Post on 25-Dec-2015
ATOMIC THEORY
In 460 B.C., a Greek philosopher, Democritus, develop the idea of atoms.
He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further?
Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms.
Democritus (460-370
BC)• Proposed that matter is made of
tiny particles and empty space• atoms = smallest part of matter• different types of atom for each
type of matter
People considered Aristotle's opinions very important and if Aristotle thought the atomic idea had no merit, then most other people thought the same also.
For more than 2000 years nobody did anything to continue the explorations that the Greeks had started into the nature of matter.
Lavoisier (1743-1794)
• Made measurements of a chemical change in a sealed container
• law of conservation of matter
Proust (1799)
• Observed that water is 11% H and 89% O.
• Law of definite proportionsdefinitions
Photo not
available.
• The principle that the elements that comprise a compound are always in a certain proportion by mass.
In the 1800's an English chemist, John Dalton performed experiments with various chemicals that showed that matter, indeed, seem to consist of elementary lumpy particles (atoms).
http://antoine.fsu.umd.edu/chem/senese/101/atoms/dalton.shtml
Postulates
1. All matter consists of tiny particles.
2.Atoms are indestructible and unchangeable.
3.Elements are characterized by the mass of their atoms.
4.When elements react, their atoms combine in simple, whole-number ratios.
• The beam is bent away from a negatively charged plate.
• Like charges repel.• The beam must be negative particles.
cathode -
anode +power source
THOMSON’S MODEL
In 1897, the English physicist J.J. Thomson discovered the electron and proposed a model for the structure of the atom. Thomson knew that electrons had a negative charge and thought that matter must have a positive charge. His model looked like raisins stuck on the surface of a lump of pudding.
“You, lab tech, just stay here and count the flashes of light. I’m going out to have a drink with the other professors” - Rutherford to Geiger
Experiment Conclusion
• The atom is mostly empty space.
• The atom contains a small, dense positive core.– Nucleus– If the nucleus was the size of a
golf ball the nearest electron would be one mile away.
– model
RUTHERFORD’S MODEL
Rutherford knew that atoms consist of a compact positively charged nucleus, around which circulate negative electrons at a relatively large distance. The nucleus occupies less than one thousand million millionth of the atomic volume, but contains almost all of the atom's mass. If an atom had the size of the earth, the nucleus would have the size of a football stadium.
Particle Symbol Charge Mass (u) Mass (g)
Proton P+ +1 1 1.67x10-24
Neutron n0 0 1 1.67x10-24
Electron e- -1 .00055 9.11x10-28
Comparing Subatomic Particles
Atomic Number• Equals the
number of protons
• defines the element– all chlorine
atoms have 17 protons
17
ClChlorine
35.45
Mass number• Protons + neutrons• Isotope have
different masses– because they have
a different # of neutrons
• Chart mass is an average of the isotopes
17
ClChlorine
35.45
Mass number• What is the unit
for mass?• Atomic mass unit • symbol = u• based on carbon-
12• 1p+= 1n0 = 1u
17
ClChlorine
35.45
BOHR’S MODEL
In 1912 a Danish physicist, Niels Bohr came up with a theory that said the electrons do not spiral into the nucleus and with some rules for what does happen.
RULE 1: Electrons can orbit only at certain allowed distances from the nucleus.
RULE 2: Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit.
BOHR-SOMMERFIELD’S MODEL
According to the Bohr-Sommerfeld model, not only do electrons travel in certain orbits but the orbits have different shapes and the orbits could tilt in the presence of a magnetic field. Orbits can appear circular or elliptical, and they can even swing back and forth through the nucleus in a straight line.
QUANTUM MECHANICAL MODEL
The visual concept of the atom now appeared as an electron "cloud" which surrounds a nucleus. The cloud consists of a probability distribution map which determines the most probable location of an electron.
ATOM
ELECTRON CLOUD NUCLEUS
ELECTRONSPROTONS NEUTRONS
NEGATIVE POSITIVE NEUTRAL
1/1900 1
ATOMIC NUMBER (Z) ATOMIC MASS (A)
divisions
particlesparticles
chargecharge
Relative mass Relative mass
is
is
If atom is neutral
N. Bohr(1885-1962)
• Proposed that the electrons orbited the nucleus .
• The further away the more energy was needed.
• Electrons only occupy orbits of certain energy.
•Carry energy not matter
•Wavelength is the distance between corresponding points on the wave.
•Wavelengths per second = frequency
•1 wavelength/second = 1 Hertz (Hz)
The Modern Model• Visible light
can be split into colored light.
• Each color has its own – frequency– wavelength– energy
• Red light has– low frequency– long wavelength– less energy
• Blue light has– high frequency– short wavelength– more energy
Gamma Rays….....Pass through most substances.
X rays…....Pass through soft tissue, but not bone.
UV light…….....causes sunburn, stopped by ozone
Light……………………………………………………..visible
Infrared………………………………………..radiant heat
Radiowaves…..Carry the sounds of radio stations.
Electrons & Light
• When energy is put into an atom, electrons absorb the energy and become “excited”.
• Electrons can absorb only fixed amounts of energy.
• When excited, electrons move into a higher energy level (state).
Electrons & Light
• The electrons return to their original ground state.
• The electrons must give off energy in the form of light.
• This light has a specific frequency (and color.)
• This frequency shows up as a line when seen through a prism.
The Modern ModelBright Line Spectra
When atoms absorb energy, from fire or electricity, the electrons give off light.•When viewed
through a prism a bright line spectra appears.•This is unique for every element.
H
HgNe
The big gaps between spectral lines represent electron
transitions from one energy level to another.
hydrogen
The groups of fine lines indicate that electrons are jumping from energy levels that are close in energy.
helium
Quantum Theory Model(Electron Cloud Model)
• Heisenberg Uncertainty Principle– You can never know
exactly where an electron is if you know how fast it is moving.
– If you know its exact location, you can’t know how fast it’s moving.
Electron Cloud Model
• Electron paths are not neat orbits.
• The region around a nucleus where an electron may be found is the electron cloud or orbital.
• An electron cloud is associated with a certain energy level.
In 1927 Heisenberg formulated an idea which agreed with test results, that no experiment can measure the position and momentum of a quantum particle simultaneously. Scientists call this the "Heisenberg uncertainty principle." This implies that as one measures the certainty of the position of a particle, the uncertainty in the momentum gets correspondingly larger. Or, with an accurate momentum measurement, the knowledge about the particle's position gets correspondingly less.
Uncertainty Principle The position of a particle as well as its attributes cannot be determined with absolute precision. In association with this fact, a particle may disappear from one place and reappear elsewhere acausally.
The velocity (causal indeterminism) is coupled with uncertainty of position (acausal indeterminism). These indeterminate factors are added to the deterministic velocity to obtain the motion of the electron as the changing
position of its "locus of activity."
• The visual concept of the atom now appeared as an electron "cloud" which surrounds a nucleus. The cloud consists of a probability distribution map which determines the most probable location of an electron. For example, if one could take a snap-shot of the location of the electron at different times and then superimpose all of the shots into one photo.
• Note: Just as no map can equal a territory, no concept of an atom can possibly equal its nature. These models of the atom simply served as a way of thinking about them, albeit they contained limitations (all models do).
What should a Model look like?
Scientific models may not always look like the actual object. A model is an attempt to use familiar ideas
to describe unfamiliar things in a
visual way.This is a painting of a young This is a painting of a young
woman by Pablo Picasso. Does woman by Pablo Picasso. Does it actually look like a young it actually look like a young
woman?woman?
Is this really an Atom?
The model above represents the most modern version of the atom.
(Artist drawing)
Many of the models that you Many of the models that you have seen may look like the one have seen may look like the one
below. It shows the parts and below. It shows the parts and structure of the atom. Even structure of the atom. Even
though we do not know what an though we do not know what an atom looks like, scientific atom looks like, scientific models must be based on models must be based on
evidence. evidence.
Electron Cloud Model
• A certain amount of electrons can exist in each energy level–1st level = 2–2nd level = 8–3rd level = 18
• Electrons in the outer most levels are valence electrons.
Sublevels & Orbitals
• Each energy level can be split into “sublevels”
• Each sublevel can be divided into “orbitals”.
Tells the amount of energy
Tells the shape of orbital
Is a specific volume around the nucleus where two electrons
exist.
1st energy level has 1 sublevel = 1s
2nd energy level has 2 sublevels = 2s, 2p
3rd energy level has 3 sublevels = 3s, 3p and 3d
4th energy level has 4 sublevels = 4s,4p, 4d and
4f
s sublevel holds maximum 2 e-p sublevel holds maximum 6 e-d sublevel holds maximum 10 e-f sublevel holds maximum 14 e-
Placing electrons in orbitals:•orbital can hold a maximum of 2 e-•orbital has the same name as the sublevel•orbitals in the “s” sublevel are “s” orbitals
2s
yz
x
Placing electrons in orbitals:•orbital can hold a maximum of 2 e-•orbital has the same name as the sublevel•orbitals in the “p” sublevel are “p” orbitals
2px 2pz2py
For Aluminunum
1s 2s 2p 3s 3p1
Electron Configurations
sublevelsublevelnumber of number of
electrons in electrons in the sublevelthe sublevel
2
2
6
2
For Manganese
1s 2s 2p 3s 3p 4s 3d 6
Electron Configurations
sublevelsublevelnumber of number of
electrons in electrons in the sublevelthe sublevel
2
2
6
2
2
5
Try these electron configurations!
10Ne =
11Na =
12Mg =
18Ar =
19K =
1s2 2s2 2p6
1s2 2s2 2p6 3s1
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p6 4s1
Déjà vu!
10Ne =
11Na =
12Mg =
18Ar =
19K =
1s2 2s2 2p6
1s2 2s2 2p6 3s1
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p6 4s1
Look! It’s Look! It’s neon!neon!
Ar?!Ar?!
Let’s make this shorter.
10Ne =
11Na =
12Mg =
18Ar =
19K =
1s2 2s2 2p6
1s2 2s2 2p6 3s1
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p6 4s1
[Ne]
[Ne]
[Ar]
Noble Gas Notation
10Ne =
11Na =
12Mg =
18Ar =
19K =
1s2 2s2 2p6
[Ne] 3s1
[Ne] 3s2
1s2 2s2 2p6 3s2 3p6
[Ar] 4s1
Try these electron configurations!
26Fe =
47Ag =
79Au =
20Ca =
40Zr =
[Ar] 4s2 3d6
[Kr] 5s2 4d9
[Xe] 6s2 4f14 5d9
[Ar] 4s2
[Kr] 5s2 4d2
• SUMMARY • 1. De Broglie first pointed out the wave-particle duality
of nature. His ideawas that all particles exhibit some wave characteristics, and vice versa.
• 2. The momentum of an object is the product of its mass and velocity. Wavelength varies inversely as momentum.
• 3. Heisenberg's uncertainty principle concerns the process of observing an electron's position or its velocity. It is impossible to know accurately both the position and the momentum of an electron at the same time.
• 4. Schrodinger developed a mathematical equation which describes the behavior of the electron as a wave. The solution set of the wave equation can be used to calculate the probability of finding an electron at a particular point.
• 5. Because of the electron's high velocity, it effectively occupies all volume defined by the path through which it moves. This volume called the electron cloud.
• 6. The principal quantum number (n = 1, 2, 3,..) is the number of energy level and describes the relative electron cloud size.
• 7. Each energy level has as many sublevels as the principal quantum number. The second quantum number (1 = s, p, d, f...) describes the of the cloud.
• 8. The third quantum number, m, describes the orientation in space of orbital. 9:10, 9
• 9. The fourth quantum number, s, describes the spin direction of an electron.
• 10. Each orbital may contain a maximum of one pair of electrons. Electrons in the same orbital have opposite spins. s:12
• 11. Pauli's exclusion principle states that no two electrons in an atom have the same set of quantum numbers.
• 12. Electrons occupy first the empty orbital giving the atom the lowest energy.
• 13. The diagonal rule can be used to provide the correct electron configuration for most atoms.
• 14. The chemist is primarily concerned with the electrons in the outer energy level. Electron dot diagrams are useful in representing the outer level electrons.