Alpha Particle

Key Stage 4

Meaning

An alpha particle (α-particle) is a type of ionising radiation made of 2 protons and 2 neutrons emitted from the nucleus of an unstable isotope.

About Alpha Particles

Alpha particles may also be referred to as alpha radiation and is written with the symbol α.

Alpha particles are a Helium nucleus.

Alpha particles have a relative atomic mass of 4 and relative charge of +2.

Alpha particles are emitted when a nucleus is too large or the ratio of protons to neutrons is too large.

Alpha particles are emitted rather than single protons or neutrons because the Helium nucleus is extremely stable.

Charge and Mass

	Scientist were able to determine the charge and mass of an α-particle by sending it between two electrically charged plates and observing its path. The α-particle moves towards the negative plate, so it must be positively charged. The rate of curvature depends on the mass:charge ratio which indicates it has a relative atomic mass of 4 and relative charge of +2.

Penetration Depth

	Alpha particles can travel around 15cm through air (STP) before colliding with and ionising atoms or molecules.

	Alpha particles can be stopped by a thin sheet of paper.

Ionising Potential

With a charge of +2, α-particles are the most ionising of the three ionising radiations. It is capable of removing two electrons from a single atom or molecule or removing 1 electron from two atoms or molecules.


When an α-particle interacts with an atom the α-particle can remove one or two electrons to ionise the atom.

Precautions

Alpha radiation is the most ionising but the least penetrating.

Alpha sources are kept inside a block of lead with a hole that only allows the alpha particles out in one direction.

Outside the body an organism can be protected from alpha radiation by keeping a distance greater than 5cm or by covering any bare skin.

Inside an organism alpha particles are the most dangerous as they are highly ionising and there is nothing inside an organism to block them ionising biological molecules like DNA. Every precaution must be taken not to contaminate the skin with the alpha source in case it finds its way into the body through a cut or the mouth.

When handling a source of alpha radiation the precautions which should be taken are:

Wear gloves - to prevent contamination.
Use tongs more than 5cm long to handle the source, never touch it - to prevent contamination and irradiation.
Aim the source away from any living organism - to prevent irradiation.
Store the source in a sealed container - to prevent contamination and irradiation.

Applications


In a smoke detector alpha particles are released between two electrodes. When the alpha particles ionise the molecules in the air the ions move between the electrodes and a current flows in the circuit. When smoke gets between the electrodes the ions become stuck in large particles of soot preventing them from moving. This stops the current in the circuit. The ammeter in the circuit is connected to an alarm which goes off when there is no current.

Equation

\(_Z^AX \rightarrow _{Z-2}^{A-4}Y + _2^4\alpha\)

\(_{92}^{238}U \rightarrow _{90}^{234}Th + _2^4\alpha\)

Example Calculations

Find the element 'X' and calculate its relative atomic mass 'A' and its relative atomic charge 'Z'. \({}_{84}^{210}Po \rightarrow {}_{Z}^{A}X + {}_2^4\alpha\)	Find the element 'X' and calculate its relative atomic mass 'A' and its relative atomic charge 'Z'. \({}_{Z}^{A}X \rightarrow {}_{86}^{222}Rn + {}_2^4\alpha\)
1. Calculate the relative atomic mass by looking at the top row of numbers. 210 = A + 4 A = 206	1. Calculate the relative atomic mass by looking at the top row of numbers. A = 222 + 4 A = 226
2. Calculate the relative atomic charge by looking at the bottom row of numbers. 84 = Z + 2 Z = 82	2. Calculate the relative atomic charge by looking at the bottom row of numbers. Z = 86 + 2 Z = 88
3. Since the relative atomic charge is the same as the atomic number look up the element on the periodic table. \({}_{82}^{206}Pb\)	3. Since the relative atomic charge is the same as the atomic number look up the element on the periodic table. \({}_{88}^{226}Ra\)

References

Key Stage 5

Meaning

Alpha radiation consists of alpha particles emitted by certain radioactive materials, each composed of two protons and two neutrons.

About Alpha Radiation

Alpha particles are helium nuclei.
They can be stopped by a sheet of paper or human skin.
Poses a health risk if alpha-emitting substances are inhaled or ingested.
Alpha radiation has a high ionizing power, causing significant damage to living cells.
Because of their high mass and charge, alpha particles have a short range in air (a few centimeters).
Alpha radiation is used in radiation therapy for treating certain cancers.
It is also used in smoke detectors and in some types of radioactive power sources (radioisotope thermoelectric generators).

Examples

Polonium-210 emits alpha particles and is highly toxic if ingested.
Alpha radiation is used in some types of smoke detectors.

Alpha Particle

Contents

Key Stage 4

Meaning

About Alpha Particles

Charge and Mass

Penetration Depth

Ionising Potential

Precautions

Applications

Equation

Example Calculations

References

AQA

Edexcel

OCR

Key Stage 5

Meaning

About Alpha Radiation

Examples