Barium (Ba) is a soft, silvery alkaline earth metal, the fifth element in Group 2 of the periodic table. It is highly reactive and is never found in nature as a free element. Its name is derived from the Greek word barys, meaning “heavy.” This article explains how to determine the number of neutrons in a Barium atom.
Atomic Building Blocks: Protons, Mass, and Neutrons
Every atom is composed of three primary subatomic particles: protons, neutrons, and electrons. Protons are positively charged particles located in the dense central nucleus. The number of protons determines the element and is known as the atomic number, which is unique to each element.
Neutrons are electrically neutral particles that reside alongside protons in the nucleus. These particles contribute significantly to the atom’s overall mass but do not change its chemical identity. The total count of protons and neutrons combined defines the mass number of a specific atom.
This mass number represents the entire mass of the nucleus, as the mass contributed by electrons orbiting the nucleus is negligible. Understanding the relationship between these particles is necessary for calculating the neutron count of any element.
The Calculation: Finding the Neutron Count in Barium
To find the number of neutrons in Barium, one must first locate its atomic number and its mass number. Barium’s atomic number is 56, meaning every Barium atom contains exactly 56 protons. This number is fixed and defines the element.
The standard atomic mass of Barium is approximately 137.33 atomic mass units (u). To find a specific mass number for the calculation, we round this value to the nearest whole number, which is 137. This rounded number represents the mass number of the most commonly represented form of Barium.
The formula for finding the neutron count is the mass number minus the atomic number. Applying this to the rounded Barium mass, we subtract the 56 protons from the mass number of 137. This calculation, \(137 – 56 = 81\), yields a neutron count of 81.
Why Barium’s Neutron Count Varies
While the calculation suggests 81 neutrons, the actual number can vary because Barium exists in nature as a mixture of different isotopes. Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. This difference in neutron count results in varying atomic masses.
Naturally occurring Barium is composed of seven primordial nuclides, which are atoms with slightly different masses. For instance, Barium-138 is the most abundant isotope, making up about 71.7% of all Barium. This particular isotope has a mass number of 138, meaning it contains 82 neutrons (\(138 – 56 = 82\)).
The fractional standard atomic mass of 137.33 listed on the periodic table is a weighted average of the masses of all these naturally occurring isotopes. Since Barium-138 with 82 neutrons is the most common form, the average mass is pulled closer to 138 than to 137. Barium atoms can have neutron counts ranging from 74 up to 82.