Atomic Mass 113: Isotopes With Mass Number 113

Mass Number vs Atomic Number

Two different numbers describe an atom and they are easy to confuse:

• Atomic number (Z) is the number of protons in the nucleus. It defines what element the atom is. Hydrogen has Z = 1, oxygen has Z = 8, and nihonium has Z = 113.
• Mass number (A) is the total count of protons plus neutrons. It is always a whole number for a specific isotope. Carbon-12 has A = 12; uranium-238 has A = 238.
• Atomic mass is the actual measured mass of an atom in atomic mass units (amu, also called daltons, Da). It is close to but not identical to A, because the protons and neutrons together weigh slightly less than their separate masses (the mass defect, related to nuclear binding energy).

So "113" can label either an element (Z = 113, nihonium) or an isotope (A = 113, for example indium-113 with 49 protons and 64 neutrons). They are distinct.

Cadmium-113

Cadmium has atomic number 48. Cadmium-113 has 48 protons and 65 neutrons, summing to mass number 113.

• Symbol: ¹¹³Cd or Cd-113
• Natural abundance: about 12.2% of natural cadmium.
• Stability: very slightly radioactive. It undergoes beta decay to indium-113 with a half-life of about 8 × 10¹⁵ years, roughly 600,000 times the age of the universe, so for practical purposes it behaves as stable. It occurs naturally as a primordial isotope.
• Notable property: a very large thermal-neutron capture cross-section (about 20,000 barns). Cadmium-113 is the main reason cadmium absorbs slow neutrons so well and is used in some reactor control rods and neutron shielding.
• Atomic mass (precise): ≈ 112.9044 u. This is slightly less than 113, for reasons explained below.

Indium-113

Indium has atomic number 49. Indium-113 has 49 protons and 64 neutrons, summing to mass number 113.

• Symbol: ¹¹³In or In-113
• Natural abundance: about 4.3% of natural indium (the more common natural isotope is ¹¹⁵In, at about 95.7%).
• Stability: stable. It is the only truly stable isotope with mass number 113. (The more abundant indium-115 is itself very weakly radioactive.)
• Atomic mass (precise): ≈ 112.9041 amu.
• Notable use: ¹¹³ᵐIn, an excited (metastable) state of indium-113 with a half-life of about 100 minutes, has been used in nuclear medicine imaging. It is obtained from a tin-113 generator.

Other Isotopes With Mass 113

Several other isotopes share the mass number 113, though most are unstable:

• Tin-113 (¹¹³Sn): radioactive, half-life about 115 days. Used as the parent for ¹¹³ᵐIn generators in some nuclear medicine workflows.
• Silver-113 (¹¹³Ag): radioactive, half-life about 5.4 hours.
• Antimony-113 (¹¹³Sb): radioactive, half-life about 6.7 minutes.
• Tellurium-113 (¹¹³Te): radioactive, half-life under two minutes.
• Various others, such as palladium-113, iodine-113 and xenon-113, with short half-lives.

This is what nuclear physicists call an isobar chain — a sequence of isotopes that all share the same mass number A = 113 but differ in proton count Z. Beta decay moves isotopes along the chain toward the most stable member, ¹¹³In. Lighter-Z members (e.g. ¹¹³Ag) decay to ¹¹³Cd, which itself decays, extremely slowly, to ¹¹³In. Heavier-Z members (e.g. ¹¹³Sn, ¹¹³Sb) decay down to ¹¹³In.

113 amu in Practical Mass Spectrometry

In mass spectrometry, peaks at mass-to-charge ratio m/z = 113 are common. They can come from:

• Singly charged ions of indium or cadmium (where their natural isotopes hit 113).
• Molecular fragments whose nominal mass adds up to 113 — for example, certain six- or seven-atom organic fragments.
• Doubly charged ions of much heavier species (m/z = 113 from a 226-amu doubly charged ion).

Identifying the source of an m/z 113 peak depends on isotope pattern, instrument resolution, and the chemistry of the sample. High-resolution instruments can distinguish 113.000 from 113.040 from 113.084, for example, helping to pin down the elemental composition.

Why Atomic Mass Is Close to But Not Equal to A

The mass number A is a count: 113 nucleons (protons + neutrons). A hydrogen atom (one proton plus one electron) has a mass of about 1.0078 u, and a free neutron about 1.0087 u. For cadmium-113 (48 protons with their electrons, plus 65 neutrons), the separate parts add up to about 113.94 u.

Real nuclei weigh less than this because some of the mass is converted to binding energy when the nucleons bond together — Einstein's E = mc² in the nuclear context. For ¹¹³Cd, the actual atomic mass is about 112.9044 u, around 1.03 u less than the sum of its parts. That mass defect corresponds to the energy holding the nucleus together.

The mass defect is why nuclear reactions release so much energy compared to chemical reactions — even small differences in atomic mass between reactants and products correspond to enormous energies.

Element 113 (Nihonium) Is Different

Element 113 has atomic number 113, meaning 113 protons in its nucleus. Its known isotopes are much heavier: from ²⁷⁸Nh, the first one made, up to ²⁸⁶Nh, the longest-lived (half-life roughly 10 seconds). That is why its atomic weight is listed as [286]. Element 113 has nothing in common, mass-wise, with the isotopes discussed above.

For the story of element 113's discovery and naming, see element 113: nihonium. For its isotopes and half-lives, see nihonium properties and isotopes. For its position in the table, see what element is 113 on the periodic table.

Common Mistakes

• Confusing mass number with atomic number. A = 113 (an isotope mass) is not the same as Z = 113 (the element nihonium).
• Treating atomic mass as exact. The mass number is exact (a count of nucleons), but the atomic mass is a measured quantity in amu and includes mass defect.
• Assuming all 113-mass isotopes are stable. Only ¹¹³In is stable. ¹¹³Cd decays with a half-life of about 8 × 10¹⁵ years, and the rest are clearly radioactive.
• Misreading mass spec peaks. An m/z 113 peak does not always mean a 113-amu species — multiply-charged ions and adducts can land at the same nominal mass.

Quick-Reference Card

• Mass number 113: 113 nucleons total (protons + neutrons).
• Most common stable isotopes at A = 113: ¹¹³In, ¹¹³Cd.
• Cadmium-113 atomic mass: ≈ 112.9044 amu.
• Indium-113 atomic mass: ≈ 112.9041 amu.
• Element with Z = 113: nihonium (Nh), whose isotopes have mass numbers 278–286, unrelated to mass number 113.
• Isobar chain: everything ends at ¹¹³In, the only stable member.

For more on 113 in chemistry and physics, see 113 on the periodic table, 113 chemical compounds, and element 113 (nihonium).