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A. Representation of an Atom
1. nucleons- description of atomic nuclei based on protons and neutrons particles
2. nuclide- used in nuclear chemistry as a reference to an atom, identified by the number of
protons and neutrons in its nucleus
3. Example:
(atomic mass)
( name of element followed by atomic mass
Ra (symbol)
which is the # of protons + neutrons)
(atomic number)
B. Mass and Energy Relationships
1. mass defect- difference between the mass of an atom and the sum of the masses of the
subatomic particles composing that atom
2. Example: helium-4:
2 protons (2 x 1.007276 amu) = 2.014552 amu
2 neutrons (2 x 1.008665 amu) = 2.017330 amu
2 electrons (2 x 0.0005486 amu) = 0.001097 amu
total combined mass: 4.032979 amu
The actual mass of the atom is measured to be 4.00260 amu! That is 0.03038 amu less
then the sum of all of its particles.
3. The mass defect is caused by the conversion of mass (m) to energy (E) when the nucleus
was originally formed.
4. nuclear binding energy- the energy that was released when a nucleus is formed from
nucleons (E = mc2 ). It can also be thought of as the amount of energy required to break
apart the nucleus; therefore, the nuclear binding energy is also a measure of the stability of
a nucleus.
5. band of stability- a stable nuclei cluster shown as a neutron to proton ratio
a. most stable nuclei are 1:1 ratio
b. higher atomic masses show close to a 1.5:1 ratio; this is accounted for because proton
electrostatic repulsion is greater than the effect of nuclear forces at this point