The element gold (Au) is a transition metal known for its distinctive metallic yellow color and resistance to corrosion. Its atomic structure holds clues to its remarkable stability and density. Understanding the makeup of its atom requires focusing on a fundamental property called the mass number. The mass number is a whole-number count that defines the nuclear composition of any specific atom.
What Defines an Atom’s Mass Number?
The mass number, symbolized by \(A\), is the total count of protons and neutrons contained within an atom’s nucleus. These particles are collectively referred to as nucleons. The mass number is always a positive integer because it represents a count of discrete subatomic particles.
The nucleus holds the vast majority of an atom’s mass, making the mass number a reliable estimate of an atom’s total mass in atomic mass units. Protons are positively charged, and their number determines the element’s identity, known as the atomic number (\(Z\)). Neutrons carry no electrical charge but contribute significantly to the atom’s mass and nuclear stability.
The mass number is calculated by summing the atomic number and the number of neutrons. Different atoms of the same element may have varying numbers of neutrons, creating different isotopes with different mass numbers.
The Unique Composition of Gold
The mass number of gold is 197. This number is derived from the composition of the only stable and naturally occurring isotope, Gold-197 (\(\text{Au}^{197}\)). Gold is a monoisotopic element, meaning virtually all naturally found gold is composed of atoms with this single mass number.
Every gold atom has an atomic number (\(Z\)) of 79, meaning its nucleus contains exactly 79 protons. To find the number of neutrons in the Gold-197 nucleus, subtract the atomic number from the mass number: \(197 – 79 = 118\) neutrons. The nucleus is therefore composed of 79 protons and 118 neutrons.
The designation “Gold-197” is the standard way to communicate this specific atomic composition. Although gold has several radioisotopes, they are not found in significant natural abundance. The mass number 197 is the accepted value because it reflects the composition of the stable form of the element.
Distinguishing Mass Number from Atomic Weight
Confusion arises because the mass number (197) is an integer, but the value listed for gold on the periodic table is a decimal: its standard atomic weight, approximately 196.96657 atomic mass units. This difference highlights the distinction between the mass number and atomic weight. The mass number is a simple count of nucleons for a single isotope.
The standard atomic weight, by contrast, is a weighted average of the atomic masses of all naturally occurring isotopes of an element. This average is weighted based on the relative abundance of each isotope found on Earth. The atomic mass of a single atom is not perfectly equal to its mass number due to the mass defect, which accounts for the binding energy holding the nucleus together.
Since gold is monoisotopic, its atomic weight is extremely close to 197. The small deviation from a perfect integer is solely due to the mass defect of the Gold-197 nucleus. For elements with multiple stable isotopes, like chlorine, the atomic weight on the periodic table can be significantly different from any single mass number.