The Nucleus
I.
The nucleus was identified by _____________ _______________________ as a region located at the
center of each atom which contains all of the positive charge in the atom, but contributes only a very
small amount to the total volume of each atom. Rutherford realized that atoms were thus mostly empty
space; with the __________________-charged nucleus at the center and ____________________charged ________________________ whizzing through the empty space around the nucleus.
II. To keep the contents of this chapter in perspective, more than 2000 years ago, Democritus (an ancient
Greek philosopher) thought that un-dividible particles he called “atomos” must exist….It just seemed
logical to him that they existed. No one believed Democritus until John Dalton, in the early 1800’s, infers
from experimental data that atoms must exist – and Dalton also believes that atoms can’t be divided.
But, within 100 years of Dalton, JJ Thomson identifies a _____________________ particle (a particle
found within an atom) which he called an “electron”. As time progresses huge strides in understanding
occur concerning the electrons and quantum theory develops in order to explain the behavior of
electrons. Today we understand that, although the electrons travels far from the nucleus, the electron is
the _________________ (least massive) of the three major subatomic particles.
III. Protons and the atomic number: The two major subatomic particles found in the nucleus
(___________________; ___________________) are about 2000 x more massive than the tiny electron.
The _________________ _________________ of an element is, by definition, equal to the number of
_____________________ found in each atom’s nucleus; thus, the number of protons in a nucleus
identifies an atom as being an atom of a particular element.
IV. Neutrons: While the number of neutrons in an atom _______ equal the atomic number, the number of
neutrons in an atom doesn’t have to equal the atomic number. Thus, the number of neutrons in an
atom doesn’t establish the identity of the atom. The neutrons are still important though, as they
contribute a substantial amount to the total mass of the atom, and impart stability to the nucleus.
V. Radioactivity: In fact, when a nucleus is ____________________, both small particles and energy that
we call_________________________ will be emitted from an atom as the nucleus tries to become more
stable. In nature, each element has its own pattern of nuclear changes; but, as long as it is only the
number of _____________________, and not the number of protons that is changing, the identity of the
atom (as a given element) remains. Marie _______________, in the early 1900’s is the foremost of the
researchers who established an understanding of the role of the nucleus in understanding radioactivity.
In fact, Marie Curie died as a result of her studies with nuclear radiation as she developed mobile X-ray
equipment to help soldiers during WWI.
VI. Isotopes of an element – isotopes of an element have (of course) the same number of protons, but they
have a ____________________ number of neutrons. Isotopes have the _____________ chemical
reactivity, as it is the arrangement and interaction between protons and electrons which causes chemical
reactivity.
VII. Isotope notation – Unless a chemist is studying or using just one particular isotope of an element, the
atoms are identified by the symbol of the element (found on the periodic table). But, when a chemist is
referring to a particular isotope, some additional information is written to the left of the symbol.
Similarly, when a chemist is not referring to one particular isotope of an element, the element’s name as
written on the periodic table suffices. But, when a chemist is referring to one particular isotope,
additional information must be included. The additional information is what is called the “mass
number”. The mass number is the _________ of the subatomic particles which contribute to the bulk of
the atom’s mass. [Remember, the electrons’ mass is negligible.] The mass number is the number of
nuclear particles (the number of ________________ + the number of _________________) in one
atom’s nucleus.
Identify the mass number and atomic number within the notations below. Then name each isotope.
1
1H
2
1H
3
1H
12
6C
14
6C
By subtracting: (mass number – atomic number), we arrive at the number of _________________ in one atom
of that isotope. In the example isotopes (above), how many neutrons are in one atom of each isotope?
___ – ___ = ___
VIII.
___ – ___ = ___
___ – ___ = ___
___ – ___ = ___
___ – ___ = ___
Average atomic mass of the atoms in an element: Is calculated from a type of calculation called a
“_________________ _________________”. The average atomic mass of an element reflects both
the natural abundance of each specific isotope present in the element, and the mass (in amu) of each
specific isotope present in the element. The unit for the average atomic mass is the _________,
which is the symbol used for “atomic mass unit”. The average atomic mass is the same as the
______________ ____________ of the element; however, the unit is “amu” instead of “g/mole”.
%
Formula for calculating the average atomic mass:
x amu isotope 1)  (% x amu isotope 2) 
(% x amu isotope 3)  etc...
100
Additionally, when the exact “amu” of each isotope is not known, an estimate may be made using
the mass number as the amu. In this case, the sig figs are at times dramatically diminished.
Final unit = amu
%
x mass number)
 (% x mass number) 
100
(% x mass number)  etc...