Defining the Atom • Elements are made of ATOMS • ATOM: The smallest particle of an element that retains the properties of the element
BELIEVING IN THE ATOM • For a long time, we had to INFER atoms existence through scientific experiments. • Now, we have the technology to SEE atoms • SCANNING TUNNEL MICROSCOPE
How small is an atom? • Now I'd like you to picture yourself on the beach. Reach down and pick up a handful of sand. How many grains of sand do you see? Could you even begin to count the number of grains of sand you are holding in that one handful? Imagine picking out one single grain of sand. There are possibly more atoms within that single grain of sand than there are grains of sand on the entire beach. • About 5 million atoms could fit in the period at the end of a sentence.
Dalton’s Flaws • Two of his points were found to be FALSE • Atoms can be broken into subatomic particles • Some atoms of the same element can have different properties
Electrons • Atoms are made up of smaller particles • Electron: The negatively charged particles of an atom • All atoms have e- (electrons)
Cathode Ray Tube • Scientists pass electricity through a glass tube with NO AIR inside (vacuum) • PURPOSE: to study the behavior of electricity without matter. • Helped DISCOVER THE ELECTRON
Cathode Ray Tube • The glass tube is hooked up to a battery. • Negative plate (cathode) hooks up to - part of battery • Positive plate (anode) hooks to + side of battery
Battery Cathode Ray + - Vacuum tube Metal Disks
Sir William Crookes • Saw a RAY going through tube • He observed that it originated from the cathode. • He called it the CATHODE RAY
BATTERY + - • Passing an electric current makes a beam move from the negative to the positive end
Battery - + • Adding charged plates outside the tube caused the beam to be deflected • Why was the beam deflected towards the positive plate?
WHAT IS THE BEAM?? • The CATHODE RAY consists of CHARGED particles • Because they came FROM the negative end (cathode) TO the positive end (anode) MUST BE – • Because they were deflected TOWARDS + plate, MUST BE – (opposites attract) Negatively charged particles = ELECTRONS http://www.youtube.com/watch?v=O9Goyscbazk http://www.youtube.com/watch?v=7YHwMWcxeX8&feature=related
Oil-drop experiment • Millikan • Sprayed oil droplets that fell btw CHARGED plates (- plate is on the bottom) • Different amounts of charge changed the droplet’s rate of fall • More charge = falls more slowly (repelled by - plate)
Millikan’s Experiment • Determined the charge of an electron is -1
Mass of an Electron • He used the charge and the charge to mass ratio to mathematically figure out the mass of an electron. • Mass of electron = 9.1 x 10-28 kg • 1840 times SMALLER than the smallest atom
Structure of the Atom (a guess) • Atoms are NEUTRAL so if they have electrons, they must also have + charge • How are electrons arranged in an atom??
Thomson’s Model - Thought the atom was arranged like PLUM PUDDING - Electrons spread evenly throughout an atom’s positive interior
QUESTIONS… • Draw the Plum Pudding model… • Why is this model of the atom called “plum pudding?”
Rutherford • Ernest Rutherford studied alpha particles (+ charged) • He shot a narrow beam of alpha particles at a thin sheet of gold foil
Rutherford • Because + and – charge were thought to be evenly distributed, + particles should go straight through foil (not attracted or repelled)
Gold Foil Atoms Alpha Particle Beam
WHAT HE SAW…Rutherford’s experiment • Most particles passed straight through the foil as if the foil was not there (as expected) • A very small fraction of alpha particles were deflected in ALL directions… some even completely reversed direction
An animation • http://www.shsu.edu/~chm_tgc/sounds/pushmovies/l2ruther.gif
What this tells Rutherford • Alpha (+) particles should not get deflected dramatically by even distribution of charge • There must be a dense, massive concentration of positive charge in the center
+ What was happening on an atomic level… Positive Center Alpha beam deflected because positive charges repel
Rutherford’s Conclusion http://www.youtube.com/watch?v=5pZj0u_XMbc http://www.youtube.com/watch?NR=1&v=ecsgC1wSp5I • Most of atom is empty space • There is a concentrated positive center: NUCLEUS • Nucleus is DENSE and has most of atom’s MASS (electrons are MUCH lighter than nucleus)
Rutherford’s model • Electrons stay with the atom because they are attracted to the positive center • Electrons orbit the nucleus – similar to planets around the sun
Rutherford eventually concluded… -Nucleus- • Contains POSIVITELY charged particles called PROTONS and NEUTRALLY charged particles called NEUTRONS http://www.youtube.com/watch?v=lP57gEWcisY&feature=related
Neutrons • Neutrons are the other particle in the nucleus • They have no charge • Its mass is nearly equal to that of the proton
Homework… • Pg 114 #7, 8, 9 and 10
Atomic Number - Atomic Number = # protons in an atom - # of protons determines kind of atom (atoms of an element always have the SAME number of protons) • The atomic number= the number of electrons in a neutral atom • Written just above the atomic symbol on the periodic table
Isotopes • All atoms in an element have the same number of protons • The number of neutrons vary • Atoms of an element with different numbers of neutrons = ISOTOPES
http://www.youtube.com/watch?v=Jdtt3LsodAQ&feature=related Isotopes • Elements in nature = a mix of isotopes • The abundances of different isotopes are constant • EX: in ANY banana, 93% K has 20 neutrons, 7% have 22 neutrons
Mass of atoms • One atomic mass unit(amu) is defined as 1/12th the mass of a carbon-12 atom. • One amu is nearly, but not exactly, equal to one proton or one neutron.
Mass Number # PROTONS + # NEUTRONS = MASS NUMBER EXAMPLE: What is the mass number for manganese if there are 30 neutrons in the nucleus?
Element Symbols • Top is mass number • Bottom is atomic number • Practice
Symbols EXAMPLE 1: Find the • number of protons • number of neutrons • number of electrons • Atomic number • Mass Number 19 F 9
Answer… • 9 protons • 10 neutrons • 9 electrons • Atomic # = 9 • Mass # = 19 • What would the mass number be of a fluorine isotope with two additional neutrons?
Symbols EXAMPLE: Find the • number of protons • number of neutrons • number of electrons • Atomic number • Mass Number 80 Br 35
Answer • Number of protons: 35 • Number of neutrons: 45 • Number of electrons: 35 • Atomic number: 35 • Mass Number: 80
Symbols If an element has an atomic number of 34, a mass number of 78, find… • number of protons • number of neutrons • number of electrons • And WRITE the Complete symbol
Answer… • 34 protons • 34 electrons • 44 neutrons • Symbol: Se 78 34
Symbols • if an element has 91 protons and 140 neutrons what is the • Atomic number • Mass number • number of electrons • Complete symbol