Electron Capture

Introduction

Electron capture is a way for some proton-rich nuclides to reduce their proton count without emitting a positron. A proton in the nucleus combines with an atomic electron, usually from an inner shell, and changes into a neutron. The daughter keeps the same mass number but has an atomic number one lower. The process shows that a change involving an electron can still be a nuclear transformation.

Core explanation

Write the particle-level change as proton + electron → neutron + electron neutrino. On the left, the proton has charge +1 and the electron −1, giving zero total charge. On the right, the neutron and neutrino are neutral. A proton and neutron each count as one nucleon, so the nucleon count also balances. At the nuclide level, ᴬ ZX + ⁰₋₁e → ᴬ Z₋₁Y + νₑ. The electron appears on the reactant side because the nucleus captures it; it is not emitted as beta-minus radiation.

Consider beryllium-7, which can capture an electron and become lithium-7. The balanced equation is ⁷₄Be + ⁰₋₁e → ⁷₃Li + νₑ. Mass-number entries give 7 + 0 = 7. Charge-number entries give 4 − 1 = 3. The parent nucleus has four protons and three neutrons; the daughter has three protons and four neutrons. A stays seven because one proton becomes a neutron.

The captured electron commonly comes from an inner atomic shell, where its probability of being near the nucleus is relatively large. Removing it leaves an electron vacancy. When an outer electron fills that vacancy, the atom can emit characteristic X-rays or transfer energy to another electron, which is then ejected as an Auger electron. These later atomic rearrangements can accompany electron capture, but they are not the neutrino-producing nuclear step itself. The exact emissions depend on the atom and its environment.

Electron capture and beta-plus decay both reduce Z by one while keeping A constant. Their particle accounting differs: beta-plus emits a positron and a neutrino, whereas electron capture consumes an atomic electron and emits a neutrino. A given nuclide may have both routes available, or only one, depending on nuclear and atomic energy conditions. Therefore a problem stating only “Z falls by one” does not uniquely identify the mechanism unless other evidence is supplied.

An ordinary atom may also gain an electron in a chemical or electrochemical process. That changes its ionic charge, not its nucleus. In electron capture, the electron participates in a nuclear reaction with a proton. The result is a new element because the proton count falls. That is the decisive difference. An ion's electron count can also affect the availability of electrons for certain capture decays; this makes electron capture a useful nuance when comparing nuclear and chemical environments, although introductory questions generally assume ordinary atoms.

The captured electron has mass even though its nuclear-equation mass-number entry is zero. A is an integer nucleon count, not a precise rest mass. The neutrino carries energy and momentum away, and the daughter can recoil. Balancing A and electric charge is necessary but does not alone tell us whether the decay is energetically allowed or what its half-life will be.

Step-by-step reasoning

1. Confirm that an electron is captured, so write it on the left of the arrow. 2. Keep parent mass number A for the daughter. 3. Reduce the parent's atomic number Z by one and identify the daughter element. 4. Add an electron neutrino on the product side for the complete basic process. 5. Check A and charge-number sums and separate later atomic X-ray emission from the capture step.

Visual explanation

Draw an electron in a close atomic shell moving into the nucleus. Inside, show a proton dot changing to a neutron dot and a neutrino arrow leaving. Label the parent “A, Z” and the daughter “A, Z − 1.” Draw a second small arrow from an outer electron into the shell vacancy to indicate possible later X-ray or Auger-electron production.

Real-world analogy

A team converts one member from role P to role N by taking in a helper from outside the team. The counted team size stays the same while the P count falls. The helper represents the captured electron, but the analogy is incomplete because the electron is a real conserved particle with charge and the nuclear process emits a neutrino.

Real-world example

Beryllium-7 is a naturally produced radioactive isotope that decays by electron capture to lithium-7. This makes a clear classroom example because the small nuclear numbers make A and charge balancing easy to see. Its isotope behavior is distinct from an ordinary beryllium atom merely accepting an electron in a chemical process.

Why?

Why does the daughter element move one place lower in atomic number? One of the nucleus's protons becomes a neutron, so the number of protons decreases by one. Atomic number is proton count, while mass number counts both proton and neutron together and therefore remains unchanged.

Common misconception

“Electron capture is the reverse of beta-minus emission because the electron moves in the opposite direction.” Both involve electrons, but beta-minus turns a neutron into a proton and emits an electron; electron capture turns a proton into a neutron by consuming an electron. Their daughter Z values change in opposite directions.

Worked example

Suppose an isotope ⁵⁵₂₆Fe undergoes electron capture. On the left write ⁵⁵₂₆Fe + ⁰₋₁e. The daughter has A = 55 and Z = 25, which is manganese, so write ⁵⁵₂₅Mn + νₑ on the right. The full basic equation is ⁵⁵₂₆Fe + ⁰₋₁e → ⁵⁵₂₅Mn + νₑ. Verify 55 + 0 = 55 + 0 and 26 − 1 = 25 + 0. The parent has 29 neutrons, and the daughter has 30: one proton became a neutron.

Quick check

1. Where is the electron written in an electron-capture equation, and what happens to Z? Answer: The electron is on the reactant side, and the daughter Z is one lower than the parent's.

Exam focus

For electron capture, keep A, reduce Z by one, and place ⁰₋₁e on the left. Distinguish this from beta-plus decay, which emits a positively charged positron on the right, and from ordinary ion formation, which leaves Z unchanged.

Advanced insight

Because electron capture uses an atomic electron, a fully ionised atom may have a different capture rate or be unable to undergo a particular capture route until electrons are available. This is one of the limited cases in which electron environment can matter to a nuclear process. It does not imply that ordinary temperature changes can freely switch every radioactive decay on or off.

Summary

Electron capture changes a proton into a neutron by absorbing an atomic electron and emitting a neutrino. The daughter has the same A and one lower Z. Atomic shell rearrangement may follow the capture, and the mechanism differs from both beta-plus emission and simple chemical electron transfer.

Practice questions

1. Complete ⁷₄Be + ⁰₋₁e → ? + νₑ. Answer: The daughter is ⁷₃Li because A remains seven and Z falls from four to three. 2. What daughter results from electron capture by ⁵⁵₂₆Fe? Answer: ⁵⁵₂₅Mn, manganese-55, with one fewer proton and one more neutron. 3. Is electron capture identical to an atom becoming a negative ion? Answer: No. A negative ion gains an orbital electron but keeps the same nucleus; capture converts a nuclear proton to a neutron and changes the element.