Download 2. atom - New Hartford Central Schools

ATOM
Early Thoughts
Aristotle-- Continuous theory
Greeks
matter is made up of
particles--4 elements
4 elements --air--fire--water-- earth
Matter can be divided indefinitely
Democritus --Discontinuous theory
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
Sphere
Uniform density
Fire ball
Cathode
Tube
Cathode ray
Negative
electrode
-cathode rays are a form of radiation that
contain an electrical charge
-rays move from negative to positive
-rays are negative (electrons)
Positive
electrode
hi gh vol tage
QuickTime™ and a
decompressor
are needed to see this picture.
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”
Negative electrons float around
Bulk of the atom is positive
Still uniform density
Rutherford’s Gold Foil Experiment
Alpha emitter
(positvie charge)
screen
Hypothesis
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.
Gold foil
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.
QuickTime™ and a
decompressor
are needed to see this picture.
>Click Me<
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
nucleons
proton
p
+1
1 amu
nucleus
neutron
n
0
1 amu
nucleus
-1
1/1836
mass of a
proton
electron
e
Mass number = # of protons + #of neutrons
Atomic number =#of protons
Protons
And
neutrons
Nuclear charge
# of Protons
shell
A
Z
X
Calculate the number of protons, neutrons, and electrons.
K
Se
39
79
19
34
P-
19
P-
34
N- 20
N- 45
E-
E-
19
Rewrite:
34
When an atom is neutral (no charge),
the number of electrons will equal the protons.
I-131
Mass #
I
53
131
P-
53
N- 78
E-
53
Ion
-has a charge (pos or neg)
Gain electrons = negative ion
35
17Cl
-
P- 17
N- 18
E- 18
Lose electrons = positive ion
40
2+
20Ca
P- 20
N- 20
E- 18
Isotope
-Atoms with the same atomic
number, but different mass
number
-same bottom, different top
-differ in the number of neutrons
C
6
14
C
12
6
Atomic Mass
-is the weighted average of all naturally occurring isotopes.
-based on the percent abundance and mass
1.
2.
3.
4.
Write percent as decimal.
Multiply decimal by the mass #.
Add them up.
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
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
2
2(n )
Energy level
Electron configuration
the distribution of the electrons in an atom
2-8-8
1st
level
2nd
level
3rd
level
Energy level
1
2
3
4
5
6
7
Max # of e’s
2(12)
2(22)
2(32)
2(42)
2(52)
2(62)
2(72)
2
8
18
32
50
72
98
In theory
We will assume
only 32 per level
kernel
represents atom
from the nucleus to
the valance shell
valence electrons
are electrons in the
outer energy level of
the atom
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.
continuous
spectrum
ROYGBIV
•white light through a prism or a
diffraction gradient
•shows all the colors is the visible spectrum
•diffraction gradient separates the light
bright line spectra
•each element gives off their own bright
line spectrum
•each line represents an electron
dropping to a lower level
•fingerprint of an element
Modern Theory -- Wave (Quantum) Mechanical Model
electron cloud
electrons don’t travel in fixed paths but are located in regions of space
called orbitals
we don’t know the exact location of the electron but can guess where it is
most likely located
QuickTime™ and a
based on probability
decompressor
are needed to see this picture.