Download elements and isotopes - vocabulary

ELEMENTS AND ISOTOPES - VOCABULARY
element
A species of atom; each atom of a particular element has a specific number of protons in
the nucleus which is the same for all atoms of the element.
isotope
A species of atom; each atom of a particular isotope has a specific number of protons and
a specific number of neutrons in the nucleus which are the same for all atoms of the
isotope, but are not necessarily equal to each other.
atomic number
The number of protons in the nucleus of one atom; the atomic number defines an
element.
mass number
The number that represents the sum of the numbers of protons and neutrons in the
nucleus of one atom; a combination of the atomic number and the mass number defines
an isotope.
atomic weight (relative atomic mass)
The average mass of all atoms of a particular element found in nature. It is also called
relative atomic mass. It is expressed in atomic mass units (amu). On the atomic mass
scale, the mass of one atom of carbon-12 is set up as a standard and is exactly 12 amu.
naturally occurring isotope
An isotope whose atoms are present and can be detected in nature. Naturally occurring
isotopes could be stable (e.g., oxygen-17) or radioactive (e.g., uranium-238). They could
be abundant or present in tiny amounts in the Earth crust, oceans, or atmosphere.
stable isotope
An isotope whose atoms have an average half-life of more than 10 billion years (1010
years; the age of Earth is about 4.5 billion years; the age of the universe is about 15
billion years). The decay of atoms of a stable isotope cannot be detected by any modern
technique.
radioactive isotope
An isotope whose atoms undergo a detectable radioactive decay. Radioactive isotopes
can be naturally occurring or synthetic.
radioactive decay
Transformation of an unstable atom that produces a more stable atom (or atoms) as well
as radiation (energy and energetic particles).
synthetic element (isotope)
An element (isotope) whose atoms have been created in nuclear reactions as a result of
human activity: nuclear reactors, supercolliders, nuclear bomb explosions. All synthetic
elements (isotopes) are radioactive.
ATOMIC WEIGHT AND ATOMIC MASS UNIT
1. What is atomic weight?
2. What is the value of one atomic mass unit (amu or u) in grams?
3. Although there is only one naturally occurring isotope of iodine, 127I, the atomic mass of
iodine given in the modern periodic table is 126.90447, but not 127. Explain.
(From College Chemistry by J.L. Rosenberg, L.M. Epstein, and P.J. Krieger)
4. At one time there was a chemical atomic mass scale based on the assignment of the value
16.0000 to naturally occurring oxygen. What would have been the atomic mass, on such a
table, of silver? (From College Chemistry by J.L. Rosenberg, L.M. Epstein, and P.J. Krieger)
5. In the article on “Chemistry” in the Ninth Edition of the Encyclopedia Britannica (published
in 1878) the author (H. A. Armstrong) says that Mendeleev had recently proposed that
uranium be assigned the atomic weight 240 in place of the old value 120 that had been
assigned to it by Berzelius, but that he himself preferred 180. Mendeleev was right. The
correct formula of pitchblende, an important ore of uranium, is U3O8.What formula was
written for pitchblende by (a) Berzelius, (b) Armstrong?
(From General Chemistry by L. Pauling)
ATOMIC THEORY WORKSHEET
1. Which of the following statements of the atomic theory proposed by John Dalton at the beginning
of the 19th century are not quite true in light of modern atomic physics? Rewrite each statement to
reflect the current understanding of the atomic theory.
An element is made up of atoms. All atoms of
a given element are identical.
Atoms cannot be created or destroyed.
All atoms of one element have the same
mass. Atoms of two different elements have
different masses.
Atoms of different elements may combine in
the ratio of small, whole numbers to form
compounds.
2. Calculate the atomic weight of neon using the data on its isotopes provided in the table below.
Isotope
Number of
Protons
Number of
Neutrons
Mass
Number
Atomic Mass
(amu)
Natural
Abundance
(atom %)
20
10
10
20
19.9924
90.48
21
10
11
21
20.9938
0.27
22
10
12
22
21.9914
9.25
Ne
Ne
Ne
K
39
19
U
235
92
Symbol
of the
Atom
(Isotope)
92
238
235
Atomic Mass
Number Number
92
Number
of
Protons
in the
Atom
7
20
146
143
Number of
Neutrons
in the
Atom
6
92
Number of
Electrons
in the
Atom
12 amu
(exactly)
235.0439
amu
Mass of
the Atom
ATOMS, ISOTOPES, ELEMENTS
chlorine
uranium
Name of
the Element
sodium-23
sulfur-34
uranium-235
Name of
the Isotope
35Cl+
37Cl+
Intensity
88
90
94
96
98
Atomic mass, amu
92
100