More nuclear Physics, then begin chemistry notes

Lecture notes 7-3-07

Atomic number, atomic mass number and symbology example: ₉₂²³⁵U, : ₉₂²³⁸U

Recall isotopes being atoms with same number of protons and different number of neutrons.  Isotope means “same place,” in this case same place in periodic table.

Half life.   The time it takes for half of a sample of radioactive elements to decay. Independent of how much you start with.  Half of what was there is gone (decayed to another element) in one half-life, so you have half of what you started with, then half of that, then half of that...on down to when only a few of the original radioactive atoms are left, and then the half-life concept is no longer valid.

For anything that has “rate of decay or growth proportional to amount present” you have this sort of exponential time relationship: a decrease for decay and an increase for growth.  (gave example eating cake in hell, where you have your cake and eat it too, always some left but you only get a tinier and tinier crumb in each bite as time passes, but of course hell is for eternity which means time doesn't pass, so never mind) 

See previous lecture notes for another example....half of a half of a half ... of a hamburger.

Fission and Fusion.  The most stable atoms are in the middle of the periodic table.  See figure 10.20 for the reason why fission and fusion produce energy.  Who discovered fission has been a controversial issue for many years.  Lise Meitner, Otto Hahn and Fritz Strassmann worked together in Berlin until M. escaped from Nazis because she was Jewish.  But she had the correct idea along with her nephew Otto Frisch.  She should have won the nobel prize too, along with Hahn.

Biological effects of radiation.  Alpha, beta, gamma, neutrons and x-rays are ionizing radiation. Ionizing means they can knock electrons out of an atom, or even hit the nucleus and change its electrical charge (adding or subtracting a proton).  The result in either case is that the atom becomes an ion, meaning it isn’t electrically neutral.  Ion means “an atom, or chemical combination of atoms, that has a net electric charge.” Absorbed nuclear radiation can cause unwanted chemical reactions, can kill cells, can cause genetic defects that will show up in a future generation.

The average US citizen today receives a small amount (about 0.2 rem) of natural and man-made radiation. It’s a small amount but it’s a long term exposure, and is also an average. About 82% of this comes from natural sources and the rest from human activities.  The rem measures dosage and its biological effectiveness, whereas the rad is 0.01 joule of energy deposited per kg of tissue.

Chemistry, the chemical elements.

Classification of matter:  elements, compounds, mixtures. 

Pure substances can be elements or compounds.  Element: a substance that is just a bunch of the same type of atoms.  Compound:  two or more elements chemically bonded in a definite mass ratio. Water H2O, Salt NaCl.

Then there’s the mixture, a type of matter that has varying proportions of two or more pure substances that are physically mixed together but are not chemically bonded.

Heterogenous mixture:  Two or more substances can be seen in a heterogeneous mixture—it is not uniform in appearance or texture.  Italian dressing and the mixed green salad you put it on are two examples.  Others are pizza and zinc mixed with sulfur (as opposed to the compound zinc sulfide).

Homogenous mixture:  There are two or more substances making up the mixture but it appears to be just one substance.  All for one, one for all.  Another name for homogeneous mixture is solution.  Solids, liquids and gases can be solutions. Air, coffee, brass (zinc and copper), saltwater are examples. 

Solutions are composed of a solute and a solvent.  The solvent is the substance present in the largest amount.  When you make coffee or tea, water is the solvent and the coffee molecules or tea molecules are the solute.  Remember, they just mix together very well, they don’t combine chemically.

Discovery of the elements>  Robert  Boyle in 1661 proposed that substances that could not be separated into components by any method were elemental or fundamental and should be called elements.  Figure 11.8 shows how the number of identifiable elements has increased over the years.  Now the elements can be broken down by nuclear bombardment, but are still called elements.

Molecules  are stable, chemically bonded configurations of atoms. 

Occurrence of the elements:  in our bodies, 65% oxygen and 18% carbon.  In  the earth, about 47% oxygen and 27% silicon.  In the universe (and in the sun) hydrogen 75% aand helium 24%, then the other stuff is only 1% of the total and comes from supernova explosions.

_________________________got this far in class on 7-3_______________________

Two or more forms of the same element that have different bonding structures in the same physical phase are called allotropes.  Carbon example, figure 11.14.

The periodic table.  The idea it’s based on is that the properties of the elements repeat in a systematic or periodic fashion. The Russian scientist Mendeleev invented it.

Like any table, it has rows and columns.  The seven rows are called periods. Similar properties in each row occur at definite places as you go across the table.  The columns are called groups.  They are groups of elements with similar chemical properties.

What kind of properties are we talking about?  Four, as far as we’re concerned.

1. metal vs non-metal.  Metals are defined as any elements that tend to lose electrons in chemical reactions. Nonmetals are elements that tend to gain or share electrons during chemical reactions.
2. electron configuration/valence…  Introducing “Shells.”  The principle quantum numbers, n, designating the main energy levels of the electrons in an atom, are referred to as shells.  The outer shell is called the valence shell and is our only concern in here.  It’s important because the number of “valence electrons” in this shell determine the chemical properties of an element.  Elements in a given group (column) in the periodic table have the same number of valence electrons and thus have similar chemical properties.  The shells fill up going from left to right in the table.  The number of shells is equal to the group number.  “octaves” ridiculed, but turned out to be true.
3. atomic size…  increases as you go down a column (group); decreases as you go across left to right—more tightly bound outer electrons, no shells being added.
4. ionization energy…increases as you go across left to right.  Again it’s because the shells are filling and the outer electrons becoming more tightly bound.