Rubidium (Rb) is an alkali metal known for its high reactivity. Its atomic number (Z=37) defines the fixed quantity of 37 protons found within the nucleus of every rubidium atom. In its most common and electrically neutral state, a rubidium atom contains exactly 37 electrons.
The Electron Count in a Neutral Atom
The number of electrons in an atom is directly related to its atomic number (37). The atomic number is defined by the count of positive charges, or protons, located in the atom’s nucleus. Protons carry a positive electrical charge, while electrons carry a negative charge of equal magnitude. For an atom to be considered neutral, the total positive charge must be perfectly balanced by the total negative charge.
This principle of electrical neutrality dictates that the number of electrons orbiting the nucleus must be identical to the number of protons within it. Since the rubidium atom possesses 37 protons, it must also have 37 electrons to achieve this balance. This count of 37 electrons is the baseline for a rubidium atom when it is isolated. The distribution of these 37 electrons across different energy levels determines how the atom will interact with other elements.
Electron Shell Configuration
The 37 electrons within a neutral rubidium atom are organized into distinct layers, or electron shells, around the nucleus. These shells represent different energy levels, with electrons closest to the nucleus occupying the lowest energy states. Rubidium’s electrons fill up five main energy levels.
The arrangement follows a specific pattern known as the shell model configuration: 2, 8, 18, 8, 1. The inner 36 electrons form a stable core structure that resembles the noble gas Krypton.
The final electron, the 37th, resides alone in the fifth and outermost shell. This single electron is called the valence electron. The presence of only one valence electron is characteristic of all alkali metals and governs rubidium’s high chemical reactivity.
Rubidium’s Ionic State
While a neutral rubidium atom has 37 electrons, it is rarely found in this state due to its high reactivity. Rubidium belongs to Group 1, making it inclined to achieve a stable electron configuration by adhering to the octet rule. The octet rule states that atoms are most stable when their outermost shell contains eight electrons, mimicking the structure of a noble gas.
To fulfill this requirement, the rubidium atom readily loses its single valence electron from the fifth shell. The loss of one negative charge results in the formation of a positive ion, symbolized as Rb+.
This newly formed ion now has 37 protons but only 36 electrons, giving it a net charge of +1. The resulting Rb+ ion has an electron count of 36, matching the stable electron configuration of the noble gas Krypton. This ionic state is the most common form of rubidium encountered in compounds.