Thallium was used as a poison in the Agatha Christie mystery story "The Pale Horse." Thallium has two possible cationic forms, +1 and \(+3 .\) The +1 compounds are the more stable. Write the electron structure of the +1 cation of thallium.

The atomic number of an element is a fundamental property that you can find on the periodic table. It is unique to each element and is denoted as 'Z'. In simple terms, it represents the number of protons found within an atom's nucleus and is paramount in defining the identity of the element. For thallium, this atomic number is 81. Since atoms are electrically neutral, this also means a neutral thallium atom contains 81 electrons.

The atomic number not only determines the element's position on the periodic table but also dictates many of its chemical properties. The number of protons (hence the atomic number) within the nucleus exerts a pull on the surrounding electrons, influencing their arrangement around the nucleus — a concept crucial for understanding electron configurations.

Cations are positively charged ions resulting from the loss of one or more electrons by a neutral atom. Thallium is interesting because it can form two cations: +1 and +3. Each form exhibits different properties and stabilities. The +1 cation, often noted as Tl^+, is more stable, making it more common in compounds.

This stability arises due to the filled subshells in the electron configuration of the Tl^+ ion, which confer a lower energy state. The stability of cationic forms is a vital consideration in various areas, including chemical reactions, where more stable ions are likely to form preferentially.

Electron configuration notation is a way to represent the distribution of electrons among the orbitals of an atom. It follows a specific sequence that dictates the order in which electrons fill up the orbitals, known as the Aufbau principle. The notation consists of a series of numbers and letters that correspond to energy levels, subshells, and the number of electrons within those subshells.

For thallium's +1 cation, this notation becomes [Xe] 4f^{14} 5d^{10} 6s^2 after losing one electron from its 6p subshell. Notice how the notation starts with [Xe], representing the noble gas core of xenon, a handy shortcut to bypass writing out the full configuration for all preceding elements. Understanding this notation allows chemists and students alike to visualize the electron distributions, predict chemical properties, and discuss the nature of chemical bonds formed by the elements.