Rubidium is a soft, silvery-white alkali metal classified as a Group 1 element on the periodic table. This element, often found in minerals like lepidolite, is highly reactive with water and air, sharing characteristics with potassium and caesium. Determining the number of neutrons requires understanding that the count is not a single value for all Rubidium atoms.
Understanding Atomic Structure and the Neutron Formula
Every atom is built from three basic subatomic particles: protons, neutrons, and electrons. Protons and neutrons are confined within the dense central nucleus, while electrons orbit the nucleus in defined shells. Protons carry a positive charge, electrons carry a negative charge, and neutrons possess no electrical charge.
The identity of a chemical element is defined by its Atomic Number (Z), which represents the count of protons in the nucleus. The Mass Number (A) provides the total number of particles found in the nucleus, encompassing both protons and neutrons. Since electrons have a negligible mass, the Mass Number effectively represents the atom’s total mass.
This relationship between the two numbers allows for a simple calculation to determine the number of neutrons an atom contains. By subtracting the Atomic Number (protons) from the Mass Number (protons and neutrons), one can isolate the neutron count. The formula for finding the number of neutrons is \(\text{Neutrons} = \text{Mass Number} (A) – \text{Atomic Number} (Z)\).
Rubidium’s Identity and Natural Isotopes
Rubidium’s chemical identity is established by its Atomic Number, which is 37. This means every atom of Rubidium contains exactly 37 protons in its nucleus. The number of protons determines the element’s chemical behavior and placement on the periodic table.
Not all Rubidium atoms are identical in mass, leading to the existence of isotopes. Isotopes are atoms of the same element that have the same number of protons but a varying number of neutrons. This difference in neutron count results in different Mass Numbers, but they remain chemically the same element.
Natural Rubidium consists predominantly of two primary isotopes. The more common form is Rubidium-85 (\(\text{Rb}^{85}\)), which makes up approximately 72.2% of all natural Rubidium. The second form is Rubidium-87 (\(\text{Rb}^{87}\)), which accounts for the remaining 27.8%. The numerical superscripts, 85 and 87, represent the Mass Numbers for these two isotopic forms.
Calculating the Neutron Count for Rubidium
Because Rubidium has two natural isotopes, it does not have a single, fixed number of neutrons. A calculation must be performed for each specific isotopic form using the \(\text{Neutrons} = \text{Mass Number} – \text{Atomic Number}\) formula. The Atomic Number (Z) for both isotopes remains 37.
Rubidium-85
For the most common isotope, Rubidium-85, the Mass Number (A) is 85. Applying the formula yields \(85 – 37\), resulting in a neutron count of 48. This means that over 72% of all Rubidium atoms have 37 protons and 48 neutrons.
Rubidium-87
Rubidium-87 has a Mass Number (A) of 87. Using the same formula, \(87 – 37\) results in a neutron count of 50. This less abundant isotope contains 37 protons and 50 neutrons.
In summary, the number of neutrons Rubidium has depends on the specific isotope being referenced. The majority of the element’s atoms, \(\text{Rb}^{85}\), contain 48 neutrons, while the minority, \(\text{Rb}^{87}\), contains 50 neutrons.