Rubidium (Rb) is a soft, silvery-white alkali metal with the atomic number 37. When considering how many neutrons this element contains, the answer is not a single, fixed number. The count of neutrons inside an atom is subject to natural variation, which is related to the stability and identity of the element itself.
Atomic Structure Basics
An atom is composed of three primary subatomic particles: protons, neutrons, and electrons. The fundamental identity of a chemical element is defined by the number of protons contained within its nucleus, known as the atomic number. For rubidium, the atomic number is 37, meaning every rubidium atom must contain exactly 37 protons.
Neutrons, which are electrically neutral particles, also reside in the nucleus alongside the protons and contribute significantly to the atom’s mass. The total number of protons and neutrons combined gives the atom its Mass Number. To determine the number of neutrons in any specific atom, one must subtract the atomic number from the atom’s Mass Number.
The Role of Isotopes in Neutron Count
The complexity in finding a single neutron number for rubidium arises from the existence of isotopes. Isotopes are variations of the same element; they have an identical number of protons (37) but differ in the number of neutrons they possess. This difference in neutron count leads to different mass numbers for the same element.
Naturally occurring rubidium on Earth is composed primarily of two distinct isotopes: rubidium-85 (\(\text{Rb}^{85}\)) and rubidium-87 (\(\text{Rb}^{87}\)). These two forms exist in a specific ratio called their natural abundance. Rubidium-85 is the more common form, accounting for approximately 72.2% of all rubidium atoms found in nature.
The less abundant isotope, rubidium-87, makes up the remaining fraction, around 27.8% of the total. This difference in abundance confirms that any sample of rubidium contains a mix of atoms with two different neutron counts.
Calculating the Neutron Count for Rubidium
To find the specific number of neutrons for either isotope, the formula is applied directly: Neutrons = Mass Number – Atomic Number. Since rubidium’s atomic number is fixed at 37, this number is subtracted from the mass number of each isotope.
For the most common form, rubidium-85, the calculation is straightforward. Subtracting the 37 protons from the mass number of 85 reveals that this isotope contains 48 neutrons (85 – 37 = 48). Given its natural abundance of over 72%, the majority of rubidium atoms encountered will have this specific neutron count.
For the second naturally occurring isotope, rubidium-87, the same calculation is performed. Subtracting the 37 protons from the mass number of 87 shows that this atom holds 50 neutrons (87 – 37 = 50). This isotope, while less common, is notable because it is slightly radioactive and is used extensively in geological dating techniques.
Therefore, the direct answer is that rubidium atoms contain either 48 or 50 neutrons.