Transcript L15-Chemistry

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The Hydrogen Atom
Other Atoms
Only 2 electrons fit in the lower shell.
The 3rd electron must fit in the next shell.
Atomic Shells
The discrete electron levels are arranged in
shells. Each shell has a maximum occupancy.
The first electronic shell can have at most 2
electrons, the second shell has room for 8
electrons and so on.
The 1st shell has the lowest energy. Thus,
elements, in their lowest energy state fill the 1st
level first, and then fill the 2nd level next. These
elements are listed in the 1st and 2nd rows of
the periodic table.
Atoms are most stable if their outer shell is full.
The electrons in outer shells are shielded by
the inner shells from the full attraction of the
nucleus. These electrons participate most
readily in chemical reactions.
Atomic Shells
How many electrons
does neutral Carbon
(6 protons) have in
its outer shell?
How many electrons
does neutral Neon
(10 protons) have in
its outer shell?
The Periodic Table
Atomic
Sizes
Ionization
Energy
Periodic or Mendelev Table
# protons
1 e- missing to fill shell (Halogens)
1 e- in outer shell (Alkali Metals)
Full outer shell (Noble Gases)
Li: solid, Cs: liquid, Ar: gas, Tc: synthetic
Bonding
There are three major ways that elements bond to form molecules.
Atoms with filled shells, the Noble
Gases, are highly inert.
Atoms with one electron in the outer
shell, and atoms with one electron
missing are, on the other hand,
highly reactive. These atoms form
ionic bonds. The alkali gives up an
electron. The halogen takes the
electron. Elements are bonded by
the electric force between the ions.
Ionic Bonds
Example of ionic bonds
Sapphire
Aluminum oxide, Al2O3
Ruby
Metallic Bonds
Atoms in a metals also
give up electrons,
however the electrons
are not transferred to
the other atom.
Instead, they are
shared by all atoms.
The electron sea allows
current to flow through
metal. Metals thus
make good conductors.
In sodium, for example, 1 out of the 11 electrons is
released so that Na has two filled shells. The extra
electrons move around the metal in a “sea” of negative
charge. This negatively charged sea moves around a
regular structure of positive Na ions.
Covalent Bonds
Certain molecules are formed by sharing
electrons. The covalent bond that forms
resembles metallic bonds in that electrons are
shared. Yet, like ionic bonds the electrons are
shared in discrete shells of the atoms and don’t
run willy nilly throughout the material.
Question
How would you covalently bond
two oxygen atoms to make O2?
Oxygen has 8 protons & electrons.
How many electrons would each O
have to share with the other?
Periodic or Mendelev Table
# protons
1 e- missing to fill shell (Halogens)
1 e- in outer shell (Alkali Metals)
Full outer shell (Noble Gases)
What kind of bonding does KBr have?
Answer
Question
How would you expect KF to be bonded?
Periodic or Mendelev Table
# protons
1 e- missing to fill shell (Halogens)
1 e- in outer shell (Alkali Metals)
Full outer shell (Noble Gases)
What kind of bonding does KBr have?
Answer
Ionic Bond: K+F-
Earth’s Atmosphere
Multiple
Covalent Bonds
Gases in Earth’s atmosphere are mainly
covalently bonded molecules or noble gases.
N2
78%
O2
21%
H 2O
0-4%
Ar
0.9%
CO2
0.035%
Ne
0.0018%
He
0.0005%
CH4
0.0001%
H2
0.00005%
O3
0.000004%
Combining C6, N7, O8
Molecular
attractions
Polar molecules are more
positively charged on one
side and more negative
on the other. This
provides a cohesion.
Mars: why
red?
Iron Oxides are present
e.g. Hematite or Fe2O3
Ozone Layer
Life on other planets
Summary
Atoms have discrete energy levels, specific to that atom.
A photon is absorbed when an electron jumps to a higher energy level.
A photon is emitted when an electron drops to a lower energy level.
The emitted/absorbed photon’s energy equals the difference between the atomic levels
involved.
Atomic levels can only fit a certain number of electrons (2 in the 1st level, 8 in the 2nd …)
The periodic table is arranged according the electronic shells and the number of
protons/electrons in the atom.
Atoms with filled shells are most stable. Atoms bond in order to achieve this
configuration.
Ionic bonding involves the transfer of electrons from one atom to another.
Covalent bonding involves the sharing of electrons by one or several atoms.
Metallic bonding involves the sharing of electrons by the entire material/metal.
Radioactive Dating
Parent
Carbon-14
Daughter
Nitrogen-14
Half Life
5,730 yrs*
Potassium-40
Argon-40
1.25 billion yrs
Uranium-238
Lead-206
4.5 billion yrs
Thorium-232
Lead-208
14 billion yrs
Rubidium-87
Strontium-87
48.8 billion yrs
Samarium-147
Neodymium-143 106 billion yrs
Uranium-235
Lead-207
704 billion yrs
*Time that it takes wood to have half the C14 of a living plant.
Swisher et al. 1992, Science
Carbon Dating
1. Solar neutrons enter Earth’s atmosphere.
2. Neutrons collide with N14 (7p, 7n),
creating C14 (6p,8n) [n + N14  p + C14 ]
3. Living bodies continually absorb C14 (e.g.
as CO2 in photosynthesis).
4. When the plant or animal dies, it no
longer assimilates C14.
5. The C14 decays (half life of 5730 yrs).
[C14  N14 + e- + ve ] (n  p + e- + ve )
6. The e- emission rate reveals the age.
Measure ages < 70,000 yrs