Barium (Ba) is a soft, silvery metallic element categorized as an alkaline earth metal on the periodic table. Its identity is fixed by the number of protons in its nucleus, but the number of neutrons can vary. Determining the neutron count for Barium requires understanding the fundamental components that make up the atom.
The Atomic Building Blocks
Every atom is composed of three primary subatomic particles: protons, neutrons, and electrons. Protons carry a positive electrical charge and reside in the atom’s dense central core, known as the nucleus. Electrons are negatively charged and occupy the vast space surrounding the nucleus in defined shells or orbitals.
The neutron is electrically neutral and is also located within the nucleus alongside the protons. The sum of the protons and neutrons in an atom’s nucleus is defined as the Mass Number (\(A\)). The number of protons alone is called the Atomic Number (\(Z\)).
The number of protons determines the element’s identity and is constant for every atom of that element. Because the nucleus accounts for almost all of an atom’s mass, the Mass Number (\(A\)) is an approximate measure of the atom’s total mass.
Barium’s Fundamental Identity
Barium is assigned the Atomic Number \(Z=56\), which is its fixed, defining characteristic. This means that every atom of Barium contains exactly 56 protons in its nucleus. Barium’s position in Group 2 of the periodic table classifies it as an alkaline earth metal.
The standard atomic mass of Barium, listed on the periodic table, is approximately 137.33 atomic mass units (amu). This value represents a weighted average of the element’s naturally occurring forms. Since the number of protons is fixed at 56, the average atomic mass indicates that the number of neutrons is not a single, fixed value for all Barium atoms.
Determining the Neutron Count
The number of neutrons (\(N\)) in any atom is found by subtracting the Atomic Number (\(Z\)) from the Mass Number (\(A\)), expressed by the formula \(N = A – Z\). To find the average number of neutrons for Barium, the average atomic mass (\(A \approx 137.33\)) is used.
Applying the calculation to Barium’s defining values yields \(137.33 – 56 = 81.33\) neutrons. Since a neutron is a discrete particle and must exist as a whole number, this fractional result is not the count for a single atom. The value 81.33 represents the average number of neutrons across a large sample of Barium atoms. This result suggests that the most common form of Barium contains 81 neutrons.
The Role of Isotopes in Neutron Variation
The fractional average neutron count exists because of isotopes. Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. All Barium isotopes contain 56 protons, but their mass numbers vary due to differing neutron counts.
The average atomic mass (137.33 amu) is a weighted average reflecting the natural abundance of each isotope. Barium naturally occurs as a mixture of seven stable or nearly stable isotopes.
Barium-138 (Mass Number 138) is the most common isotope, accounting for approximately 71.7% of naturally occurring Barium. This isotope contains 82 neutrons (\(138 – 56 = 82\)). The second most common is Barium-137 (Mass Number 137), which contains 81 neutrons (\(137 – 56 = 81\)). Less abundant isotopes include Barium-136 (80 neutrons) and Barium-135 (79 neutrons).