The mass number is a specific measurement used to identify and characterize a single atom of a chemical element. Understanding this concept requires distinguishing it from the element’s atomic weight, which is the value typically seen on the periodic table. This distinction is relevant when discussing gold, as its unique natural composition makes determining its standard mass number straightforward.
What Defines the Mass Number
The mass number, symbolized by the letter A, represents the total count of particles found within the nucleus of an atom. These particles are the protons and the neutrons, collectively known as nucleons. Since it is a simple count of these subatomic particles, the mass number is always a whole number.
This measurement is fundamentally different from the atomic weight, which is the decimal number listed for an element on the periodic table. The atomic weight is a calculated, weighted average of the masses of all the naturally occurring forms of an element. Most elements exist in nature as a mixture of various forms, each having a different mass number due to a varying count of neutrons. The mass number applies to only one version of an atom, providing a precise measurement for that specific nuclear structure.
The Mass Number for Gold
The standard mass number for the element gold (Au) is 197. This number represents the combined count of protons and neutrons in the nucleus of the most common gold atom. Every gold atom possesses 79 protons, which is the atomic number that defines the element itself.
To determine the number of neutrons in the standard gold atom, one subtracts the atomic number (79) from the mass number (197). This calculation reveals that the nucleus of the standard gold atom contains 118 neutrons. This specific combination of 79 protons and 118 neutrons is the basis for Gold-197.
A quick reference to the periodic table shows that the atomic weight of gold is approximately 196.97 atomic mass units. This value is extremely close to the whole number 197, confirming that the Gold-197 atom is the most abundant and stable form found in the environment. The small difference is due to slight variations in mass that occur when nucleons bind together in the nucleus.
Why Gold Has Only One Stable Isotope
Gold is categorized as a monoisotopic element, meaning that 100% of the naturally occurring gold found on Earth consists of only one stable form: Gold-197. This characteristic simplifies the chemistry of gold, as there is no need to account for a natural mixture of different atomic masses. This is why the mass number for gold is consistently cited as 197.
While only one naturally stable isotope exists, scientists have synthesized several dozen other forms of gold atoms in laboratories. These other forms have different mass numbers, ranging from 169 to 210, because they contain a different number of neutrons. However, these non-standard gold atoms are highly unstable and undergo radioactive decay relatively quickly. Therefore, Gold-197 remains the single, definitive answer to the question of gold’s mass number.