Chapter 2: An Evolving Model

Transcription

1 Physical Science 4010 Nuclear Technology Chapter 2: An Evolving Model

2 Introduction The universe is made up of two things: Matter (physical, tangible stuff) Energy (more than just the ability to do work) In reality, matter and energy are the same things and can be converted into each other (with great difficulty). For the purpose of this course, consider them distinct (for now...). Take a chunk of pure matter (gold, iron, helium). Divide your chunk in half. Divide it in half again. Keep dividing it in half many, many, times... Eventually, you'll have just one particle of gold, iron, or helium. A single unit that you cannot divide in half. That is an atom.

3 Introduction An atom follows a simple model, in two parts: Nucleus: small volume, large mass Electron Cloud: large volume, small mass Today, we're quite confident that this description of the atom is correct. We have a lot of experimental evidence to support it. This model has made many successful predictions. nucleus electrons However, this is a recent development. History is full of failed atomic models. This chapter will review that history. It will also serve as an illustration of the scientific method.

4 The Ancient Greeks Leucippus, a philosopher, was the first on the scene. He had the idea that matter was made of infinitely small particles that could not be divided. He coined the word atom (atomos means indivisible in Greek). Democritus, his disciple, ran with those ideas. He made several specific claims: Atoms combine with one another in many different ways. Atoms are always moving. There are gaps between atoms (it is discontinuous). In fact, all these ideas are true, to an extent. However, this was all just a hypothesis, because Democritus had no way to test his ideas. Hypothesis: An idea or suggestion about how to explain something. With enough time and effort, we may find enough evidence to prove it. Then it becomes a theory.

5 The Ancient Greeks Aristotle and Empedocles disagreed with Leucippus and Democritus. They disliked the ideas of atoms and discontinuity. Empedocles proposed the four elements model. Basically, everything is made up of some combination of earth, air, fire, and water. Aristotle added the four basic phenomena to the model. Everything is influenced by moisture, dryness, cold, and heat. Again, although this sounds silly, the idea is sound. Matter really is made up of elements on the periodic table. However, there are many more than four. The problem is that all this was done by philosophers, not scientists. Why? Because the scientific method didn't exist yet. Therefore, science didn't exist. Philosophy is about thinking, not experimenting or observing.

6 The Renaissance For a couple of thousand years, atomic theory didn't advance. The Greeks were followed by the Roman Empire, which was more concerned with war and politics than with philosophy. When the Roman Empire fell, the Dark Ages started and they were a bad time all around. The Dark Ages ended with the Renaissance, when things became nice and bright again. The creation of the scientific method is formally credited to Rene Descartes, the same guy who gave us the Cartesian Plane. However, he just assembled ideas that had already been used informally for the past thousand years by philosophers and experimenters from all over the world.

7 The Scientific Method Although it has many definitions, the scientific method is essentially a way to gather new knowledge reliably. Start by formulating a hypothesis. Run experiments or make observations to test the hypothesis. Check if your results are consistent with the hypothesis. If they are consistent, great! Show your results to other people so they can repeat your tests to confirm your results. If they are not consistent, change your hypothesis and try again. Make a prediction that is consistent with your hypothesis. Check to see if your prediction is true. If it is true, great! Show your results to other people so they can repeat your tests to confirm your results. If it is not true, change your hypothesis and try again. Repeat this process until everyone in the scientific community agrees with your hypothesis. Congratulations! You now have a theory!

8 The Alchemists Alchemy was chemistry's little brother. Early scientists took a more systematic approach to converting one material into another material. They tried to convert lead into gold. They tried to find miraculous cures for diseases. They tried to achieve immortality. Of course, they failed miserably at all of it. But their systematic methods and recordkeeping laid a foundation for the real science of chemistry. Ultimately, the alchemists were looking for a quickfix to the problems of their times. Their objectives would require centuries of research to achieve (if ever), not just years.

9 John Dalton ( ) John Dalton was a chemist in the 19 th century. He was investigating the nature of chemical reactions and what makes different types of matter what they are. He combined two ideas: Democritus's idea of matter being divided into atoms. Empedocles's idea of matter coming in different elements. He created a new concept that some matter was actually a combination of different elements, called a compound. He also proposed that atoms of the same element are identical. For example... Oxygen is an element. The oxygen we breathe is made of two oxygen atoms. Water is a compound of one oxygen atom and two hydrogen atoms. H O O O H

10 John Dalton ( ) Dalton also measured the mass of many elements. He managed to determine which elements were heavier and which ones were lighter. This was a first step towards the periodic table of the elements. However, Dalton was still limited by 19 th century technology. He couldn't measure the real mass of atoms. He arbitrarily gave hydrogen, the lightest element, a mass of 1. Everything else was measured relative to hydrogen. He also couldn't visualize atoms, or determine their structure. All he knew is they were small particles with different identities.

11 Sir Joseph Thomson ( ) Sir Joseph Thomson was a British physicist studying electricity. He set up an electric current between two electodes in a vacuum tube. However, when he placed the negative end of a magnet near the current, it got pushed away (like charges repel each other). + Thomson concluded that electricity was made of negative particles, which he called electrons. He also concluded that electrons were part of the atom. And since atoms have no overall charge, there must also be positive particles (foolishly, he didn't name them).

12 Sir Joseph Thomson ( ) Thomson was still limited by technology, though. He realized that the atom had both positive and negative charge. However, he couldn't speculate on how they were arranged. He assumed that the atom was a big, positivelycharged sphere with negatively charged particles scattered inside. He called this the plumpudding model of the atom, which is a British dessert with raisins in it. The pudding represents the positive sphere, while the raisins represent the electrons scattered within the sphere

13 Ernest Rutherford ( ) Ernest Rutherford was a colleague of Thomson. To test Thomson's plumpudding model, he ran an experiment. Rutherford fired a beam of small, fast, positive particles at a thin sheet of gold. If the plumpudding model was correct, then the solid atoms should act like a brick wall and block the positive particles. But no! Most of the beam went right through the foil, like it wasn't there... Only a few particles bounced back.

14 Ernest Rutherford ( ) Rutherford concluded that the atom was not, in fact, solid, but mostly made up of empty space. The empty space allowed most of the beam to pass right through. The positive particles of the atom (which he name protons ) were in a tiny nucleus in the core of the atom. Very occasionally, the positive beam would hit the nucleus and bounce back, repulsed by the same positive charge. Note: This is considered one of the most important experiments in the history of science, for two reasons: It was extremely elegant and simple to carry out. It provided a huge insight into the structure of the atom.

15 Niels Bohr ( ) Rutherford had established that the protons were concentrated in the nucleus of the atom, but where exactly were the electrons? Niels Bohr was a Danish physicist. He proposed that the electrons occupied definite orbits around the nucleus. He named these rings energy levels or shells. We ll speak about these in more detail later

16 James Chadwick ( ) But if protons are positively charged and they are all concentrated in the nucleus, what keeps them from flying apart due to the repulsion of having the same charge? James Chadwick, a student of Ernest Rutherford, proposed that there was yet another particle involved that had no charge. He called this a neutron, for neutral. They are found inside the nucleus, with the protons. They use a force of attraction that would only be discovered later, called a nuclear force, to overcome the force of repulsion between protons

17 Quantum Theory Later in the 20 th century and up to today, scientists have used more and better technology to refine atomic theory even more. Quantum theory is the modern version, which describes protons, electrons, and neutrons as waves rather than particles. Much of the recent research was at the Large Hadron Collider. A huge ring, 8 km in diameter and several meters thick, buried about 100 meters below ground. Underneath the FranceSwitzerland border, near Geneva. The single largest machine ever built. Different particles are accelerated up to enormous speeds and smashed into each other to see what happens.

18 The Modern Simplified Atomic Model The model of the atom we use currently is essentially that proposed by Chadwick. The proton, neutron, and electron are three of the many subatomic particles, particles that are smaller than an atom. Protons and neutrons are collectively called nucleons, because they are found in the nucleus An atom always has as many electrons as it has protons, so the charge balances.

19 The Modern Simplified Atomic Model Some factoids about atoms and their components... Electrons are much smaller than protons and neutrons. A proton's mass is 1840 times greater than the mass of an electron. A neutron's mass is slightly greater than the mass of a proton. In fact, it's approximately heavier by the small mass of an electron So basically, m neutron = m proton + m electron The charge on a proton and electron are equal and opposite. So basically, charge neutron = charge proton + charge electron = 0 It's almost as if a neutron is just a proton and electron combined...

20 The Modern Simplified Atomic Model Some factoids about atoms and their components... Diameter of a nucleus is about cm. That's cm Diameter of an atom is about 5x10 8 cm. That's cm Therefore, the whole atom is 50,000 times bigger than the nucleus Also, the nucleus accounts for about 99.97% of the mass of the atom.

21 Electron Configurations Electrons occupy specific circular paths around the nucleus. What Bohr called energy levels or shells. Each energy level has a limited capacity of how many electrons it can hold. 1 st level: 2 electrons 2 nd level: 8 electrons 3 rd level: 8 electrons We would write the electron configuration of Argon (atomic number 18) this way: Argon: 2e, 8e, 8e 18 p+ 22 n Argon

22 Electron Configurations Electrons will always try to occupy the lowest available energy level. Therefore, we can build the electron configuration by counting up. For example: 1 p+ 0 n 12 p+ 12 n Hydrogen: 1e 7 p+ 7 n Magnesium: 2e, 8e, 2e Nitrogen: 2e, 5e Page 2.29, exercise

23 Summary (p.2.38) Ancient Greeks Leucippus: Matter is discontinuous, made of indivisible atoms. Democritus: Atoms are always moving and combine to make different materials. Empedocles: Matter is continuous and made of four elements. Aristotle: Matter is also made of four phenomena. Alchemists Had crazy objectives but reasonable techniques. Started modern day chemistry. Modern science Dalton: Brought back ideas of atoms and elements. Thomson: Discovered electron and created plum pudding model. Rutherford: Discovered proton and nucleus with his goldfoil experiment. Bohr: Discovered that electrons exist in energy levels. Chadwick: Discovered the neutron in the nucleus.