Whole number rule
Encyclopedia
The whole number rule states that the masses of the elements are whole number multiples of the mass of the hydrogen
atom. The rule can be formulated from Prout's hypothesis
put forth in 1815. In 1920, Francis W. Aston demonstrated through the use of a mass spectrometer that apparent deviations from the rule are predominantly due to the existence of isotopes; they are secondarily due to binding energy
, as mass defect. The modern form of the whole number rule is that the atomic mass
of a given elemental isotope
is approximately the mass number
(number of protons plus neutrons) times an atomic mass unit
(approximate mass of a proton, neutron, or hydrogen-1 atom). This rule predicts atomic mass
es of various nuclide
s and isotopes to accuracies greater than 99%.
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atom. The rule can be formulated from Prout's hypothesis
Prout's hypothesis
Prout's hypothesis was an early 19th century attempt to explain the existence of the various chemical elements through a hypothesis regarding the internal structure of the atom...
put forth in 1815. In 1920, Francis W. Aston demonstrated through the use of a mass spectrometer that apparent deviations from the rule are predominantly due to the existence of isotopes; they are secondarily due to binding energy
Binding energy
Binding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...
, as mass defect. The modern form of the whole number rule is that the atomic mass
Atomic mass
The atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....
of a given elemental isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
is approximately the mass number
Mass number
The mass number , also called atomic mass number or nucleon number, is the total number of protons and neutrons in an atomic nucleus. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion...
(number of protons plus neutrons) times an atomic mass unit
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...
(approximate mass of a proton, neutron, or hydrogen-1 atom). This rule predicts atomic mass
Atomic mass
The atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....
es of various nuclide
Nuclide
A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
s and isotopes to accuracies greater than 99%.
See also
- William ProutWilliam ProutWilliam Prout FRS was an English chemist, physician, and natural theologian. He is remembered today mainly for what is called Prout's hypothesis.-Biography:...
- Atomic massAtomic massThe atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....
- Binding energyBinding energyBinding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...
- Law of definite proportionsLaw of definite proportionsIn chemistry, the law of definite proportions, sometimes called Proust's Law, states that a chemical compound always contains exactly the same proportion of elements by mass. An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have...
- Mass numberMass numberThe mass number , also called atomic mass number or nucleon number, is the total number of protons and neutrons in an atomic nucleus. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion...