THE mass of an atom is concentrated in a very small central portion of the atom which is called the atomic nucleus. The atomic nucleus is made up of electrically positive protons and electrically neutral neutrons. Surrounding the atomic nucleus are the electrically negative electrons. The masses and charges of these three fundamental constituents of atoms are given below:

Electron          -1                 0
Proton           +1                 1
Neutron           0                 1

ONLY ELECTRONS ARE INVOLVED IN CHEMICAL CHANGE OR REACTIONS.

THE NUCLEI OF ATOMS ARE NOT AFFECTED BY CHEMICAL REACTIONS.

IN PARTICULAR, THE ELECTRONS IN THE OUTERMOST SHELL ARE ALTERED DURING CHEMICAL CHANGE OR REACTION.

ISOTOPES AND ATOMIC MASSES

For many of the chemical elements there are several known isotopes. Isotopes are atoms with different atomic masses which have the same atomic number. The atoms of different isotopes are atoms of the same chemical element; they differ in the number of neutrons in the nucleus.

Chemists sometimes find it necessary to specify the atomic mass of an isotope. This is done by writing the atomic mass as a superscript preceding the atomic letter symbol. [¹⁴C]

Atoms of the same chemical element all have essentially the same chemical properties and reactivity but they do not always have the same mass because, although the number of protons in the nucleus is the same for all atoms of the same element, the number of neutrons is not.

The number of electrons also may vary, but only if the atom ionizes, and in any case the relative mass of the electron is very much less than that of a proton or neutron. The loss or gain of electrons is often ignored.

However, the mass of the neutron is large enough that for any element a difference of one neutron is significant. As a consequence the molar masses of the different isotopes of an element are significantly different. Most elements as they occur naturally on earth are mixtures of several isotopes.

Moles of Atoms

Historical background.  The atomic mass of an element is a relative quantity. Originally the atomic mass of hydrogen, the lightest of the elements, was taken to be one and the atomic masses of all other elements were measured in relation to the atomic mass of hydrogen. This later proved to have been a poor choice. Not only does hydrogen naturally consist of more than one isotope, but there was the additional question (particularly among early chemists) as to whether monatomic hydrogen or diatomic hydrogen should be taken as having atomic mass one.

After some effort, and one major false start with oxygen, chemists and physicists agreed on a common scale of relative atomic mass. Carbon of isotopic mass twelve was assigned an atomic mass of exactly twelve, and all other atomic masses whether of isotopes or of elements were specified relative to carbon of atomic mass twelve. This had the effect of making the relative atomic mass of hydrogen 1.0079...rather than exactly 1.0000.... The difference of less than 1% is too small to matter in many approximate chemical calculations, but it is large enough to be significant when accurate work must be done.

Physicists, and some chemists, measure the masses of individual atoms in kg, g, or atomic mass units. For most chemists, however, the mass of a single atom is inconveniently small and the molar mass of a substance is used. The molar mass of an atom is the mass of a very large number of identical atoms--one mole of atoms. One mole of atoms is by definition that number of atoms which exist in exactly twelve grams of carbon of isotopic mass twelve (¹²C). This number is called the Avogadro number;  the best current determination of its value is 6.0221 x 10²³. Moles of atoms and molecules are so central to chemistry that they will be used continually throughout all courses in chemistry.

Molar Atomic Masses of Elements

The molar mass of an atom is simply the mass of one mole of identical atoms. However, most of the chemical elements are found on earth not as one isotope but as a mixture of isotopes, so the atoms of one element do not all have the same mass.

Chemists therefore distinguish the molar atomic mass of an isotope, which is the mass of one mole of the identical atoms which form that isotope, from the molar atomic mass of an element, which is the mass of one mole of the atoms of the various isotopes of that element having the natural abundance as they are found on earth.

For many elements the variation found in the natural abundance's limits the accuracy with which the molar atomic mass of that element can be known. Those elements for which this is true are indicated in the table of atomic masses.

Chemists deal with elements as they are naturally found, and so the atomic mass of a particular isotope is of less interest than the weighted mean molar atomic mass of the individual isotopes which is the molar atomic mass of the naturally occurring element.

This property has open been called the atomic weight or chemical atomic weight of the element. The weighted mean molar atomic mass of an element as it naturally occurs will be referred to simply as the atomic mass of the element from now on.

Example. The relative abundance's of the isotopes ⁶Li and ⁷Li in naturally occurring lithium can be computed as follows. Their atomic masses are 6.0151214 and 7.0160030, respectively. The atomic mass of naturally occurring lithium given in the table of atomic masses of the elements is 6.941g/mol. If the relative abundance of ⁶Li is 7.5% and the relative abundance of ⁷Li is 92.5%, then the atomic weight of Lithium (for the entire population of lithium atoms) is:

(.075 x 6.0151214) + (.925 x 7.0160030) = 0.451  +  6.49 or 6.941