Understanding the Atomic Structure of Gold
Gold, a precious and unreactive metal, has been valued across civilisations, including ancient India, for its luster and resistance to corrosion. Its unique properties are fundamentally linked to its atomic structure. To comprehend the behaviour of gold, such as its malleability or its ability to resist tarnishing, understanding its atomic makeup is essential.
Atomic Number and Mass Number
Every atom is defined by its atomic number (Z), which represents the number of protons in its nucleus. For Gold, the atomic number (Z) is 79. This means every atom of gold contains 79 protons.
The mass number (A) of an atom is the total number of protons and neutrons in its nucleus. While gold has several isotopes, the most common and stable isotope is Gold-197 ($^{197}\text{Au}$). Therefore, its mass number (A) is typically considered 197.
Subatomic Particles in a Neutral Gold Atom
Based on its atomic and mass numbers, the number of subatomic particles in a neutral gold atom can be determined:
- Protons: As the atomic number (Z) is 79, a gold atom contains 79 protons. These positively charged particles reside in the nucleus.
- Electrons: In a neutral atom, the number of electrons is equal to the number of protons. Thus, a neutral gold atom possesses 79 electrons. These negatively charged particles orbit the nucleus in specific energy levels or shells.
- Neutrons: The number of neutrons is calculated by subtracting the atomic number from the mass number (A - Z). For Gold-197, the number of neutrons is 197 - 79 = 118 neutrons. Neutrons are neutral particles also found in the nucleus.
Electron Configuration
The electron configuration describes how electrons are distributed among the atomic orbitals. For Gold (Au), with 79 electrons, its ground state electron configuration is an exception to the strict Aufbau principle, exhibiting a more stable arrangement.
The full electron configuration for a neutral gold atom is: $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6 4d^{10} 4f^{14} 5s^2 5p^6 5d^{10} 6s^1$
This configuration can also be represented using the noble gas notation, referencing Xenon (Xe), which has 54 electrons: $[Xe] 4f^{14} 5d^{10} 6s^1$
The deviation from the expected configuration ($[Xe] 4f^{14} 5d^9 6s^2$) to $4f^{14} 5d^{10} 6s^1$ occurs because a completely filled $5d$ subshell ($5d^{10}$) provides extra stability to the atom.
Valence Electrons
Valence electrons are the electrons located in the outermost principal energy shell of an atom. These electrons are primarily involved in chemical bonding.
For Gold, based on its electron configuration $[Xe] 4f^{14} 5d^{10} 6s^1$: The outermost principal energy shell is the 6th shell, which contains 1 electron in the $6s$ orbital. Therefore, the primary valence electron for gold is the $6s^1$ electron.
While the $6s^1$ electron is the most readily lost electron, the $5d$ electrons can also participate in bonding, especially when gold exhibits higher oxidation states (e.g., +3). However, for a fundamental understanding of valence, the $6s^1$ electron is considered the principal valence electron.