Selenium (Se) is a chemical element found on the periodic table in Group 16, known as the chalcogens. It is classified as a non-metal and is widely distributed in the Earth’s crust, existing as a solid at room temperature.
Selenium is an essential trace element for human health, incorporated into selenoproteins. These proteins play a role in various bodily functions, including antioxidant defense systems and the regulation of thyroid hormones. Understanding the atom’s fundamental structure, especially its electron count, explains its unique chemical behavior and biological importance.
Determining the Total Electron Count
The total number of electrons in a neutral atom of any element is determined by its atomic number (Z), which is the count of protons found within the atom’s nucleus. For the atom to be electrically neutral, the positive charge from the protons must be balanced by an equal number of negatively charged electrons.
Selenium has an atomic number of 34, meaning every selenium atom contains exactly 34 protons in its core. Consequently, a neutral selenium atom must also possess a total of 34 electrons to maintain a zero net charge. This number defines the element Selenium.
The total electron count remains 34 unless the atom loses or gains electrons to form an ion. When selenium forms an ion, the number of electrons changes, but the number of protons remains 34.
The Importance of Valence Electrons
Not all 34 electrons are equally important for chemical reactions; valence electrons govern an atom’s bonding behavior and reactivity. These are the electrons situated in the outermost electron shell, farthest from the nucleus. A neutral selenium atom has 6 valence electrons.
The six valence electrons are chemically significant because atoms tend to react in ways that allow them to achieve a stable configuration of eight outermost electrons, known as the octet rule. Since selenium has six, it strongly seeks to gain two additional electrons to complete its outer shell. Gaining two electrons forms the stable selenide ion (\(\text{Se}^{2-}\)), which is a very stable state with a full octet.
The desire for an octet explains selenium’s common oxidation states, which are \(+6\), \(+4\), and \(-2\). The \(-2\) state results from gaining two electrons, while the \(+4\) and \(+6\) states involve sharing or losing some or all of the six valence electrons. This chemical versatility allows selenium to form a wide range of compounds and function within the active sites of selenoproteins.
Electron Shell Arrangement
The 34 total electrons of selenium are organized into distinct layers of energy known as electron shells or principal energy levels. These shells are numbered starting from the one closest to the nucleus, and the arrangement follows a sequential filling pattern.
The first shell holds 2 electrons, the second contains 8, and the third accommodates 18 electrons. The remaining 6 electrons occupy the fourth and outermost shell, which is the valence shell.
This distribution is summarized by the shell structure notation 2, 8, 18, 6. This arrangement also introduces the concept of subshells (\(s\), \(p\), and \(d\)) within the main shells. Selenium’s full electron configuration is \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^4\). The outermost electrons (\(4s^2\) and \(4p^4\)) constitute the six valence electrons.