Chemistry Objective: SWBAT describe the changes made to the model of the atom over time. Chemistry Warmup:

Transcription

1 Chemistry Objective: SWBAT describe the changes made to the model of the atom over time. Chemistry Warmup: 1. Pick up a set of the notes for today from the first lab table. 2. Take out your lab activity from yesterday.

2 Analyze & Conclude 1. How did the mass of the product, zinc chloride, compare with the mass of the zinc? 2. How can you account for the difference in mass? - remember that according to the law of conservation of mass - the mass of a compound is equal to the sum of the individual masses of each element making ][[=it up. 3. What happened to the zinc`? I know that it dissolved - so don t tell me that. Think about what actually happened to it. 4. Why were the flask and its contents heated? remember your separation techniques. Apply and Assess 1. Chemists have determined that zinc chloride is 48% zinc. Use this information to compute the mass of zinc in your product. How does this mass compare with the mass of zinc you started with? example-> if your total mass of the ZnCl2 was 0.45 grams then 0.45 x 48% = grams of zinc should have been your started amount 2. If the difference in the question above is greater than 0.04 g, how can you account for it? 3. How does this experiment support the law of conservation of matter/mass? 4. How could you modify the procedure so that the law of conservation of matter/mass is better demonstrated?

3 The first Atomic Model, proposed by John Dalton in the early 1800 s, was based upon three laws. Law of Definite Composition -> states a compound contains the same elements in exactly the same proportions by mass regardless of size of sample or source of compound. ex. Sucrose - no matter how big the sample is 42.1% carbon, 51.4% oxygen and 6.5% hydrogen. Law of Conservation of Mass -> states that when two or more elements react to produce a compound, the total mass of the compound is the same as the sum of masses of the individual elements. ex. 24 g Mg + 32 g O2 --> 56 g MgO Law of Multiple Proportions -> applies to different compounds made from the same elements and states that the mass ratio for one of the elements that combines with a fixed mass of other elements can be expressed in small wholenumbers. ex. H2O = 16 g O : 2g H H2O2 = 32 g O : 2 g H 8 g O : 1 g H 16 g O : 1 g H

4 Models of the Atom In the early 1800's, John Dalton was the first person to develop a model of the atom. Experimental data at the time led Dalton to propose five (5) postulates as to the nature of the atom. Dalton's 5 Postulates Matter consists of definite particles called "atoms." Atoms are indestructible. In chemical reactions, the atoms rearrange but they do not themselves break apart. The atoms of one particular element are all identical in mass (and other properties). The atoms of different elements are different in mass (and other properties). When atoms of different elements combine to form compounds, new and more complex particles form. However, their constituent atoms always are present in a definite numerical ratio. Law of Multiple Proportions This picture is a good representation of what Dalton believed to be true.

5 Chemistry Objective: SWBAT describe the changes made to the model of the atom over time. Word of the Day: Law of Definite Composition: The principle stating that the proportion of elements in a specific compound is a fixed quantity.

6 Since Dalton based his model on chemical evidence, he was did not know about the electrical nature of the atom. But his model of the atom was an important beginning. Approximately 100 years after Dalton's work with the model of the atom, evidence was becoming quite clear that atoms were not solid spheres. Atoms had components. By 1903, it had been discovered that the atom was not the smallest particle of matter. In 1897 the physicist J. J. Thomson developed a series of experiments designed to study the nature of electric discharge in a highvacuum cathode-ray tube an area being investigated by numerous scientists at the time.

7 Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of "bodies much smaller than atoms" that he calculated as having a very large value for the charge to mass ratio. Later he estimated the value of the charge itself. In 1904 he suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. Since the particles in the beam were bent towards the positive plate Thomsom could deduce that the beam itself was made up negative charges. He also placed a small pinwheel in the CRT itself and was able to observe that the pinwheel spun because it was being hit by something. From this Thomson proposed a new model of the atom that he named after an English dessert of that time. His Plum Pudding model of the atom consists of small pockets of negative energy located within a ball of positive matter, very similar to the blueberries in a blueberry muffin.

8 About 10 years later Ernest Rutherford tested Thomson's hypothesis by devising his "gold foil" experiment. Rutherford reasoned that if Thomson's model was correct then the mass of the atom was spread out throughout the atom. Then, if he shot high velocity alpha particles (helium nuclei) at an atom then there would be very little to deflect the alpha particles. He decided to test this with a thin film of gold atoms. As expected, most alpha particles went right through the gold foil but to his amazement a few alpha particles rebounded almost directly backwards. Rutherford s Gold-Foil Experiment

9 Ernest Rutherford interpreted these results and suggested a new model for the atom. He said that Thomson's model could not be right. The positive charge must be concentrated in a tiny volume at the center of the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source. On this model, the electrons orbited around the dense nucleus (center of the atom). So please remember the nucleus of the atom contains ALL of the Positive Charge in the atom, MOST of the Mass of the atom and VERY LITTLE of the Volume of the atom.

10 However, the naive planetary model has several difficulties, the most serious of which is the loss of energy by synchrotron radiation. That is, an accelerating electric charge emits electromagnetic waves which carry energy; thus, with each orbit around the nucleus, the electron would radiate away a bit of its orbital energy, gradually spiraling inwards to the nucleus until the atom was no more. Niels Bohr He also knew about the existence of line spectra from chemical elements and stumbled across Balmer's numerology for the hydrogen spectrum, and in a flash came up with a workable model of the atom. The model asserts that: The planetary model is correct. When an electron is in an "allowed" orbit it does not radiate. When an electron absorbs energy from incident electromagnetic radiation, it "quantum jumps" into a higher energy allowed state. When an electron is in a higher energy state, it can quantum jump into a lower energy state, emitting all of its energy as a single photon of electromagnetic energy.

11 Bohr s Model of the Atom This model isn't concerned with a physical picture of electron motions, so it is hard to describe visually, but perhaps the picture of a staircase will help. A ball sitting on a stair has to be on one step or another. It can't be in between steps. If it falls down one step, it loses energy. Energy must be supplied to lift it up a step. Notice how the electrons are within a given orbit which corresponds to a certain energy level. Bohr's model gave a very satisfactory understanding of the way atoms and light are related, and it led to a very successful understanding of the way electrons are involved in chemical reactions

12 Chemistry Objective: SWBAT describe the changes made to the model of the atom over time. Word of the Week Assignment: Using the words of the week, DESCRIBE the laws John Dalton used as a basis for his atomic model and how they are related to that model.

13 Chemistry Objective: SWBAT describe the changes made to the model of the atom over time. Warm-up: Identify the various models of the atom based on the following pictures. A B C

14 Image standing on the side of a lake with a stone in your hand. When that stone is thrown into the water it causes ripples. Now image your friend standing only a few feet away with another stone. If both of you throw your stones at the same time - the ripples will do two things: 1. they will come together and magnify each other or 2. they will collide with each other and cancel each other out. Light behaves in much the same way. If it passes through slits, some of the waves will amplify each other and some will cancel each other. But what if the light is made up of something or has a particle nature. Erwin Schrödinger showed that electrons are really waves that have properties like a particle. What do you mean electrons are waves?! I thought they were particles!

15 Electron cloud is a term used for introducing the concept of wavefunction. This idea corresponds to delocated electrons moving or standing like clouds around the atomic nuclei. This is indeed a better image than the very common image provided by the Bohr model. This representation is related to the idea that the electrons are not precisely located around the atomic nuclei but must instead be described by probability amplitudes or wave functions which provides the probability to find electrons in a given region of space. These mathematical functions of the coordinates of all electrons are often expressed in terms of electron configurations which are in turn expressed in terms of atomic orbitals. So the electrons no longer exist on a specific path but more in region of space around the nucleus or an orbital. The probability of finding a specific electron in that orbital is said to be about 90%. Heisenberg's Uncertainty Principle Werner Karl Heisenberg stated in 1927 that certain specific pairs of variables cannot be measured simultaneously with high accuracy. Most importantly, he pointed out that within an atom, it is possible to measure the position, or the momentum, of a subatomic particle such as an electron. However, it is not possible to measure both of them at the same time, because the measuring process interferes to a substantial degree with what is being measured.

16 So the Quantum Theory of the Atom, also known as the Electron Cloud Theory is more concerned with describing the electrons of the atom than the nuclues itself. The Quantum Theory uses quantum numbers to describe the location of the electrons. n Quantum Numbers Principal Quantum Number describes the energy level of the electron l ml ms Secondary Quantum Number Tertiary Quantum Number Spin Quantum Number describes which sublevel within the energy level describes the axis of the electron describes the spin of the electron Pauli Exclusion Principle -> This principle basically states that no two electrons in the same atom can have the same four (4) quantum numbers.

17 Atomic Theory Review 1. According to Dalton s Model of the Atom, why would isotopes not exist? 2. What properties of the electron were determined by J.J. Thomson in his experiments using cathode-ray tubes? 3. What unexpected results did Rutherford s gold-foil experiment produce? How did Rutherford explain these results? 4. Describe a significant weakness in Rutherford s model of the atom. 5. What is an energy level of an atom? Why are energy levels described as quantized? 6. What is an orbital?