Introduction to Thulium
Thulium, symbolized as Tm, is a chemical element with an atomic number of 69. It is classified as a lanthanide, a group of elements often referred to as rare earth metals. Despite the name “rare earth,” these elements are not exceptionally rare in the Earth’s crust, but they are typically dispersed and not found in concentrated deposits, making their extraction challenging and expensive. Thulium is the second least abundant of the lanthanides. In India, rare earth elements, including traces of thulium, can be found in monazite sands, particularly along the coastal regions like Kerala, Odisha, and Andhra Pradesh, which are significant sources of these strategic minerals for various technological applications.
Atomic Structure of Thulium
The atomic structure of Thulium can be understood by examining its fundamental subatomic particles: protons, neutrons, and electrons.
Number of Protons, Neutrons, and Electrons
- Atomic Number (Z): The atomic number of an element defines the number of protons in the nucleus of its atoms. For Thulium (Tm), the atomic number is 69.
- Therefore, a Thulium atom contains 69 protons.
- Number of Electrons: In a neutral atom, the number of electrons is equal to the number of protons to maintain electrical neutrality.
- Thus, a neutral Thulium atom contains 69 electrons.
- Mass Number (A): The most stable and naturally occurring isotope of Thulium is Thulium-169 ($^{169}$Tm), which has a mass number of 169. The mass number represents the total number of protons and neutrons in the nucleus.
- Number of Neutrons: The number of neutrons can be calculated by subtracting the atomic number (number of protons) from the mass number.
- Number of neutrons = Mass Number - Atomic Number
- Number of neutrons = 169 - 69 = 100 neutrons.
Electron Configuration
The electron configuration describes the distribution of electrons of an atom in its atomic orbitals. For Thulium (Tm) with 69 electrons, the electron configuration can be written using the noble gas notation, starting from Xenon (Xe), which has 54 electrons.
The electron configuration of Thulium is:
[Xe] 4f¹³ 6s²
This configuration indicates:
- [Xe]: Represents the electron configuration of the noble gas Xenon, which accounts for 54 inner-shell electrons (1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶).
- 4f¹³: After the Xenon core, 13 electrons occupy the 4f subshell. The 4f subshell can hold a maximum of 14 electrons.
- 6s²: Two electrons occupy the 6s subshell, which is the outermost principal energy level.
Valence Electrons
Valence electrons are the electrons located in the outermost shell of an atom. These are the electrons that are primarily involved in chemical bonding and determine an element’s chemical properties.
For Thulium, the outermost principal energy level is the 6th shell, which contains the 6s electrons. The 4f electrons, although being filled, are considered largely inner-shell electrons due to their shielding by the 5s, 5p, and 6s electrons.
Therefore, for the purpose of high school chemistry and understanding common chemical reactivity, the 2 electrons in the 6s subshell are typically considered the valence electrons of Thulium. While Thulium commonly exhibits a +3 oxidation state (implying the involvement of one 4f electron in addition to the two 6s electrons), the 6s electrons are the most readily available for chemical reactions.