Look for selenium in the periodic table. (a) Write the complete electron configuration for an atom of selenium. (b) Write the shorthand electron configuration for an atom of selenium. (c) How many valence electrons does selenium have? (d) Underline the valence electrons in the shorthand electron configuration.

The periodic table is a fundamental tool in the study of chemistry, organizing all known elements in order of increasing atomic number. Elements are arranged into rows called periods and columns known as groups. Similar properties recur periodically when elements are arranged by atomic number, primarily because elements in the same group have the same number of electrons in their outermost shell, known as valence electrons.

When looking at the arrangement, one can notice that elements are also divided into blocks, corresponding to the subshell that is being filled with electrons during the electron configuration: s, p, d, and f blocks. The properties of each element, including reactivity and the type of bonds it can form, are vastly influenced by its electron configuration. Thus, understanding the layout of the periodic table aids immensely in predicting how elements will behave in chemical reactions.

Valence electrons are the electrons that reside in the outermost shell of an atom. These electrons play a pivotal role in chemical reactions, as they are the ones involved in forming chemical bonds. For selenium, an element located in group 16 of the periodic table, it is the electrons in the 4s and 4p orbitals that are the valence electrons.

Selenium, like other elements in its group, typically has six valence electrons. This configuration influences selenium's chemical properties, including its ability to gain or share electrons during reactions. Recognizing the number of valence electrons helps predict the types of bonds selenium will form and its reactivity with other elements.

The Aufbau Principle is a fundamental concept in quantum chemistry, providing a guideline for predicting the ground state electron configuration of an element. According to this principle, electrons are added to an atom's orbitals in order of increasing atomic energy level. The order in which orbitals are filled is based on their relative energies, starting with the lowest energy orbital, 1s, and proceeding to higher energy levels in a defined sequence.

The sequence, often remembered with the help of a diagram or mnemonic, is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on, following the pattern implied by the Madelung rule. In the case of selenium, this principle dictates the electron configuration process, filling the lower energy orbitals before the higher ones, resulting in its ground state electron configuration.

The noble gas shorthand notation is a simplified way of representing an element's electron configuration. Instead of writing out the entire configuration from the 1s orbital, the nearest noble gas preceding the element in the periodic table is used to represent the filled electron shells. This method simplifies the electron configuration, making it shorter and easier to read.

For selenium, the noble gas prior to it in the periodic table is argon (Ar), with an atomic number of 18. The shorthand notation starts with the symbol [Ar], followed by the electron configuration from that point forward. In selenium's case, the shorthand is [Ar] 4s² 3d¹⁰ 4p⁴. This notation accentuates the valence electrons because they determine the chemical behavior of the atom and are typically the only electrons involved in chemical bonding.