Bromine (Br) is a halogen element known for its distinctive properties. It is one of only two elements that exist as a liquid at room temperature, appearing as a dense, reddish-brown fluid with a strong, irritating odor. As a highly reactive nonmetal, it readily forms compounds with many other elements. Understanding bromine requires examining the subatomic particles that make up its atoms: protons, neutrons, and electrons.
Defining Bromine Through Its Protons
The identity of an element is fixed by the number of protons found in the nucleus of its atoms. This count is known as the atomic number, which is represented by the symbol \(Z\). Bromine is defined by having exactly 35 protons in its nucleus, placing it specifically on the periodic table as element 35. If an atom gains or loses a proton, it fundamentally changes into a different element entirely. For example, an atom with 34 protons is Selenium, while an atom with 36 protons is Krypton. The 35 protons exert a positive charge that dictates the atom’s behavior and structure, making the atomic number the single, unchanging identifier for all bromine atoms.
How to Calculate Bromine’s Electrons
In a neutral bromine atom, the number of negatively charged electrons orbiting the nucleus must perfectly match the number of positively charged protons within the nucleus. Since bromine is defined by its 35 protons, a neutral bromine atom must therefore contain 35 electrons. This balance ensures the atom has no net electrical charge, which is the standard state for an element as listed on the periodic table. However, atoms can gain or lose electrons to achieve a more stable configuration, forming what are called ions. Bromine is a halogen, meaning it belongs to a group of elements that requires only one additional electron to complete its outermost electron shell. When a neutral bromine atom gains this single electron, it forms a bromide ion, which is represented by the symbol \(\text{Br}^-\). The bromide ion, therefore, has 35 protons and a total of 36 electrons, resulting in a net charge of \(1-\). The electron count for bromine is thus either 35 for the neutral atom or 36 for the common bromide ion.
The Variation in Bromine’s Neutron Count
Unlike the fixed number of protons, the number of neutrons within a bromine nucleus can vary, resulting in different versions of the element known as isotopes. Neutrons are subatomic particles with no electrical charge, and they contribute mass to the atom alongside the protons. The total count of protons plus neutrons is called the mass number of a specific isotope. To find the number of neutrons for a particular isotope, one must subtract the atomic number (35 protons) from the mass number.
The most common natural forms of the element are Bromine-79 (\(\text{Br}\)-79) and Bromine-81 (\(\text{Br}\)-81). For the Bromine-79 isotope, the neutron count is calculated by subtracting 35 from 79, which equals 44 neutrons. For the slightly heavier Bromine-81 isotope, subtracting 35 from 81 yields 46 neutrons. These two stable isotopes occur in nature in roughly equal proportions, which is why the average atomic mass listed on the periodic table is approximately 79.904. This fractional number is the weighted average mass of all naturally occurring isotopes. Therefore, bromine atoms most commonly have either 44 or 46 neutrons.
Atomic Summary of the Bromine Atom
Bromine’s atomic structure is defined by a fixed number of protons, but its other subatomic particles vary depending on its state. All bromine atoms contain 35 protons, which is the element’s fundamental identifier. For neutral bromine, the electron count is 35, matching the proton count to maintain a zero net charge. The bromide ion (\(\text{Br}^-\)) contains 36 electrons. The neutron count is variable based on the isotope: Bromine-79 has 44 neutrons, and Bromine-81 has 46 neutrons. Therefore, a bromine atom will have 35 protons, 35 or 36 electrons, and 44 or 46 neutrons.