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Atomic Theory:


    1.    Name and describe the three subatomic particles in an atom.

    2.    Determine the number of protons, neutrons, and electrons in an atom or ion.

    3.    Define isotope and atomic mass.

Key Terms:

    proton    neutron    atomic mass unit    atomic number    ion    mass number    isotope    atomic mass

Web Resources:



Atoms are made of even smaller subatomic particles called protons, neutrons, and electrons.  

(web.buddyproject.org/web017/ web017/images/atom.JPG)
protons - have a positive charge, found in the nucleus and have an atomic weight of one
electrons - have a negative charge, move in the space around the nucleus, the number is equal to protons in a neutral atom and they have no appreciable mass
neutrons - have no net charge, found in the nucleus, and have an atomic weight of one

Atomic Numbers:

    The atomic numbers indicated on the periodic table indicate the number of protons in the nucleus.

(sciencemadesimple.com/ img_static/atom.gif)

The number of protons determines the positive charge of the atom
The number of protons determines the atoms identity
The electrons and neutrons of an atom can change but it cannot change the amount of protons and still be of the same element
In neutral atoms the # of protons = # of electrons


Whenever an atom gains or looses electrons it becomes an ion.  You can find the net charge of the atom by subtracting the number of electrons from the atomic number.

If the result is positive (atomic # > electrons) - cation - overall positive charge
    example: Ca    20(p) - 18(e) = 2+    or    Ca2+
If the result is negative (atomic # < electrons) - anion - overall negative charge
    example: O     8(p) - 10(e) = 2-        or    O2-


Most elements in the first two rows of the periodic table have at least two known isotopes.  

    Example:    hydrogen-1            hydrogen-2            hydrogen-3

(www.cat.cc.md.us/~cminnier/ hisotop.gif)

Normally found in mixtures in nature in specific ratios
Isotopes have the same number of protons and electrons but differ in the number of neutrons.  (mass - protons = neutrons)
Isotopes are named after their masses (neutrons + protons) 

    Chemical Notation:

          (mass #)   37Cl1-  (charge)

        (atomic #)  17

Atomic Mass:

The atomic mass of an atom is expressed in atomic mass units (amu).  This unit is derived from the carbon atom and is measured to be 1/12 of the mass of the carbon-12 atom.  Mass of an atom is found by adding the protons and neutrons of an atom.  Since most atoms exist in isotopes with known ratios and neutrons do not have the exact weight as protons, the mass number expressed on the periodic table will usually not agree exactly with the amu number.  

Is the sum of the protons + neutrons
can be expressed in amu
The amu expresses the most common isotope of the atom.

Determining the amu in atoms of multiple isotopes:

Look at the three common isotopes of silicon.  Multiply the masses of the isotopes by their fractional abundances (percent found in nature) and add the products together.

Element Symbol Mass amu Fractional Abundance Contributing Mass Average Atomic Mass
  Si-28 27.977 92.21% 25.80  
Silicon Si-29 28.976 4.70% 1.36 28.09
  Si-30 29.974 3.09% 0.93  


Last modified: September 19, 2003