Understanding the Atomic Structure of Astatine
Astatine (At) is a chemical element with atomic number 85. It is a metalloid and belongs to Group 17 of the periodic table, classifying it as a halogen. Astatine is the rarest naturally occurring element in the Earth’s crust, found only in minute traces as a decay product of heavier elements. All isotopes of astatine are radioactive, with the most stable isotope, Astatine-210, having a half-life of only 8.1 hours. Its extreme rarity and radioactivity make its study challenging.
Atomic Number and Subatomic Particles
The atomic number (Z) of an element defines the number of protons in its nucleus. In a neutral atom, the number of electrons is equal to the number of protons. The number of neutrons can vary, leading to different isotopes of an element.
Protons
For Astatine (At), the atomic number is 85. Therefore, the number of protons in an astatine atom is 85. These 85 protons reside in the nucleus of the atom.
Electrons
In a neutral astatine atom, the number of electrons is equal to the number of protons. Thus, a neutral astatine atom contains 85 electrons. These electrons occupy various energy shells or orbitals surrounding the nucleus.
Neutrons
The number of neutrons in an astatine atom varies depending on the specific isotope. Astatine has no stable isotopes, and its mass numbers range from 191 to 229. The mass number (A) is the sum of protons and neutrons (A = Z + N). For example, for the isotope Astatine-210 ($^{210}\text{At}$): Number of neutrons = Mass Number - Atomic Number = 210 - 85 = 125 neutrons. For the isotope Astatine-211 ($^{211}\text{At}$), which is sometimes used in medical research: Number of neutrons = 211 - 85 = 126 neutrons.
Electron Configuration
The electron configuration describes the arrangement of electrons in the atomic orbitals of an atom. Electrons fill orbitals according to the Aufbau principle, Hund’s rule, and the Pauli exclusion principle.
Full Electron Configuration
Astatine has 85 electrons. Its full electron configuration is: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^2 4f^{14} 5d^{10} 6p^5$
Condensed Electron Configuration
To simplify the representation, the electron configuration of the noble gas preceding the element can be used. For astatine, the preceding noble gas is Xenon (Xe), which has an atomic number of 54. The electron configuration of Xenon is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6$. Therefore, the condensed electron configuration for astatine is: $[\text{Xe}] 4f^{14} 5d^{10} 6s^2 6p^5$
Valence Electrons
Valence electrons are the electrons located in the outermost principal energy level of an atom. These electrons are primarily involved in chemical bonding. For astatine, the outermost principal energy level is the 6th shell. From the condensed electron configuration, $[\text{Xe}] 4f^{14} 5d^{10} 6s^2 6p^5$, the electrons in the 6th shell are $6s^2$ and $6p^5$. The total number of valence electrons for astatine is 2 (from $6s$) + 5 (from $6p$) = 7. This corresponds to its position in Group 17 of the periodic table, characteristic of halogens.