The (Almost) Modern Atom Model. SUMMARY from previous lecture Energy comes in different forms. ENERGY IS ALWAYS CONSERVED.

Transcription

1 Announcements The midterm 1 exam scores will be available on OWL at 5:00 PM today. Login to OWL and click the Course Grades link to see your score. Your score has already been scaled to correspond to the usual grading scale: : A 80-89: B 70-79: C 60-69: D <60: F OWL Homework 3 will be available tomorrow around noon. Homework 3 will be due Sunday, Oct. 19 before midnight. Astro 101, 10/9/2008 Do the behaviors of matter and energy, observed on the Earth, apply to the whole cosmos? What do we know about matter and energy on Earth? SUMMARY from previous lecture Energy comes in different forms. Kinetic Energy Bulk motion Thermal Energy Potential Energy Gravitational potential energy Chemical potential energy Electrical potential energy Nuclear potential energy Mechanical potential energy Radiative Energy (Light) We ll spend at least one entire class on this one. ENERGY IS ALWAYS CONSERVED. Some forms of energy are more easily controlled than others, e.g., chemical potential energy can be easily stored for a rainy day, but heat is hard to hold on to Electrical charge: a fundamental property of matter Oppositely charged particles attract each other Similarly charged particles repel each other Like energy, charge is always conserved The proton defines the basic unit of charge: Proton charge = +1 Electrons have the precise opposite charge of the proton electron charge = -1 Most ordinary atoms have equal numbers of protons and electrons and consequently zero net charge, i.e., they are electrically neutral The (Almost) Modern Atom Model nucleus p + n proton (positive charge) neutron ( neutral - no charge) electron (negative charge) 1

2 A particular element always has the same number of protons, i.e., the #protons defines the element. Hydrogen has 1 proton. Carbon has 6 protons, oxygen has 8, etc. The vast majority of the mass is in the nucleus. nucleus p + n electron (negative charge) The size of an Atom Although it is the smallest part of the atom, most of the atom s mass is contained in the nucleus. The electrons do not orbit the nucleus; they are smeared out in a cloud which give the atom its size. proton mass = 1836x electron mass neutron mass = x proton mass Hydrogen Atom Magnified by Periodic Table of the Elements There are atoms in a drop of water. This is comparable to the number of stars in the entire Universe! (nucleus) atomic number = #protons atomic mass no. = #protons + #neutrons 2

3 Hydrogen Helium p + p + p + n n atomic number = 1 atomic mass number = 1 atomic number = 2 atomic mass number = 4 Hydrogen Deuterium isotope p + n What if an electron is missing? ion p + p + n n atomic number = 1 atomic mass number = 2 atomic number = 2 atomic mass number = 4 He +1 3

4 What if two or more atoms combine to form a particle? molecule Phases of Matter Matter can take many forms depending mostly on its temperature, density, and internal energy. p + p + 8p + 8n H 2 O (water) As temperature increases, i.e., as the internal kinetic energy is increased, the strength of the bonding between particles decreases. Phases of Matter Depend on: TEMPERATURE Increasing temperature increases the average speed of the particles at the microscopic level It is more difficult for particles to bond when moving more rapidly Phases of Matter Depend on: DENSITY density = mass volume The higher the density, the greater the probability that the particles will collide and stick together As temperature increases, i.e., as the internal kinetic energy is increased, the strength of the bonding between particles decreases. When matter is compressed, its density increases and it can change from gas to liquid to solid. 4

5 Phases Transitions of Matter Many phase transitions are quite familiar: Freezing: liquid solid Melting: solid liquid Boiling: liquid gas However, substances never have a single temperature; there is always a range of temperatures in matter. Consequently, we can also have: Sublimation: particles escape from the solid phase directly into the gas phase Evaporation: most particles are cool and remain in the liquid phase, but a few have enough energy to escape to the gas phase Who cares? Physicists (astronomers are just a type of physicist) seem to be obsessed with boring details about temperature, density, blah, blah, blah Why is all this stuff important? Consider what happens when we cool things down using liquid nitrogen If we smack a liquid with a hammer, it can absorb the energy in a variety of ways... If we smack a solid with a hammer, there are fewer ways that it can absorb the energy Energy from impact Energy from impact Weaker bonds here After impact, loosely bound particles in the liquid phase can splash around, and absorb the energy Something has to give. If there is a place where the bonds are weaker, that s where the solid will break apart. 5

6 The world is strange on the microscopic level. Consider how the energy of an object can change on the macroscopic scale: A 100 kg truck moving at 100 km/s has an easily calculated amount of kinetic energy: KE = 1/2 mv 2 = 5 x 10 5 Joules We can change the speed of the truck by any amount, e.g., we can increase the speed to 101 km/s, km/s, 150 km/s, (whatever) and the kinetic energy will change accordingly. Likewise, we can lift the truck off the ground to any height, and its gravitational potential energy will increase according to a simple equation: GPE = mhg GPE = mass x height x gravitational acceleration One might think an electron on an atom would behave the same way, but it does NOT Hydrogen p + atomic number = 1 atomic mass number = 1 The electron bound to an atom is not simply a mass moving at any velocity. The electron is attracted to the nucleus due to its opposite charge. Therefore one might expect it to have potential energy analogous to gravitational potential energy. The bound electron can store potential energy, but in a more complicated way than gravitational potential energy. Atom Energy Levels nucleus Energy Electrons can be in different orbits of certain energies, called energy levels. Different atoms have different energy levels, set by quantum physics. Quantum means discrete! The electron energy levels are like a staircase... The electron can step from one level up or down to another level, but the bound electron cannot have an energy between levels Allowed energy levels for the bound electron Bound electrons prefer to be in the lowest energy level, which is called the ground state. Electron cloud: Add just the right amount of energy, and the electron will jump to a higher energy excited state. If enough energy is given to the electron, it will be removed from the atom altogether resulting in an ION and a FREE ELECTRON. Unlike excitation, ionization does not require a discrete quantum amount of energy. The energy only needs to exceed some amount called the ionization potential which depends on the atom being ionized. 6

7 Review: phases of matter Plasma: free electrons and free nuclei. Ionization: atoms broken apart. Dissociation: molecules broken appart. Gas. Liquid. Solid. 7