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ATOM
Early Thoughts
Greeks
matter is made up of
particles--4 elements
4 elements --air--fire--water-
-- earth
Aristotle-- Continuous theory
Democritus --Discontinuous theory
Matter can be divided indefinitely
Matter can not be divided indefinitely Atoms-indivisible
Rich -Paradis
ATOMIC THEORIES Dalton’s Atomic Theory
• All elements are composed of individual atoms. •All atoms of a given element are identical • Atoms of different elements are different. •Compounds are formed by the combination of atoms of different elements (Law of Definite Proportions) Reactions are rearrangements of atoms
Fire ball
Sphere Uniform density
high v oltage
Cathode
Tube
Negative
electrode Positive
electrode
Cathode ray
-cathode rays are a form of radiation that
contain an electrical charge
-rays move from negative to positive
-rays are negative (electrons)
JJ Thomson’s Model used the cathode ray tube to show one of the smaller units (particle)
that make up the atom Thomson theorized that an atom contains small negatively charged
particles -------electrons
His model is the “Plum Pudding”
Still uniform density
Bulk of the atom is positive
Negative electrons float around
Rutherford’s Gold Foil Experiment
Hypothesis Alpha emitter
(positvie charge) screen
Gold foil
Because the atom has uniform density, the alpha
particles will pass straight through with little to no
deflection.
Like a bullet through a stick of butter.
Rutherford’s Gold Foil Experiment
Results Conclusion
The atom does not have uniform density.
Because the alpha particles passed through, there must
be empty space.
Because the alpha particles were bounced back, they must
have hit something dense.
Because the alpha particles were deflected, the dense part
must have a positive charge.
Rutherford’s model
Dense positive
center
Mostly empty or unoccupied space
Electrons float around randomly in the empty space
Chadwicks
-discovered the neutron
-when doing studies, he found that the atom often weighed twice as much as he
anticipated with the number of protons
-he determined that there must be some other particle with the same mass as a
proton but had no charge
-neutrons are located in the dense positive center (nucleus)
Particle Symbol Charge Mass Location
proton p + 1 1 amu nucleus
neutron n 0 1 amu nucleus
electron e - 1 1/1836
mass of a
proton
shell
nucleons
Nuclear charge
Mass number = # of protons + #of neutrons
Atomic number =#of protons X A
Z
Protons
And
neutrons
# of Protons
K 39
19 Se 79
34
I 131
53 P-
N-
E-
19
20
19
P-
N-
E-
34
45
34
P-
N-
E-
53
78
53
I-131
When an atom is neutral (no charge),
the number of electrons will equal the protons.
Calculate the number of protons, neutrons, and electrons.
Mass # Rewrite:
Ion -has a charge (pos or neg)
Gain electrons = negative ion
Lose electrons = positive ion
P-
17Cl - 35
N-
E-
20Ca2+ 40
P-
N-
E-
17
18
18
20
20
18
Isotope -Atoms with the same atomic number, but different mass number -same bottom, different top -differ in the number of neutrons
C 12
6
C 14
6
Atomic Mass
1. Write percent as decimal. 2. Multiply decimal by the mass #. 3. Add them up. 4. Units will be atomic mass units (amu)
75% Cl-35 25% Cl-37
.75 x 35 = 26.25
.25 x 37 = 9.25
35.50 amu
-is the weighted average of all naturally occurring isotopes.
-based on the percent abundance and mass
Bohr
Model
•proposed a model of the atom showing a dense nucleus with electrons found in surrounding orbits. •Bohr model shown shows a nucleus surrounded by electrons in circular orbits. •Each electron must posses just the right amount of energy to keep it in place around the nucleus.
energy levels principal energy levels shells
Electron configuration
the distribution of the electrons in an atom
2(n2) Energy level
2-8-8
1st
level
2nd
level
3rd
level
Energy level Max # of e’s
1 2(12) 2 2 2(22) 8 3 2(32) 18 4 2(42) 32 5 2(52) 50 6 2(62) 72 7 2(72) 98
In theory
We will assume
only 32 per level
quantum-- a discrete amount of energy -- each energy level represents a specific quanta -- to move from one level to another, you must use that much energy
Ground state all electrons are in the lowest possible level will follow the pattern: 2-8-18 Pattern on periodic table
Excited state when an electron absorbs energy and jumps to a higher level can’t stay there pattern skips 2-7-1 NOT on periodic table WAS: 2-8 (moved 1 e- to the next level)
Atoms can not stay excited, when electrons drop to a lower level, energy is released in the form of light(photon). Each element gives off a specific color and spectrum.
kernel valence electrons
represents atom from the nucleus to the valance shell
are electrons in the outer energy level of the atom
continuous
spectrum bright line spectra
R O Y G B I V
•white light through a prism or a
diffraction gradient
•shows all the colors is the visible spectrum
•diffraction gradient separates the light
•each element gives off their own bright line spectrum •each line represents an electron dropping to a lower level •fingerprint of an element