Unit II Atomic Structure

proton, neutron and electron Obj. 1…Dalton’s Atomic Theory Four postulates (1808)... 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

Obj. 2…Atomic Scientists J. J. Thomson discovered electrons (e-) in 1897. 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 (+).

Obj. 2 cont... E. Goldstein discovered positive particles in atoms He observed rays traveling in the opposite direction of cathode rays. These particles were termed ‘ protons’ (p+) by Ernest Rutherford in 1920. James Chaddwick 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.

Obj. 2 cont... Robert Millikan furthered Thomson’s work by describing the electron more in depth. all e- carry exactly one unit of negative charge. mass of an e- is 1/1840 the mass of a hydrogen atom (p+).

Obj. 3 and 4…The Bohr-Rutherford Model Ernest Rutherford conducted the famous ‘gold foil experiment’ (1911) which concluded that: 1. atoms are mostly space. (football arena) 2. atoms have a solid nucleus at the center which contains most of the mass. This overturned the accepted ‘plum pudding model’ of the time.

Obj. 3 and 4 cont... The Gold Foil Experiment... Stats... (BBs and cream cheese!) Stats... 98% of particles went straight through (expected) 2% of particles deflected at large angles 0.01% of particles deflected straight back (canon balls & tissue!)

Obj. 3 and 4 cont... After Rutherford’s findings, Niels Bohr further explained the atom by concentrating on the e- (1913). Why don’t the (-) e- fall into the (+) nucleus? 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. The progression of the atom... Rutherford Rutherford-Bohr Quantum Plum Pudding

PROPERTIES OF SUBATOMIC PARTICLES Obj. 5…Subatomic Particles PROPERTIES OF SUBATOMIC PARTICLES PARTICLE SYMBOL ELECTRICAL CHARGE RELATIVE MASS ACTUAL MASS (g) Electron Proton Neutron e- -1 1/1840 9.11 x 10-28 p+ +1 1 1.67 x 10-24 n0 1 1.67 x 10-24

Obj. 6…Characteristics of Elements K 19 Potassium 39.098 atomic # element symbol element name atomic mass

Obj. 6 cont... Atomic #: # of p+ in an atom identifies element (change atomic # = change of element). # p+ = # e- in neutral atom ** (+) charge = less e- than p+ ** (-) charge = more e- than p+ Atomic mass: (a.k.a. mass #) mass of the nucleus p+ + n0 units are a.m.u. (atomic mass unit)

X X Obj. 7…Atomic Symbols OR There are two ways to represent elements: Symbol Form: # X atomic mass element symbol (# of p+) atomic # OR # X atomic mass element symbol Shorthand Form: name of element followed by atomic mass. Ex... Aluminum - 27 Nitrogen - 14 Carbon - 14 You can find mass #, atomic #, # of n0, and # of e- with either notation!

Obj. 8…Atomic Calculations All mass of the atom is in the nucleus. - only p+ and n0 are in the nucleus. - if you know the mass of any atom, you can find the # of n0. - if you know the # of n0 and the # of p+, you can find the mass. Ex... electrical charge on atom 31 P 15 +1 Manganese - 55 element = _______________ Phosphorus symbol = ______________ Mn atomic mass = ___________ 31 a.m.u.s atomic mass = __________ 55 a.m.u.s atomic # = ______________ 15 atomic # = ____________ 25 # of p+ = _______________ 15 # of p+ = ______________ 25 # of n0 = _______________ 16 # of n0 = ______________ 30 # of e- = _______________ 14 # of e- = ______________ 25

Obj. 9…Isotopes X Neon - 20 Fluorine - 20 Neon - 22 Isotopes: atoms of the same element with different atomic masses. - different # of n0 !!! Three isotopes of Carbon: Carbon - 12 (6p+, 6n0) element of life extremely rare radioactive…carbon dating Carbon - 13 (6p+, 7n0) Carbon - 14 (6p+, 8n0) Note: atomic # will NEVER change in isotopes… only mass and # of n0 do! Which of the following are isotopes of the same element? Neon - 20 Neon - 22 Fluorine - 20 22 X 12 +3 10 25 -1

Obj. 10…Average Mass of Isotopes Isotopes are naturally occurring. The mass # of an element (periodic table) is the weighted avg. of all isotopes that exist in nature. - 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 avg. mass... Step 1 : mass x abundance for each isotope Step 2 : add the two values from step 1 together 43.42 62 .93 x .69 = 43.42 + 20.13 64 .93 x .31 = 20.13 63.55 g/mole

Obj. 10 cont... The average 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 closest to 16, therefore, Oxygen-16 is the isotope that is most abundant in nature.