BACK Atomic Structure
I Early Models of the Atom
1. Greek philosopher
2. world made up of indivisible particles
3. called them atomos - atom
4. today - atom: smallest particle of an element that retains the chemical identity of that element.
B. Aristotle and others
1. opposing idea
2. matter is continuous - never ending
3. persisted until 17th century
C. ended with the introduction of scientific observations and experimentation.
1. Lavoisier's Law of Conservation of Matter
2. Proust's Law of Constant Composition - compounds always composed of elements in the same propor
tions by mass.
D. Dalton, 1803
1. proposed the Atomic Theory
a. each element composed of atoms
b. atoms of an element are identical
c. atoms cannot be created nor destroyed in a reaction.
d. given compounds have the same kind and number of atoms.
2. has been modified with new discoveries.
II Discovering Atomic Structure
A. static electricity
1. Dalton thought atoms were solid spheres - Billiard ball model
2. B. Franklin objects can have positive or negatives charges.
3. Faraday suggested atoms related to electricity.
B. cathode rays and electrons
1. cathode rays - rays that stream from the cathode(-) to an anode(+) in vacuum tube when connected to a
battery - CRT - Crookes tube
a. stream of particles
b. magnets deflects the ray
c. carries a negative charge
3. JJ Thomson conducted series of experiments page 98
a. measured deflection of ray by magnetic and electric fields.
b. used different gases and different electrode materials - same effects.
c. deflections mathematically predictable.
d. concluded ray was negative particles from the cathode
e. impact: atoms are not indivisible - contain subparticles.
f. calculated the charge to mass ratio: 1.76 x 10 8 coulombs/gram
g. later discovered the proton using the CRT
h. proposed the Plum pudding model
a. determined the charge of an electron
b. used the oil drop apparatus: page 99
c. determined the electron charge is 1.60 x 10 -19 C.
d. calculated electron mass is 9.11 x 10 -28 g.
1. discovered by Becquerel, 1896
2. accompanies changes in the atom.
a. placed radioactive material in front of electric field.
b. some deflected towards - plate called alpha(* ) radiation.
c. some deflected towards + plate called beta (ß) radiation
d. later discovered gamma ( ) radiation - no deflection
D. the nuclear atom
1. JJ Thomson approached Rutherford for evidence for his model
a. designed the gold foil experiment page 101
b. beam of alpha aimed at thin gold foil
c. most went straight through
d. surprised small % deflected - some almost straight back
e. concluded most atomic mass concentrated in the center or nucleus.
f. proposed the nuclear model.
g. implies atoms are mostly empty space.
h. agrees with gold foil observations.
III Modern Atomic Theory
A. atoms composed of protons, electrons,and neutrons
B. subatomic made up of even smaller unit called: quarks, mesons, gluons, etc.
C. structure of the atom
1. central nucleus
a. composed of protons and neutrons
b. protons have positive charge - equal to electron but opposite
c. neutrons are neutral - about same mass as protons
a. Bohr + Rutherford thought electrons orbit nucleus
b. planetary model
c. incorrect - electrons do not move in discrete paths
d. move within specific spatial areas - forms electron cloud
D. properties location charge mass amu
1. protons nucleus +1.6x10-19C (1) 1.673x10-24g (1)
2. neutrons nucleus 0 1.675x10-24g (1)
3. electrons electron cloud -1.602x10-19C (1) 9.109x10-28g (0)
E. atomic number
1. Moseley found that each atom has unique number of protons.
2. determine the identity of an atom
3. called the atomic number (Z)
4. atoms are neutral therefore number of electrons = number of protons.
1. atoms can lose or gain electrons.
2. atoms more electrons than protons - anions - negative charge
3. atoms more protons than electrons - cations - positive charge
4. Mg 12 protons -----------------------> 12 protons
12 electrons if loses 2 electrons = - 10 electrons
+ 2 charge
5. Mg +2 cation
1. disproves Dalton's 2nd postulate - atoms of an element are identical.
2. same atomic number but different number of neutrons.
3. H 1P 0N most abundant 99.99%
H 1P 1N rare .0085%
H 1P 2N rarer .0015%
4. distinguished by mass number
Chlorine - 35 35 Cl
Chlorine - 37 37 Cl
5. problem 56 Fe 2+ P: 26
26 N: 56-26 =30
e: 26- 2 = 24
H. the mass of an atom
1. based on a standard - carbon
2. measured in amu
3. amu = 1/12 carbon
4. element's mass in amu proportional to cabon's mass in grams.
5. amu mass calculation uses the isoopes' fractional abundance in nature.
6. average mass called atomic mass.
IV Changes in the Nucleus
A. results from a nuclear reaction contradicts Dalton's 3rd postulate
B. changes the number of neutrons and protons in the nucleus.
C. produces radiation
D. radioactivity - spontaneous emission of radiation.
E. nuclear stability
1. most atoms are stable
2. nucleus held together by strong nuclear forces.
a. short range
b. hold quarks together in the nucleus.
c. counters the electric fore which repels protons.
3. neutrons add additional attractive force in the nucleus.
4. elements 1-20 equal number of P and N.
5. elements 20-83 increasing number of N needed.
6. beyond 83 - neutrons insufficient - radioactive.
7. too many neutrons or too few neutrons compared to common isotope are radioactive - beta.
F. types of radioactive decay
a. composed of 2 protons and 2 neutrons
b. high velocity
c. similar to helium without electrons 4 He 2+
d. low penetrating power
e. low health problems
a. high speed electrons
b. nuclear origin
c. neutron become a proton and an electron.
d. electron propelled out of the nucleus.
e. 0 e-
f. 100 times more penetrating than alpha - damage skins.
a. energy similar to x-rays
b. produced with alpha and beta.
d. very penetrating and dangerous.
4. radioactive decay - emitting radiation due to nuclear change.
5. nuclear equation
a. alpha decay
1. 226 Ra ----> 222Rn + 4
88 86 2
b. beta decay
1. 131 I -----> 131 Xe + 0
53 54 -1
c. sum of mass numbers and atomic numbers are the same before and after the