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 Thomson
Through the use of cathode rays, Thompson was able to discover a negatively charged particle with almost no mass
Discovered electrons
“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 See the experiment
Gold Foil Experiment What was used
Alpha (α) particles – positively charged Same as Helium nuclei (4
2He2+)
Gold Foil – only a few atoms thick
What 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
Dots represent valence electrons
Examples:1. K
2. Ca
3. S
4. Ar
5. Ca2+
6. S2-
Warm-up Questions1. Who is credited with discovering the nucleus?
a. Thompson
b. Rutherford
c. Bohr
d. Dalton
2. Which subatomic particle was discovered first?
3. Draw the Bohr Diagram for 39K+.
Electron Configurations Distribution of electrons Given in the lower left hand corner of the
periodic table (below the atomic number)
Rules1. 1st PEL can hold 2 electrons2. 2nd PEL can hold 8 electrons3. 3rd PEL can hold 18 electrons4. 4th PEL can hold 32 electrons5. No more than 8 electrons are in the outermost
principle energy level This equal 8 valence electrons, the maximum amount of
valence electrons possible
6. 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 2e-
1st 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
4d5s
4p3d
4s
3p3s
2p2s
1s
5p
6s
5d4f
6p
6d5f
7s
7p
Incr
easi
ng E
nerg
y
Low Energy
High Energy
Energies of
Sublevels
Diagonal Method
Examples
1. C
2. Al
3. Sc
4. Fe
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)– Every element has a unique spectra, so it can be
used for identification
• Spectroscope– An instrument that breaks light into colored bands – Emission Spectrum
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