The number of electrons in a neutral iodine atom is determined by its atomic number, which is found on the periodic table. The atomic number represents the count of protons in the nucleus. In an electrically neutral atom, the number of electrons orbiting the nucleus is exactly the same as the number of protons.
The Atomic Number Determines the Count
Iodine is designated by the chemical symbol ‘I’ and has the atomic number 53. This means a neutral iodine atom contains 53 protons within its nucleus. Due to electrical neutrality, the positive charge from these protons must be balanced by an equal number of negative charges. Therefore, a neutral iodine atom contains 53 electrons.
This count of 53 electrons remains constant for any neutral atom of iodine, even though the number of neutrons can vary (isotopes). This specific number of electrons dictates iodine’s chemical behavior and how it interacts with other atoms.
The Electron Arrangement in Shells
The 53 electrons are organized into distinct layers, or electron shells, around the nucleus. These shells are numbered starting from the innermost shell (\(n=1\)) and moving outward. The complete arrangement of electrons in a neutral iodine atom follows the sequence \(2, 8, 18, 18, 7\) across five occupied main shells.
The first four shells are completely filled. The outermost shell, or valence shell, is the fifth shell (\(n=5\)), where the remaining seven electrons are found. These seven valence electrons are primarily involved in chemical reactions and bonding.
The precise distribution is detailed by the electron configuration notation. The full configuration for iodine is \(1s^22s^22p^63s^23p^64s^23d^{10}4p^65s^24d^{10}5p^5\). This confirms the seven valence electrons are located in the \(5s\) and \(5p\) subshells. The shorthand notation uses Krypton (\(\text{Kr}\)), the nearest preceding noble gas, resulting in \([\text{Kr}]5s^24d^{10}5p^5\).
The Role of Valence Electrons
Iodine belongs to Group 17 of the periodic table, known as the halogens (including fluorine, chlorine, and bromine). All halogens share the characteristic of having seven valence electrons in their outermost shell. This arrangement makes them highly reactive, as atoms strive to achieve a stable octet of eight electrons.
Having seven electrons means iodine is just one electron short of this stable, noble-gas configuration. This deficit drives its chemical behavior, making it an electron acceptor in most reactions. Iodine has a strong tendency to gain a single electron.
The Iodide Ion (\(I^-\))
When an iodine atom gains the single electron needed to complete its valence shell, it forms the iodide ion (\(I^-\)). This resulting species carries a single negative charge. The addition of one electron changes the total electron count from 53 to 54.
The iodide ion’s electron count of 54 makes it isoelectronic with the noble gas Xenon (\(\text{Xe}\)), which also has 54 electrons. Its new electron configuration, \([\text{Kr}]5s^24d^{10}5p^6\), signifies a completely filled \(5s\) and \(5p\) subshell, achieving the stable octet structure. This charged state is the form in which iodine is most often encountered, such as the trace mineral required for thyroid hormone synthesis.