Introduction to Thulium
Thulium (Tm) is a chemical element with atomic number 69. It is classified as a lanthanide, a group of rare earth elements known for their similar chemical properties and often occurring together in mineral deposits. Thulium was discovered in 1879 by Swedish chemist Per Teodor Cleve, and its name is derived from “Thule,” an ancient Greco-Roman name for a mythical land generally thought to be Scandinavia. Thulium is not found as a free element in nature but occurs in small quantities in various minerals, such as monazite and bastnäsite, which are mined in countries like China, the United States, Australia, and India. While not as widely used as some other rare earth elements, thulium has specialized applications, particularly in portable X-ray devices and solid-state lasers.
Basic Atomic Structure
An atom of thulium contains specific numbers of subatomic particles:
Protons
The atomic number (Z) of thulium is 69. Therefore, a thulium atom always contains 69 protons in its nucleus. The number of protons defines the element’s identity.
Electrons
For a neutral atom, the number of electrons equals the number of protons. Thus, a neutral thulium atom has 69 electrons orbiting its nucleus.
Neutrons
The most common and stable isotope of thulium is Thulium-169. The mass number (A) represents the total number of protons and neutrons in the nucleus. To determine the number of neutrons, the atomic number is subtracted from the mass number: 169 (mass number) - 69 (protons) = 100 neutrons. Other isotopes of thulium exist but are generally less stable and have different numbers of neutrons.
Electron Configuration
The electron configuration describes the arrangement of electrons in the atomic orbitals around the nucleus. For thulium, with 69 electrons, the ground state electron configuration can be written using the noble gas notation:
[\text{[Xe] 4f}^{13} \text{ 6s}^2]
This notation indicates that the inner electrons are arranged like those of a xenon (Xe) atom, which has 54 electrons. Following the xenon core, the remaining 15 electrons fill the 4f and 6s orbitals in a specific order:
- [Xe]: Represents the electron configuration of Xenon: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶.
- 4f¹³: Thirteen electrons occupy the 4f subshell. This subshell can hold a maximum of 14 electrons.
- 6s²: Two electrons occupy the 6s subshell. This subshell can hold a maximum of 2 electrons.
The full electron configuration for thulium is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 4f¹³ 6s².
Valence Electrons
Valence electrons are the electrons located in the outermost electron shell of an atom, typically involved in chemical bonding. For thulium, a lanthanide element, identifying valence electrons requires considering the energy levels of its outermost orbitals.
The primary valence electrons are the two electrons in the 6s subshell. These 6s² electrons are the most readily available for chemical reactions. However, in lanthanides, the 4f electrons are also close in energy to the 6s and 5d orbitals. While the 4f electrons are largely shielded by outer shells, they can participate in chemical bonding, leading to variable oxidation states. The most common oxidation state for thulium is +3, which typically involves the loss of the two 6s electrons and one 4f electron. Therefore, while the 6s² electrons are the primary participants, it is understood that the element’s chemical behavior often involves electrons from the partially filled 4f subshell as well.