how many electrons do you expect the ion that selenium is likely to form to have?
The correct answer and explanation is:
Selenium (Se) has an atomic number of 34, meaning a neutral selenium atom contains 34 electrons. Selenium is in group 16 of the periodic table, also known as the chalcogens. Elements in this group tend to gain two electrons to achieve a stable octet configuration similar to the noble gas that follows them, which is krypton.
When selenium forms an ion, it most commonly forms the selenide ion, Se²⁻. This ion has gained two extra electrons compared to the neutral atom. Therefore, the selenide ion has 34 + 2 = 36 electrons.
The reason selenium gains two electrons relates to its electron configuration. The neutral selenium atom has the configuration [Ar] 3d¹⁰ 4s² 4p⁴. The 4p subshell is partially filled with 4 electrons but can hold up to 6 electrons. By gaining two electrons, selenium fills the 4p orbital to six electrons, resulting in a full valence shell of eight electrons (4s² 4p⁶), which is energetically more stable. This full valence shell is characteristic of noble gases, and atoms tend to gain, lose, or share electrons to achieve this stable configuration.
The selenide ion is more stable due to the completed octet in the outer shell, which lowers the overall energy of the ion. In many chemical compounds, selenium will exist in this -2 oxidation state. Although selenium can also form other oxidation states, such as +4 and +6, the Se²⁻ ion is the most common anion formed when selenium gains electrons.
In summary, the selenium ion most commonly formed is Se²⁻, which has 36 electrons. This ion achieves a stable electron configuration by gaining two electrons to complete its valence shell, resulting in greater stability.