Download Theory of Atomic Structure

Theory of Atomic
Structure
Greeks – Democritus, Leucippus





Over 2000 years ago
All matter is composed of tiny particles
These particles are so small that they cannot be
broken down (indestructible)
Named these particles atoms (from the Greek
work for indestructible)
Common Greek theory was that all matter
consisted of four "elements" - earth, air, fire, and
water
John Dalton

1803 - Proposed an "atomic theory" with spherical
solid atoms based upon measurable properties of
mass



All elements are composed of atoms, which are
indivisible and indestructible particles
All atoms of the same element are exactly alike; in
particular they all have the same mass
Joining atoms of two or more elements forms
compounds. In any compound, the atoms are joined
in a definite whole number ratio (H2O, 2 to 1 ratio)
Dalton

Did account for the following laws:




Law of conservation of mass (a chemical change
is simply a rearrangement of atoms)
Law of definite proportions (atoms are combined
in definite ratios)
Law of multiple proportions (elements can
combine in different ratios to form new
compounds – H2O, H2O2
Did not account for isotopes
J.J Thompson

Through the use of cathode
rays, Thompson was able to
discover a negatively
charged particle with almost
no mass
 Discovered

electrons
See Cathode Ray Tube
“Plum Pudding” Model (1890)
 Negative
charges were
scattered throughout the
positive atom
Ernest Rutherford


Discovered that atoms have
a positive, dense nucleus
surrounded by negative
“empty” space (the electrons
are in the “empty” space)
Gold Foil Experiment – 1909
(chem ASAP)
Gold Foil Experiment
 What

Alpha (α) particles – positively charged


Same as Helium nuclei (42He2+)
Gold Foil – only a few atoms thick
 What

was used
was done
Shot a beam of α particles through the
gold foil
Gold Foil Experiment
 What
happened?
The majority of the particles went straight
through the foil and exposed the film
behind it
 A few particles were deflected back from
the foil and scattered

Gold Foil Experiment

What does this mean?

Since the α particles are (+) charged, they must
have been pulled through the foil by a strong (-)
charge


The atom was mostly negatively charged “empty”
space
Since some α particles bounced back, they must
have hit a positive part of the foil

The atom must have a small positive part located
in the center
Gold Foil Experiment

Atomic Theory (Conclusion)


An atom consists of a very small, positively
charged nucleus and the rest is negatively
charged empty space. This empty space must
contain the electrons.
Lets see the experiment again!
Bohr Model – Neils Bohr 1919
Electrons resemble our solar
system
 Electrons revolve (orbit)
around the nucleus
 Principle energy levels (PEL)
= rings around the nucleus

Principle Energy Levels




The only regions in which electrons can be
found
Each level can only contain a specific
number of electrons (2 in the 1st, 8 in the 2nd)
Electrons farther away from the nucleus
have higher energy
Number of PEL’s is given by the period
number (found on the periodic table)
James Chadwick


1932 – discovered neutrons
Using alpha particles he discovered a
neutral atomic particle with a mass close to
a proton.
Valence Electrons – outer electrons




Outermost electrons give an atom its
properties
These are the electrons that are involved in
reactions
Last number in the electron configuration
Kernel electrons – inner electrons
Electron Dot Symbols / Lewis Dot
Diagrams


1.
2.
3.
4.
5.
6.
Dots represent valence electrons
Examples:
K
Ca
S
Ar
Ca2+
S2-
Electron Configurations


Distribution of electrons
Given in the lower left hand corner of the
periodic table (below the atomic number)
Rules
1.
2.
3.
4.
5.
1st PEL can hold 2 electrons
2nd PEL can hold 8 electrons
3rd PEL can hold 18 electrons
4th PEL can hold 32 electrons
No more than 8 electrons are in the outermost
principle energy level

6.
This equal 8 valence electrons, the maximum amount of
valence electrons possible
The electrons are added one at a time to the
unfilled principle energy levels
Examples

Ca: 2-8-8-2
What does this mean?




4 principle energy levels contain electrons
1st PEL contains 2e-, 2nd and 3rd PEL contain
8e-, and the 4th PEL contains 2e1st and 2nd are filled, 3rd and 4th are not
completely full
Ca has 2 valence electrons, and 18 kernel
electrons
Examples:
1.
C
2.
Li
3.
F
Detailed Electron Configurations

Within each principle energy levels there
are sublevels
 Each s sublevel contains 2 electrons
 Each p sublevel contains 6 electrons
 Each d sublevel contains 10 electrons
 Each f sublevel contains 14 electrons
PEL
Sublevels
# of electrons
1
s
2
2
s, p
2+6=8
3
s, p, d
2 + 6 + 10 = 18
4
s, p, d, f
2 + 6 + 10 + 14 = 32
Example

Calcium:


1s22s22p63s23p64s2
What does this mean???


The number in front of the sublevel indicates the
PEL (4s = 4th PEL, s sublevel)
The number of electrons is given by the
superscript (4s2 = 4th PEL, s sublevel, 2 e-)
Rules

Fill the lowest energy sublevels first


See energy chart and diagonal method
No more than 8 electrons can be in the
outermost principle energy level
Examples
1.
C
2.
Al
3.
Ca
4.
Ni
Ground State




Electrons are in the lowest available energy
levels
This is how the electron configuration is
written on the periodic table
Normal configuration
Stable
Excited State



When an electron gains energy, the
electron is at a higher energy state (excited
state)
Electrons absorb energy and move to new
higher energy levels
UNSTABLE
Excited State



When an electron returns from the excited
state to the ground state, energy is released
The energy is often emitted in the form of
light
The wavelength (color) of the light can be
used to identify the element because each
element absorbs and releases a specific
amount of energy
Quantum Theory




Electrons can only absorb (or release) specific
amounts of energy called quanta
This energy corresponds to the differences
between energy levels
Energy is always absorbed (or released) in
definite amounts rather than in a continuous
flow
A quantum of radiation is called a photon
Flame Tests


Every element absorbs and hence emits
different amounts of energy (different
colors)
Can be used for identification
Spectra/Spectroscope
• Spectra
– The photons of light that are given off can be
broken down into lines (spectral lines)
• Spectroscope
– An instrument that breaks light into colored bands
– Every element has a unique band length
(arrangement)
Modern Orbital Theory (Wave
Mechanical/Electron Cloud Model)


Electrons do not have a specific path or
location – they have probabilities of being
located there
Regions of high probabilities are called
orbitals
 Electrons
are located in orbitals