Atoms and Bonds

I. Atoms

A. Matter

    1. Elements are different forms of matter which have different chemical and physical properties, and can not be broken down further by chemical reactions.
    2. The smallest unit of an element that retains the properties of that element is an ATOM.

B. Properties of Atoms

    1. Nucleus (protons (charge = +1, mass = 1) and Neutrons (charge = 0, mass ~1).  The number of protons distinguishes one element from another (Atomic number).
    2. Cloud of electrons (-) Shells and orbitals: Shells 1, 2, 3 have 1, 4, 4 orbitals, holding 2, 8, and 8 electrons Distance of orbital correlates with energy of electron farther away from nucleus, more energy. Electrons can absorb or release energy and move to an outer orbital or inner orbital. Without an input of energy, an atom will lose energy and approach it's lowest energy state. Binding properties are largely governed by the number of electrons in the outermost orbital.
    3. Mass = # protons + # neutrons.
    4. Charge = number of protons relative to number of electrons If unequal, then the atom has a net charge and is called an ion.
    5. Space: The orbitals are 10,000 x the width of the nucleus, so an atom is mostly space
    6. Radioactive Decay:
            Isotopes: Isotopes are atoms with extra neutrons...thus they are heavier.  Some isotopes are stable; others decay (lose the extra neutrons). Radioisotopes (those that decay) emit energy (radiation) when they loose a neutron. This change in the nucleus is unaffected by environmental conditions (chemical reactions, temperature, etc.), and thus the rate of decay is constant. Since the rate is constant, if we can measure the amount of parent and daughter isotopes, and we know the rate of change, we can determine the amount of time that has passed for "this much change" to occur.

            Gamma decay -   neutron emits energy as a photon - no change in neutron number, mass, or element.
            Alpha decay - loss of an alpha particle  (2 protons and 2 neutrons) from the nucleus.  This changes the mass and element.
                    (Uranium with 92 protons decays to Thorium with 90 protons)
            Beta decay - a neutron changes to a proton, and an electron is emitted. This changes only the element (determined by the number of protons.), but not the mass.
                    (C14 is an example here... 6 protons and 8 neutrons. With the conversion of a neutron to proton N14 is produced - 7 proton and and 7 neutrons)

                a. Principle:
                   - measure amt of parent and daughter isotopes = total initial parental
                   - with the measureable1/2 life, determine time needed to decay this fraction
                   - K40-Ar40  suppose 1/2 of total is Ar40 = 1.3by

                (Now, you may be thinking, "be real"! How can we measure something that is this slow?)

                 - Well, 40 grams of Potassium (K) contains:
                                 6.0 x 1023 atoms (Avogadro's number, remember that little chemistry tid-bit?).

                  So, For 1/2 of them to change, that would be:
                                 3.0 x 1023 atoms in 1.3 billion years (1.3 x 109)

                  So, divide 3.0 x 1023  by 1.3 x 109 = 2.3 X 1014 atoms/year.

                  Then, divide 2.3 x 1014 by 365 (3.65 x 102) days per year = 0.62 x 1012 per day ( shift decimal = 6.2 x 1011)

                  Then, divide 6.2 x 1011 by 24*60*60 = 86,400 seconds/day: (= 8.64 x 104) = 0.7 x 107 atoms/second

                   0.7 x 107 = 7 x 106 = 7 million atoms changing from Potassium to Argon every second!!!

                 This radiation is detectible and measureable...and when it has been measured over the last 100 years, it is always the same.  So, not only is there theoretical justification for expecting a constant decay rate, tests have confirmed this expectation. It is unaffected by any known physical change in the environment... freeze it, heat it, pressurize it... no change in the rate of decay.
 

II. Bonds

A. Molecules

    1. atoms chemically react with one another and form molecules - the atoms are "bound" to one another by chemical bonds.
    2. Bonds form because atoms attain a more stable energy state if their outermost shell is full. It can do this by loosing, gaining, or sharing electrons.  This is often called the 'octet rule' because the 2nd and 3rd shells can contain 8 electrons.

B. Covalent Bonds

Share electons in pairs: H2, H2O, etc. These are the primary bonds in biologically important molecules - they can be non-polar (shared evenly) -H2 - or polar (shared unevenly...creating a charge differnce across the molecule).

C. Ions

If the attraction between nuclei is very unequal, the shared electrons can be stripped from one of the atoms and taken by the other. This creates charged particles (ions) which may then be attracted to one another based on their opposite charge. (NaCl)

D. Hydrogen Bonds

These are weak ionic bonds, weak attraction between a partially charged Hydrogen Atom (+ charge) and a negatively charged molecule or the negative portion of a molecule.

 


  study questions

1.  What is the 'octet rule'?  How and why does it govern atomic binding?

2. How do polar and non-polar covalent bonds differ?

3. What is a hydrogen bond, and why are they so common?

4. What is radioactive decay?  If the half-life of Potassiium to argon decay is 1.3 by, and we observe a rock with a ratio of K:Ar of  1/8: 7/8, how old is it?