SUMMARY OUTLINE: ATOMIC STRUCTURE
MODELS
A) Dalton

Atoms are indivisible so no idea of protons, neutrons, or electrons

Atoms of the same element are alike, but different elements have different atoms

Atoms of different elements combine in fixed proportions to make compounds

No atoms are destroyed or created ONLY REARRANGED in a chemical reax
B) Thomson (plum-pudding model)

Subatomic particles (protons and electrons) exist

Protons are uniformly distributed in an atom (pudding) with electrons (raisins or plums)
embedded in this positive sphere
C) Rutherford (gold-foil experiment)

Few alpha particles (+’s) deflected by gold atoms so Thomson’s model replaced with an
atom with an extremely small nucleus or core of protons (otherwise high chance alpha
particles would have hit them and deflected back).

Therefore atoms are mostly “empty” space, where electrons reside
D) Bohr (orbit or planetary model)

By this time neutron discovered and placed inside nucleus with protons

Electrons are in fixed orbits or energy levels
E) Quantum Mechanical (electron cloud model)

Electrons exisit “in-between” the “orbits” in sublevels and orbitals…defined regions of
space
PROTONS, NEUTRONS, AND ELECTRONS
a) Basics
Particle
proton
CHARGE
+1
MASS
1 amu
SYMBOL
1 H
LOCATION
Nucleus*
1
neutron
0
1 amu
Nucleus
1
n
0
electron
-1
“Zero”
e
Outside
(1/1836xP)
nucleus
-1
*nuclear charge = the total number of protons in an atom of an element
0
b) Atomic # The number of protons in an atom of an element; identifies the element
c) Mass #
The total number of protons and neutrons in an atom; accounts for an
atom’s mass
d) Calculating p, n, and e for ATOMS

#p = the atomic number

ATOMS ARE NEUTRAL BECAUSE

If mass# =p+n

and atomic = # p___

then Mass# (-) atomic # =

Example
# of positives= # of negatives, so #p= #e
n
52
Cr0

p=24, n= 28 (52-24) e=24 (p=e for atoms)
24

e) Calculating p,n, and e for IONS: CHARGED particles or atoms that have lost or
gained electrons.

Cations= positively charged ions, or ions that have LOST electrons

Anions= negatively charged ions, or ions that have GAINED electrons

Example
31
P3-

p=15, n= 16 (31-15) e=18 (15 + 3)
15

WEIGHTED AVERAGE
a) Concept

Isotopes, or atoms of the same element with DIFFERENT #’s of neutrons, mean
that atoms of the same element will have different masses
Ex.
neon-20
Abundance:

neon-21
20
neon-22
21
Ne
Ne
90.51%
0.27%
22
Ne
9.22%
Which mass should REPRESENT Neon on the periodic table ? Answer: include
the maases of ALL the isotopes BUT give more WEIGHT to the most ABUNDANT
Isotope.
b) Calculating weighted average:
Iso #1
(Mass x abundance)
( 20 x 0.9051)
iso #2
iso #3
+ (mass x abundance)
+
+
+
(21 x 0.0027)
(mass x abundance) = weighted avg
(22 x 0.0922) = 20.187 amu
Note that to take a % of a mass you divide the % value by 100