Ions are charged atoms or molecules that have gained or lost one or more electrons. A neutral atom exists when the number of positively charged protons equals the number of negatively charged electrons. When this balance is disrupted, the atom acquires a net electrical charge.
The classification of an ion depends on the nature of this charge. A cation carries a net positive charge, occurring when a neutral atom loses electrons. Conversely, an anion carries a net negative charge, resulting from a neutral atom gaining electrons.
Bromine’s Atomic Structure and Electron Tendency
The neutral Bromine atom (\(\text{Br}\)) possesses an atomic number of 35, meaning it contains 35 protons and 35 electrons. Bromine has seven valence electrons in its outermost shell.
Bromine is located in Group 17 of the periodic table, known as the Halogens. Atoms strive for chemical stability, often achieved by having a full outer shell of eight valence electrons, a state referred to as the octet rule.
Since Bromine has seven valence electrons, it is only one electron short of achieving this stable octet. Gaining a single electron requires significantly less energy than the alternative, which would be to lose all seven valence electrons. This preference dictates that Bromine strongly tends to attract and accept one additional electron from a neighboring atom.
Bromide: The Ion Formed by Bromine
Bromine readily accepts a single electron to achieve stability, transforming the neutral atom into a charged particle. When the \(\text{Br}\) atom gains this negative charge, it becomes the Bromide ion (\(\text{Br}^-\)). The resulting ion has 35 protons and 36 electrons, creating a net charge of negative one.
Since the Bromide ion carries a net negative charge, it is classified as an anion. The formation of the \(\text{Br}^-\) anion is chemically favorable because the ion achieves the stable electron configuration of the noble gas Krypton.
It is important to distinguish between the element and the ion. “Bromine” refers to the neutral element, which can exist as a diatomic molecule (\(\text{Br}_2\)). “Bromide” specifically refers to the ion, \(\text{Br}^-\), which is the charged species that participates in ionic compounds.
Bromine’s Role in Common Compounds
The Bromide anion is commonly found in a variety of simple ionic compounds, which are formed when it pairs with a positively charged metal cation. For instance, in the compound Sodium Bromide, the metal sodium readily donates its single valence electron to Bromine. Sodium loses one electron to become the positive cation \(\text{Na}^+\), while Bromine gains that electron to become the negative anion \(\text{Br}^-\).
The resulting compound, \(\text{NaBr}\), is a stable salt held together by the strong electrostatic attraction between the oppositely charged ions. Another common example is Potassium Bromide (\(\text{KBr}\)), where the Potassium cation (\(\text{K}^+\)) similarly bonds with the Bromide anion (\(\text{Br}^-\)). These compounds illustrate the characteristic behavior of Bromine, which acts as the electron acceptor and forms the negative anion component of the salt.