CHAPTER 4: Matter is Made up of Atoms ATOMS & THEIR STRUCTURE Aristotle thought matter was made of air, earth, fire and water. Democritus (250 B.C.)- Said the world is made of empty space & tiny particles called atoms DEVELOPMENT OF THE ATOMIC THEORY (3 S CIENTISTS INVOLVED ) 1. Lavoisier (1743-1794)- Law of Conservation of Matter The Father of Modern Chemistry 2. P ROUST (1799) Law of definite proportions- elements that make up compounds are found in fixed proportions Joseph Proust 3. D ALTON ’S ATOMIC THEORY 1. All matter is made of atoms. 2. Atoms are indivisible 3. All atoms of 1 element are alike, but are different than atoms of other elements Was he right? 1. All matter is made of atoms. 2. Atoms are indivisible 3. All atoms of 1 element are alike, but are different than atoms of other elements DISCOVERY OF ATOMIC STRUCTURE 4 S CIENTISTS JJ Thomson (1897) 1. Discovered the electron during his vacuum tube/cathode ray experiments. 2. JJ’s Plum Pudding model 3. When studying Ne he discovered isotopes - atoms of an element that are alike chemically, but differ in mass. *Same # protons, different # neutrons 4. His work led to the discovery of the neutron NAGAOKA Saturnian model or early planetary model RUTHERFORD A. Gold foil experiment 1. shot positive particles through a thin sheet of Au. 2. Most went straight through, but others were deflected. WHY ? http://www-outreach.phy.cam.ac.uk/camphy/nucleus/nucleus6_1.htm CONCLUSIONS DRAWN FROM THE GOLD FOIL EXPERIMENT 1. Most of the atom is empty space. 2. Discovery of the nucleus - small, dense, positively charged central core of the atom NEILS BOHR (1913) 1. Given credit for the planetary model of the atom. 2. Was more specific than Nagaoka because he concluded that electrons are arranged in E levels. BOHR ’S STATEMENTS ABOUT ELECTRONS 1. Said that electrons absorb E & move to higher states. 2. These “excited” electrons give off that E as light waves & fall back to a lower E level. Neils Bohr’s Atomic Model An excited electron jumps to an outer E level. It falls back, releasing light waves. + The “packet of light” released is called a photon . The atom is not flat! Nucleus Electron Orbit Energy Levels Our View of Electrons Today E levels are not neat orbits E levels are spaces around the nucleus where electrons are most likely to be found Dalton JJ Thomson Nagaoka Rutherford Today’s model Bohr Subatomic Particles Subatomic particle Mass in µ Charge Protons 1.01 Positive Neutrons 1.01 Neutral Electrons 0.0005 Negative Mass of atoms is measured in atomic mass units (µ, or amu) Determining Atomic Structure 1. Atomic number- the # of protons in the nucleus & the # of electrons if the atom is neutral 2. Atomic mass/Mass number - the sum of protons + neutrons in the nucleus Carbon 12.011 Atomic mass C 6 Atomic number # protons = 6 # neutrons = 12-6= 6 # electrons = 6 You try it How many protons, neutrons, and electrons are present in nitrogen? p = 7 e = 7 n = 14 -7=7 Try bromine p = 35 e = 35 n = 45 Why isn’t atomic mass a whole number? It is a weighted average of all isotopes of an element. The isotope most common in nature is the one that has a mass closest to the one found on the periodic table. Ex: Most C has a mass of 12. The most common isotope of carbon is therefore C-12 Electrons in Energy Levels 1st E level can hold a max of 2 electrons 2nd E level can hold 8 3rd can hold 18 4th can hold 32 Drawing atoms Beryllium 9.012 Draw an atom of beryllium #p= 4 #n=9-4=5 Be #e=4 4 # valence electrons = 2 Valence electrons = # electrons in outermost E level Try to draw an atom of phosporus on your own Valence = 5 P=15 N=16 This is P-31. P=15 N=16 This is P-33. The only thing different is the P=15 # of neutrons N=18 Drawing ions This is a neutral To draw a +1 atom of P-31 ion, take away 1 electron P=15 Now the N=16 atom has a +1 charge Drawing ions To draw a -1 ion, add 1 electron P=15 Now the N=16 atom has a -1 charge Drawing ions What do you do to make the atom have a -3 charge? P=15 N=16 IN SUMMARY You have learned about: The development of the atomic theory The discovery of atomic structure How to construct a model of an atom of any element using the periodic table The End!!!!!!!!!.