Mercury (Hg) is a unique element known for existing as a liquid metal at standard room temperature. This dense, silvery element has been used for centuries, though its toxicity is now widely recognized. Understanding the fundamental structure of the mercury atom is necessary to comprehend its chemical behavior. The primary question concerns the number of neutrons contained within its atomic nucleus.
Defining the Atomic Components
Atoms are built upon three fundamental subatomic particles: protons, neutrons, and electrons. Protons and neutrons are clustered in the central nucleus, while electrons orbit in energy shells. The number of protons defines an element and acts as its unique identifier. This count is the Atomic Number, symbolized by \(Z\).
For mercury, the Atomic Number (\(Z\)) is 80, meaning every atom contains 80 protons. The number of neutrons can vary and is determined by the atom’s overall mass. The total number of protons and neutrons combined is called the Mass Number (\(A\)). Protons and neutrons contribute almost all of an atom’s mass.
To determine the count of neutrons (\(N\)), one subtracts the Atomic Number (\(Z\)) from the Mass Number (\(A\)). The calculation is \(N = A – Z\). Since \(Z=80\) is fixed for all mercury atoms, the neutron count depends entirely on which version of the element’s nucleus is being examined.
Calculating Neutrons in Standard Mercury
To determine the number of neutrons, it is most practical to consider mercury’s most common stable form. Mercury exists naturally as a mixture of stable forms, called isotopes. The most abundant is Mercury-202, which makes up nearly 30% of naturally occurring mercury atoms.
The number 202 represents the specific Mass Number (\(A\)) for this isotope. Applying the formula \(N = A – Z\), the number of neutrons is found by subtracting \(Z=80\) from \(A=202\). This calculation reveals that a standard Mercury-202 atom contains 122 neutrons (\(202 – 80 = 122\)). This figure is the most direct answer, corresponding to the most frequently encountered stable form.
The average atomic weight (approximately 200.59) is not used for this calculation. This value is a weighted mean of the mass numbers of all stable isotopes based on their natural abundances. Using the average atomic weight results in a non-whole number of neutrons, which is impossible for a single nucleus.
The Range of Mercury Isotopes
The neutron count is not constant because mercury exists in nature as a combination of seven different stable isotopes. An isotope possesses the same number of protons (80) but a different count of neutrons. This variation means mercury atoms have differing mass numbers, though they remain chemically identical.
The stable mercury isotopes range from Mercury-196 to Mercury-204, translating to a neutron count spanning from 116 to 124. For instance, Mercury-196 contains 116 neutrons (\(196 – 80\)), and Mercury-204 contains 124 neutrons (\(204 – 80\)). Other stable forms include Mercury-198 (118 neutrons), Mercury-200 (120 neutrons), and Mercury-201 (121 neutrons).
The range of 116 to 124 neutrons demonstrates that the answer depends on the specific isotope referenced. Despite these nuclear differences, all stable isotopes of mercury exhibit identical chemical properties. This is because chemical behavior is governed by the number of electrons, which equals the fixed number of 80 protons in a neutral atom.