Notes on Half-Lives and Isotopes, 12/13/10:

Vocab Words for today:  Half-life,  Atom, Isotope, Neutron, Proton, Electron

A half-life is the length of time it takes for 1/2 of a parent isotope to decay into a daughter isotope.

Examples:   
  • Carbon 14 is an isotope of Carbon which has a half-life of 5730 years.  In 5730 years, you could expect 1/2 of a sample of Carbon 14 to decay
    into Nitrogen 14 (Important Note, Mr. Ward mispoke in class today saying it turned into Carbon 12.  This was a mistake.  It decays into Nitrogen
    14.)
  • Rubidium 87 is an isotope of Rubidium which has a half-life of 44.8 BILLION years.  In 44.8 billion years you could expect 1/2 of a sample of
    Rubidium 87 to decay into Strontium 87, its daughter isotope.

Atom:  (Comes from the Greek word for "indivisible").  The smallest unit of matter that could be considered an element.  Atoms have a dense region in
their center made of positively charged protons and neutrally charged neutrons.  Surrounding this "atomic nucleus" is a cloud of electrons.

Examples:  Carbon and Nitrogen















Atomic Number:  As you can see, the atomic number is above the letter for the element and equals the number of protons (and electrons, usually).  
The number of protons in an atom
DEFINES what type of element an atom is.  NO MATTER WHAT else is going on with an atom, the atomic number
always defines what sort of element it is.  Carbon is always has 6 protons and Nitrogen always has 7 protons.

Atomic Mass:  This is the average mass of this type of element's atom.  If you round the number, you will be able to find out the number of neutrons
in addition to the number of protons.  
Number of Protons (Atomic Number) + Number of Neutrons = Rounded Atomic Mass Number.

This means that Carbon 12, the most stable and most abundant form of Carbon, has 6 protons and 6 neutrons (as well as a usual number of 6
electrons).

This also means that
Nitrogen 14, the most stable and most abundant form of Nitrogen, has 7 protons and 7 neutrons (as well as a usual number
of 7 electrons).

Isotope:  All atoms are isotopes.  Isotopes are different versions of the same element which have the same number of protons (Atomic
Numbers) and different Atomic Masses (different numbers of neutrons)
.  Some isotopes are stable and some are unstable.  The unstable
ones decay into more stable ones.

Exampe 1:  Hydrogen, the most common element in the Universe, has 3 different isotopes.  Each has 1 proton and generally 1 electron, but
they have different atomic masses because they have different numbers of neutrons.

1st Hydrogen Isotope:  Protium (most common)-->  1 proton and 1 electron.  There is NO neutron and its atomic mass is 1.
2nd Hydrogen Isotope:  Deuterium (rare)-->  1 proton, 1 neutron and 1 electron.  The 1 neutron + the 1 proton makes its atomic mass 2.
3rd Hydrogen Isotope:  Tritium (extremely rare and made famous in Spiderman 2)-->  1 proton, 2 neutrons, and 1 electron.  The 2 neutrons + the 1
proton makes it
s atomic mass 3.

Example 2:  Carbon 14 decays into Nitrogen 14 with a half-life of 5730 years.

Carbon 14 has 6 protons and 8 neutrons.
Nitrogen 14 has 7 protons and 7 neutrons.

Where does Carbon 14 come from?
Carbon 14 is made in the outer Earth atmosphere where cosmic rays bombard the different
elements in our air.  Since Nitrogen is the most common element in our atmosphere, it is the
most likely to get whacked by a cosmic ray.  Nitrogen is usually in a stable isotope form called
Nitrogen 14, with 7 protons and 7 neutrons.  If it gets whacked by a neutron, it can lose a proton
and gain a neutron, leaving it with 6 protons and 8 neutrons.  Since the number of protons
determines what sort of element it is, it has now become Carbon instead of Nitrogen.
However, it has a larger mass than usual:  6 protons plus 8 neutrons give it an atomic mass of 14.