CHARACTERISTICS OF ATOMS
DOE-HDBK-1015/1-93
Fundamentals of Chemistry
CH-01
Rev. 0
Page 4
Chemical Symbols
At one time chemists used various symbols, similar to shorthand, for the atoms of the
different elements. These symbols were very cumbersome and were replaced by
abbreviations of the names of the elements. Each element has been assigned a specific
one or two letter symbol based on the first letter of its chemical name. Because there are
several elements with the same first letter, it is often necessary to add the second letter
to the symbol. In some cases the symbol comes from an abbreviation for the old latin
name of the element. For example, Fe stands for iron (ferrum) and Cu for copper
(cuprum). The first letter of the chemical symbol is always capitalized. If the symbol
has two letters, the second letter is always lowercase.
Atomic Number
The number of protons in the nucleus plays such an important role in identifying the
atom that it is given a special name, the atomic number. The symbol Z is often used for
atomic number (or number of protons). Hydrogen has an atomic number of 1 and
lawrencium has an atomic number of 103. The atomic number is also equal to the
number of electrons.
Atomic Mass Number
The sum of the total number of protons, Z, and the total number of neutrons, N, is called
the atomic mass number. The symbol is A. Not all atoms of the same element have the
same atomic mass number, because, although the Z is the same, the N and thus the A are
different. Atoms of the same element with different atomic mass numbers are called
isotopes.
Atomic Weight
In Table 1, the masses of atomic particles are given in atomic mass units (amu). These
units represent a relative scale in which the mass of the isotope carbon-12 is used as the
standard and all others are related to it. Specifically, 1 amu is defined as 1/12 the mass
of the carbon-12 atom. Since the mass of a proton or a neutron is approximately 1 amu,
the mass of a particular atom will be approximately equal to its atomic mass number, Z.
The atomic weight of an element is generally more useful than isotopic masses. The
atomic weight of an element is defined as the weighted average of the masses of all of
its natural occurring isotopes. The atomic weight of the elements are listed in Table 2.
The elements that have their atomic weights in parentheses are unstable. For these
elements, the atomic weight of the longest living isotope is used rather than the average
of the masses of all occurring isotopes.
