Electrons, the negatively charged particles orbiting the nucleus, govern the behavior of all matter and are responsible for every chemical interaction. A select few of these electrons, known as valence electrons, hold the primary responsibility for chemical reactivity. Understanding the precise number of these electrons for a given element like Selenium (Se) is key to understanding its unique chemical properties.
What are Valence Electrons?
Valence electrons reside in the outermost electron shell of an atom, also called the valence shell. These electrons possess the highest energy and are the farthest from the positively charged nucleus. This distant location makes them the sole participants in the formation of chemical bonds when two atoms interact.
Electrons not in the outermost shell are called core electrons; they are tightly bound to the nucleus and remain uninvolved in chemical reactions. The distinction between core and valence electrons determines an atom’s chemical identity and its combining power, or valency. Atoms achieve stability by filling their outermost shell with a specific number of electrons.
This drive for stability is formalized by the octet rule, which states that many atoms seek to have eight electrons in their valence shell. Elements can achieve this stable configuration by either transferring or sharing their valence electrons with other atoms. An atom’s tendency to gain, lose, or share its valence electrons determines whether it will form an ionic bond or a covalent bond with its reaction partner.
Using the Periodic Table to Determine the Count
The periodic table is organized to allow for the quick determination of valence electron counts for the main group elements (Groups 1, 2, and 13 through 18). These elements follow a predictable pattern. The number of valence electrons an atom possesses is directly related to the vertical column, or group, in which it is placed.
For elements in Groups 1 and 2, the group number itself corresponds exactly to the number of valence electrons. For the elements in Groups 13 through 18, the count is determined by looking at the last digit of the group number. For instance, Group 13 elements have three valence electrons, while Group 17 elements, known as the halogens, have seven.
Selenium (Se) is located in Group 16 of the periodic table, directly below oxygen and sulfur. Applying the rule for the higher-numbered groups, the last digit of “16” is six. This placement reveals that every element in this column, including Selenium, has six outermost electrons. This shared trait explains why elements within the same group exhibit similar chemical behaviors.
Selenium’s Electron Configuration and Chemical Behavior
A neutral atom of Selenium (Se) has six valence electrons. Selenium has 34 total electrons, distributed across its electron shells in the configuration 2, 8, 18, 6. This shell model shows the six electrons occupying the final, outermost shell (the fourth principal energy level).
The detailed electron configuration specifies the subshells where these electrons reside: \([Ar] 3d^{10} 4s^2 4p^4\). The six valence electrons are specifically the two electrons in the \(4s\) subshell and the four electrons in the \(4p\) subshell. The ten electrons in the \(3d\) subshell are considered core electrons because they are not in the highest principal energy level.
The presence of six valence electrons dictates Selenium’s chemical behavior, as it seeks to achieve a stable octet of eight electrons. The most favorable way for Selenium to complete this shell is by gaining two additional electrons. This results in Selenium commonly forming a stable ion with a negative two charge (\(Se^{2-}\)), or forming two covalent bonds by sharing electrons with other nonmetals. This reactivity pattern is similar to that of its group neighbors, oxygen and sulfur.