Charge on an Alpha Particle — Value, Formula & Explanation

The charge on an alpha particle is:

• In terms of elementary charge: +2e • In coulombs: +3.204 × 10⁻¹⁹ C • In esu (electrostatic units): +9.615 × 10⁻¹⁰ esu

Where e = 1.602 × 10⁻¹⁹ C (charge of one proton)

Calculation: Charge = number of protons × charge per proton Charge = 2 × (+1.602 × 10⁻¹⁹ C) Charge = +3.204 × 10⁻¹⁹ C

The charge is positive because the alpha particle has 2 protons but zero electrons. Neutrons carry no charge, so only protons contribute to the net charge.

An alpha particle is a helium-4 nucleus emitted during radioactive alpha decay. It is one of the three main types of radioactive radiation (alpha, beta, gamma).

Composition: • 2 protons (positive charge) • 2 neutrons (no charge) • 0 electrons • Total nucleons: 4 (mass number = 4) • Atomic number: 2 (same as helium)

Symbol: • ⁴₂He²⁺ or ⁴₂α • Sometimes written as α²⁺ to show the +2 charge

An alpha particle is identical to a helium-4 nucleus. The only difference between an alpha particle and a helium atom is that the helium atom has 2 electrons orbiting the nucleus, making it electrically neutral. The alpha particle has lost both electrons, giving it a +2 charge.

The charge on an alpha particle comes entirely from its protons:

Particle | Number | Charge per particle | Total charge Proton | 2 | +1.602 × 10⁻¹⁹ C | +3.204 × 10⁻¹⁹ C Neutron | 2 | 0 | 0 Electron | 0 | — | 0

Net charge = +3.204 × 10⁻¹⁹ C + 0 + 0 = +3.204 × 10⁻¹⁹ C = +2e

Key points: • Protons are the only charged particles in the alpha particle • Each proton has a charge of +e = +1.602 × 10⁻¹⁹ C • 2 protons give a charge of +2e • Neutrons are electrically neutral — they contribute zero charge • There are no electrons to cancel out the positive charge • Therefore, the alpha particle has a net positive charge of +2

Along with charge, the mass of an alpha particle is frequently asked in exams:

• Mass: 6.644 × 10⁻²⁷ kg • Mass in amu: 4.001506 u (atomic mass units) • Mass in MeV/c²: 3727.379 MeV/c² • Approximately 4 times the mass of a proton • Approximately 7344 times the mass of an electron

Mass calculation: Mass ≈ 2 × mass of proton + 2 × mass of neutron Mass ≈ 2 × (1.673 × 10⁻²⁷) + 2 × (1.675 × 10⁻²⁷) Mass ≈ 3.346 × 10⁻²⁷ + 3.350 × 10⁻²⁷ Mass ≈ 6.696 × 10⁻²⁷ kg (approximate, before binding energy correction) Actual mass = 6.644 × 10⁻²⁷ kg (slightly less due to nuclear binding energy — mass defect)

The difference between the sum of individual nucleon masses and the actual alpha particle mass is the mass defect, which corresponds to the nuclear binding energy via E = mc².

The specific charge (charge-to-mass ratio) of an alpha particle is an important value in physics:

Charge-to-mass ratio (e/m) = charge ÷ mass e/m = 3.204 × 10⁻¹⁹ C ÷ 6.644 × 10⁻²⁷ kg e/m = 4.822 × 10⁷ C/kg

Alternatively: e/m = 4.822 × 10⁷ C kg⁻¹

Comparison of charge-to-mass ratios: • Alpha particle (α): 4.822 × 10⁷ C/kg • Proton (p): 9.579 × 10⁷ C/kg • Electron (e): 1.759 × 10¹¹ C/kg

The alpha particle has the lowest charge-to-mass ratio among these three because it is very heavy (4 nucleons) relative to its charge (+2). The electron has the highest charge-to-mass ratio because it is extremely light.

Significance: A lower charge-to-mass ratio means alpha particles are deflected less in electric and magnetic fields compared to beta particles (electrons).

Complete list of alpha particle properties:

• Charge: +2e = +3.204 × 10⁻¹⁹ C • Mass: 6.644 × 10⁻²⁷ kg (≈ 4 amu) • Speed: typically 5–7% of the speed of light (≈ 1.5 × 10⁷ m/s to 2.1 × 10⁷ m/s) • Kinetic energy: typically 4–9 MeV • Penetrating power: LOW — stopped by a sheet of paper or a few cm of air • Ionising power: HIGH — strongly ionises air and other materials • Range in air: 2–10 cm (depending on energy) • Deflection in electric field: deflected towards the negative plate (because positively charged) • Deflection in magnetic field: deflected according to the left-hand rule (positive charge) • Effect of electric/magnetic field: less deflection than beta particles (due to high mass and lower charge-to-mass ratio) • Nature: particulate radiation (has mass and charge) • Identical to: helium-4 nucleus (⁴₂He²⁺)

Alpha particles are emitted during alpha decay of heavy, unstable nuclei. In alpha decay, the parent nucleus loses 2 protons and 2 neutrons (an alpha particle), becoming a daughter nucleus.

General equation: ²ᴬ_Z X → ᴬ⁻⁴_(Z-2) Y + ⁴₂He (alpha particle)

Examples:

1. Uranium-238 decay: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He Uranium-238 emits an alpha particle to become thorium-234

2. Radium-226 decay: ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He Radium-226 emits an alpha particle to become radon-222

3. Polonium-210 decay: ²¹⁰₈₄Po → ²⁰⁶₈₂Pb + ⁴₂He Polonium-210 emits an alpha particle to become lead-206

In each case: • Mass number decreases by 4 • Atomic number decreases by 2 • The emitted alpha particle carries away +2e charge and 4 amu mass

Property | Alpha (α) | Beta (β⁻) | Gamma (γ) Identity | Helium-4 nucleus | Electron | Electromagnetic wave (photon) Charge | +2e (+3.204 × 10⁻¹⁹ C) | −1e (−1.602 × 10⁻¹⁹ C) | 0 (no charge) Mass | 6.644 × 10⁻²⁷ kg (≈ 4 amu) | 9.109 × 10⁻³¹ kg (≈ 1/1836 amu) | 0 (massless) Speed | 5–7% of c | up to 99% of c | speed of light (c) Penetrating power | Low (stopped by paper) | Medium (stopped by aluminium sheet) | High (reduced by thick lead/concrete) Ionising power | High (strongest) | Medium | Low (weakest) Deflection in fields | Small deflection (towards −ve plate) | Large deflection (towards +ve plate) | No deflection Range in air | 2–10 cm | up to a few metres | Very large (kilometres) Symbol | ⁴₂He²⁺ or α | ⁰₋₁e or β⁻ | γ

Key insight: Alpha particles have the highest charge (+2e) and highest mass, which gives them the strongest ionising power but the weakest penetrating power. They lose energy quickly because they interact strongly with matter.