1. Atoms, Elements & Isotopes
Atomic Structure
Atoms, Elements & Isotopes

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

3. 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. E. Goldstein discovered positive particles in atoms in 1886.
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.
Atomic Scientists

5. Atomic Scientists
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. 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. The Bohr-Rutherford Model
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 (canon balls & tissue!)

8. The Bohr-Rutherford Model
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...
Plum Pudding
Rutherford
Rutherford-Bohr
Quantum

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

10. Characteristics of ElementsK 19
Potassium
39.098
Atomic Number
Element Symbol
Element Name
Atomic Mass

11. Characteristics of Elements
Atomic Number : Number 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 Number)
mass of the nucleus
p+ + n0
units are a.m.u. (atomic mass unit)

12. X X Atomic Symbols OR There are two ways to represent elements:
Symbol Form: # X
atomic mass
atomic #
(# of p+)
element symbol 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 number, atomic number, number of n0, and number of e- with either notation!

13. X X Atomic Symbols OR There are two ways to represent elements:
Symbol Form: # X
atomic mass
atomic #
(# of p+)
element symbol # X
atomic mass
element symbol OR
Shorthand Form:
name of element followed by atomic mass.
Ex... Aluminum – 27
Find the following elements on the Periodic Table: Beryllium, Copper, Sulfur, Neon and write them in both notations.

14. X Atomic Symbols Using the symbol form for neutral elements:# X
atomic mass
element symbol
(# of p+)
atomic #
Now find for each element the:
atomic mass
atomic number
# of p+ (same as atomic#)
# of n0 (mass# - atomic#)
# of e- (same as # of protons)
Ex...
1. Beryllium-
2. Copper -
3. Sulfur-
4. Neon -

15. Atomic Calculations P atomic # = atomic # = symbol = __Mn
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
Manganese - 55 31 P 15 +1
symbol = __Mn
atomic mass = __55 a.m.u.s
atomic # = 25
# of p+ =
# of n0 = 30
# of e- =
element = Phosphorus
atomic mass = __31 a.m.u.s
atomic # = 15
# of p+ =
# of N0 = 16
# of e- = 14

16. Isotopes X Neon - 20 Fluorine - 20 Neon - 2212 +3 10 25 -1
Neon - 20
Neon - 22
Fluorine - 20
Isotopes: atoms of the same element with different atomic masses.
- different # of n0 !!!
Three isotopes of Carbon:
Carbon (6p+, 6n0) element of life
Carbon (6p+, 7n0) extremely rare
Carbon (6p+, 8n0) radioactive…carbon dating
Note: atomic # will NEVER change in isotopes… only mass and # of n0 do!
Which of the following are isotopes of the same element?

17. 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 = %
Copper - 65 (mass = %
To calculate avg. mass...
Step 1 : mass x abundance for each isotope
Step 2 :add the two values from step 1 together
x .69 =
x .31 =
63.55 g/mole

18. Average Mass of Isotopes
The average mass of an element is closest to the isotope that is most plentiful in nature.
Ex...
Three isotopes of Oxygen:
Oxygen %
Oxygen %
Oxygen %
The avg. mass (from P.T.) is closest to 16, therefore, Oxygen-16 is the isotope that is most abundant in nature.