1. Unit II
Atomic Structure

2. 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

3. 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 (+).

4. 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.

5. 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+).

6. 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.

7. 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!)

8. 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

9. 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

10. Obj. 6…Characteristics of ElementsK 19
Potassium
39.098
atomic #
element symbol
element name
atomic mass

11. 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)

12. 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!

13. 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

14. 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

15. 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 = g/mole)
69%
Copper - 65 (mass = 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
x .69 =
43.42 +
20.13
x .31 =
20.13
63.55 g/mole

16. 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 %
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.