Unit 3

1. Unit 3 Atomic Structure

2. Democritus Atomic Theory Democritus (Greek philosopher, ~442 BC) • His theory: Matter could not be divided into smaller and smaller pieces forever; eventually the smallest possible piece would be obtained. • This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.” • To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes.

3. proton, neutron and electron 1. All elements are composed of tiny, indivisible particles called ‘atoms’. isotopes exist 2. Atoms of the same element are identical... each element is unique 3. Atoms of different elements can physically mix or chemically combine (compounds). 4. Chemical rxns. occur when atoms are separated, joined, or rearranged. • Much of this theory is still accepted, with 2 exceptions Dalton’s Atomic Theory • Four postulates (1808)... * *

4. He passed an electric current through a glass tube filled with gas. • He discovered that a beam of negative charges traveled from the cathode (-) to the anode (+). Scientists J. J. Thomson (British physicist) discovered electrons (e-) in 1897.

5. These particles were termed ‘protons’ (p+) by Ernest Rutherford in 1920. Scientists Eugen Goldstein (German physicist) discovered positiveparticles in atoms in 1886 • He observed rays traveling in the opposite direction of cathode rays. James Chadwick (English physicist) discovered neutrons (n0) in 1932. • His discovery was based on the fact that different atoms’ atomic mass and atomic # (# of protons) were not adding up.

6. Robert Millikan furthered Thomson’s work by describing the electron more in depth. Scientists • all e- carry exactly one unit of negative charge. • mass of an e- is 1/1840the mass of a hydrogen atom (p+).

7. Bohr-Rutherford Model 2. atoms have a solid nucleus at the centerwhich contains most of the mass. • This overturned the accepted ‘plum pudding model’ of the time. Ernest Rutherford (British physicist) conducted the famous “Gold Foil Experiment” (1911) which concluded that: 1. atoms are mostly empty space.

8. cont... • The Gold Foil Experiment... • Stats... • 98% of particles went straight through (expected) • 2% of particles deflected at large angles • 0.01% of particles deflected straight back

9. The modern view of the atom was developed by Ernest Rutherford (1871-1937).

10. Results of foil experiment if Plum Pudding model had been correct.

11. What Actually Happened

12. Bohr proposed that the e- travel on concentric orbits around the nucleus. • Each orbit has a fixed energy (energy level) and e- do not lose energy. Plum Pudding Quantum Rutherford-Bohr Rutherford cont... • After Rutherford’s findings, Niels Bohr (Danish physicist) further explained the atom by concentrating on the electrons (1913). Why don’t the (-) electrons fall into the (+) nucleus? • The progression of the atom...

13. Plum Pudding Quantum Rutherford-Bohr Rutherford History of Atomic Theory… Stick figure style http://www.youtube.com/watch?v=QbWKF9uDF7w

14. ACTUAL MASS (g) RELATIVE MASS ELECTRICAL CHARGE SYMBOL PARTICLE Electron Proton Neutron Subatomic Particles PROPERTIES OF SUBATOMIC PARTICLES e- -1 9.11 x 10-28 1/1840 p+ +1 1.67 x 10-24 1 same same 1 n0 0 1.67 x 10-24

15. 19 K Potassium 39.098 Characteristics of Elements atomic # element symbol element name average atomic mass

16. cont... • Atomic #: • # of p+ in an atom • identifies element (change atomic # = change of element). A.P.E. At # = #of p+ = #of e- • # p+ = # e- in neutral atom ** (+) charge = less e- than p+ ** (-) charge = more e- than p+ • Atomic mass: (a.k.a. mass #) M.A.N. Mass = At # + #of n0 • mass of the nucleus • p+ + n0 • units are a.m.u. (atomic mass unit)

17. atomic mass X # element symbol atomic # # (# of p+) X atomic mass # element symbol • You can find mass #, atomic #, # of n0, and # of e- with either notation! Atomic Symbols • There are two ways to represent elements: • Symbol Form: OR • Shorthand Form: name of element followed by atomic mass. • Ex... Aluminum - 27 Nitrogen - 14 Carbon - 14

18. - if you know the # of n0and the # of p+, you can find the mass. electrical charge on atom P +1 31 15 Atomic Calculations • All mass of the atom is in the nucleus. - if you know the mass of any atom, you can find the # of n0. • Ex... Manganese - 55 Phosphorus element = _______________ Mn symbol = ______________ 31 a.m.u. atomic mass = ___________ 55 a.m.u. atomic mass = __________ 15 atomic # = ______________ 25 atomic # = ____________ 15 # of p+ = _______________ 25 # of p+ = ______________ 16 30 # of n0 = _______________ # of n0 = ______________ 25 14 # of e- = _______________ # of e- = ______________

19. Isotopes: atoms of the same element with different atomic masses. element of life extremely rare radioactive…carbon dating • Note: atomic # will NEVERchange in isotopes… only mass and # of n0 do! Neon - 20 X +3 X X -1 22 22 25 Fluorine - 20 12 10 10 Neon - 22 Isotopes - different # of n0 !!! • Three isotopes of Carbon: (6p+, 6n0) Carbon - 12 (6p+, 7n0) Carbon - 13 (6p+, 8n0) Carbon - 14 • Which of the following are isotopes of the same element?

20. The mass # of an element (periodic table) is the weighted avg. of allisotopes that exist in nature. abundance 63.55 g/mole Average Mass of Isotopes • Isotopes are naturally occurring. - abundance of isotope is just as important as mass! • Ex... Natural copper (Cu) consists of 2 isotopes ... Copper - 63 (mass = 62.930 g/mole) 69% Copper - 65 (mass = 64.930 g/mole) 31% • To calculate average mass... mass × abundance for each isotope Step 1 : add the two values from step 1 together Step 2 : 43.42 62.93 x 0.69 = ______ 43.42 + 20.13 64.93 x 0.31 = ______ 20.13

21. cont... • The average atomic mass of an element is closest to the isotope that is most plentiful in nature. • Ex... Three isotopes of Oxygen: Oxygen - 16 99.759% Oxygen - 17 0.037% Oxygen - 18 0.204% • The avg. mass (from P.T.) is 15.999 a.m.u., therefore, Oxygen-16 is the isotope that is most abundant in nature.