroyed.
14greek word meaning cannot be cut Democritus 400 BCLooked at sand on the beach. Cut sand in half and got fewer and fewer grains of sand.What was the smallest piece?He called it atomos =greek word meaning cannot be cut
16Democritus 400 BC According to Democritus atoms are: InvisibleIndivisibleSolidEternalSurrounded by an empty spaceinvisibleindivisible(no void inside)eternal because they are perfectto explain their movement and changes in densityto explain the diversity observed in nature)
17Democritus 400 BC continued: Have an infinite number of possibleshapes.Each type of atom had a different size.
19Aristotle’s Idea 300 BCAll substances are made of 4 elements: Fire, Air, Earth, and Water
20Aristotle’s Idea 300 BCThere were also four qualities: dryness, hotness, coldness, and moistness.Fire was dry and hot, while water was moist and cold, etc.Each of these elements move naturally in a line to their "proper place," where it will be at rest.
22Aristotle 300 BCWater sits on top of the earth, he explained, because it is lighter, yet air floats above the water because it is lighter still—and fire, lightest of all, rises highest. Furthermore, he claimed that the planets beyond Earth were made up of a "fifth element," or quintessence, of which little could be known.
23Democritus vs Aristotle Ancient Greeks accepted Aristotle’s ideas and rejected Democritus.What holds the particles together?Democritus could not answer this questionRemained that way until the 17th century
24Important Discoveries Law of Conservation of MassLaw of Definite ProportionsLaw of Multiple Proportions
25Law of Conservation of Mass (Matter) Lavosier measured the mass of chemicals before and after a chemical reaction and found that the weight did not change.In a chemical reaction, matter is neither created nor destroyed.
26Law of Definite Proportions Also called Law of Constant CompositionProposed by Joseph ProustElements always react and combine with one another in the same proportions.
27Law of Definite Proportions A chemical compound is always composed of the same combination of atoms -copper carbonateCuCO3
28Law of Definite Proportions Water has the formula H2O. This means that water in the ocean, lakes, or in our sinks always contains 2 atoms of Hydrogen for every 1 atom of Oxygen. What percent of water is Hydrogen and what percent is Oxygen?H: 2g/18g = 11% O: 16g/18g = 89%
29Law of Multiple Proportions If two elements form more than one compound between them, then the ratios of the weights of the two atoms will be ratios that can be reduced to small whole numbers.
31John DaltonQuakerColor BlindrEnglish school teacher and public lecturer by the age of 12.As a Quaker, Dalton led a modest existence, although he received many honors later in life. In tribute, more than 40,000 people marched in his funeral procession.Quaker1 in 300 could readColor blind - of color-blindness came to be known as Daltonism
32John Dalton’s Atomic Theory Dalton’s Theory was a return to the ideas of DemocritusDalton turned the idea into a scientific theory that could be testedNot all of Daltons ideas are still true today. Some ideas were modified.
33John Dalton’s Atomic Theory: All matter is composed of atoms.Atoms of a particular element have identical properties. Elements of a different element have different properties.Atoms cannot be divided or destroyed.Atoms combine to form compounds.During a chemical reaction atoms are rearranged.
34John Dalton’s Atomic Theory: All matter is composed of atoms.Atoms of a particular element have identical properties. Elements of a different element have different properties.Atoms cannot be divided or destroyed.Atoms combine to form compounds.During a chemical reaction atoms are rearranged.
35JJ ThomsonExcuse me... how can you discover a particle so smallthat nobody has ever seen one?J.J. was very awkward with his fingers, and I found it very necessary not to encourage him to handle the instruments! But he was very helpful in talking over the ways in which he thought things ought to go." -- H. F. Newall, onetime assistant to the young Professor Thomson
57Thomson's “Plum Pudding Model" Electrons are red.Negative electronsAre embedded in aBlue positive atom.
58All atoms contain electrons. J.J. Thompson’s ModelCathode rays are beams of negatively-charged particles called electrons.All atoms contain electrons.Atoms also contain an equal and opposite positive charge.
59Ernest Rutherford"All science is either physics or stamp collecting."Rutherford, Ernest ( ): Born in New Zealand, Rutherford studied under J. J. Thomson His work constituted a notable landmark in the history of atomic research as he developed Bacquerel's discovery of Radioactivity into an exact and documented proof that the atoms of the heavier elements, which had been thought to be immutable, actually disintegrate (decay) into various forms of radiation.Rutherford is best known for devising the names alpha, beta, and gamma rays to classify various forms of "rays" which were poorly understood at his time (alpha and beta rays are particle beams, while gamma rays are a form of high-energy electromagnetic radiation ).Cavendish Laboratory, in CambridgeStudied under JJ ThomsonWorked with Gieger, Bacquerel's
60Ernest Rutherford’s Model He became a student of a teacher named Professor J J Thomson at Cambridge University in England
61Ernest Rutherford’s Model Rutherford studied how gold atoms interacted with radioactivity.
62Radioactivity is the processes by which unstable atoms emit subatomic particles(radiation).
63Made of 2 protonsand 2 neutrons.Charge of +2, and amass of 4Relatively slow andheavy.
64Charge of minus 1Mass is very small.They are the same asan electron.They are fast, and light.
65Gamma rays arewaves, not particles.They have no massand no charge.
66Penetrating PowerAlpha particles are easy to stop, gamma rays are hard to stop.
67FluorescentScreenLead blockUraniumGold FoilHere’s how it looked.
69Rutherford Expected:The alpha particles to pass through without changing direction.Because…?…the positive charges were thought to be spread out evenly. Alone they were not enough to stop the large alpha particles.
81Rutherford’s Atom1. Most of the mass of an atom must be located in a small volume at the center of the atom (the nucleus). 2. The nucleus is made of positively charged particles called protons. 3. The electrons move in a large volume which is mostly empty space.
82Problems with Rutherford’s Atom According to "classical" theory the electrons should lose energy by radiating electromagnetic radiation, as they are accelerated electric charges.They should spiral into the nucleus.
842 years after Rutherford…. Neils Bohr 1885 - 1962 Born in Sweden -Went to England to study at Cambridge but he didn’t like ThomsonHe met Rutherford and liked him so he started working with himCame up with the idea of energy levelsBecause he was part Jewish when Hitler came to power and invaded Sweden he had to leave. He first went to England then he came to Los Alamos
89ChadwickIn 1932, Chadwick proved the existence of neutrons - elementary particles devoid of any electrical charge.Located in the nucleus(Rutherford also put out the idea that there could be a particle with mass but no charge)EnglishWorked under RutherfordLater, he found out that a German scientist had discovered the neutron at the same time. But Hans Falkenhagen (Rostock) was afraid of publishing his results. When Chadwick learned of Falkenhagen's discovery, he offered to share the Nobel Prize with him. Falkenhagen was modest and refused this honour.
91Chadwick’s Atom Modern Atom 1. Most of the atom's volume is occupied by electrons. 2. The number and arrangement of electrons in an atom determine its chemical properties.
92Chadwick’s Atom3. The identity of an element is determined by the number of protons in the nucleus.4. Different isotopes of elements exist and differ only in the number of neutrons and hence the mass of the atom.
105An isotopes contribution is determined by its relative abundance. Atomic Mass-An isotopes contribution is determined by its relative abundance.
106Cs 133 55 EXAMPLE Atomic number = protons and electrons How many protons, neutrons and electrons are found in an atom of13355CsAtomic number = protons and electronsThere are 55 protons and 55 electronsMass number = sum of protons and neutrons133 – 55 = 78There are 78 neutrons
120IsotopesAtoms of a given element with differing numbers of neutrons are called isotopes.
121IsotopesAn atom is still the same element if it is missing an electron. The same goes for isotopes. They are still the same element. They are just a little different from every other atom of the same element.
122An isotopes contribution is determined by its relative abundance. Atomic Weight-An isotopes contribution is determined by its relative abundance.
123Atomic numbers are whole numbers Mass numbers are whole numbersThe atomic mass is not a whole number.
124Calculating Atomic Mass (% abundance of isotope 1)(mass of isotope 1) +(% abundance of isotope 2)(mass of isotope 2) +(% abundance of isotope 3)(mass of isotope 3) + ...
125Calculating Atomic Mass Answer the following questions:"How many naturally occurring isotopes does carbon have?""What is the abundance of each of the isotopes?"
126The sum of all the fractions of abundance IsotopeAtomic MassRelative AbundanceC-1212.0098.93C-1313.001.07The sum of all the fractions of abundanceof each naturally occurring isotopesshould equal 1.00 or 100%.
127atomic mass of carbon =(0.9893)( amu)+(0.0107)(13.00 amu)= amu amu= amu
128What is the atomic mass of Lithium Isotope Atomic RelativeMass AbundanceLiLi
129What is the atomic mass of Lithium 0.0759*6.015 =* =0.45656.48286.9393