An alpha (α) particle emitted in radioactive decay is best described as which of the following?

Introduction / Context:
Many radioactive elements, especially heavy nuclei such as uranium and radium, decay by emitting alpha particles. Understanding what an alpha particle really is helps explain why alpha radiation has relatively low penetrating power but high ionising power, and why it carries a specific positive charge and mass. This question asks you to choose the correct physical description of an alpha particle from several options that include hydrogen atoms, helium nuclei, protons and electrons.

Given Data / Assumptions:

• Alpha particles are emitted from the nuclei of certain heavy radioactive elements.
• They carry a charge of plus two and have a relatively high mass compared with electrons.
• A helium 4 nucleus consists of two protons and two neutrons.
• A hydrogen atom consists of one proton in the nucleus and one electron around it.
• A beta particle is a high speed electron or positron, not an alpha particle.

Concept / Approach:
Experimental measurements show that alpha particles have a charge of plus two and a mass approximately four times that of a proton. These values match the properties of a helium 4 nucleus, which contains two protons and two neutrons and no electrons. When a heavy nucleus emits an alpha particle, its atomic number decreases by two and its mass number decreases by four, exactly as if a helium nucleus had been removed. The approach is to match this description with the correct option and to distinguish alpha particles from other nuclear or atomic species such as hydrogen atoms, single protons and electrons.

Step-by-Step Solution:
Step 1: Recall that an alpha particle carries a charge of plus two and a mass number of four. Step 2: A particle with mass number four and charge plus two must contain two protons and two neutrons with no electrons. Step 3: This composition is exactly that of a helium 4 nucleus, often written as 4He2 plus. Step 4: A hydrogen molecule, H2, consists of two neutral hydrogen atoms, not a charged nuclear particle, so it does not match the known properties of alpha particles. Step 5: A single proton has charge plus one and mass approximately one atomic mass unit, so it cannot account for the alpha particle mass and charge. Step 6: A high speed electron is a beta particle with very low mass and negative charge, not an alpha particle. Step 7: Therefore, the correct description of an alpha particle is that it is a helium nucleus with two protons and two neutrons.

Verification / Alternative check:
Alpha particle scattering experiments, such as the famous Rutherford scattering experiment, used alpha particles emitted from radioactive sources. When alpha particles strike thin metal foils, their deflection patterns match the behaviour expected for heavy, positively charged helium nuclei. When alpha emitting sources are allowed to neutralise, helium gas can be detected, further confirming the link between alpha particles and helium nuclei. These observations strongly support the conclusion that an alpha particle is essentially a helium 4 nucleus stripped of its electrons.

Why Other Options Are Wrong:
Option A, a hydrogen molecule, describes two whole hydrogen atoms and does not match the charge and mass of an alpha particle. Option C, a neutral hydrogen atom, has only one proton in the nucleus and mass number one, so it is much lighter than an alpha particle. Option D, a single proton, also has mass number one and charge plus one, which does not match the properties of alpha radiation. Option E, a high speed electron, describes a beta particle, which has negative charge and extremely low mass. Only option B correctly describes an alpha particle as a helium nucleus with two protons and two neutrons.

Common Pitfalls:
Students sometimes confuse alpha particles with protons because both are positively charged, but they differ in charge and mass. Another common error is to mix up alpha and beta radiation, especially when learning about radioactive decay types for the first time. To avoid confusion, remember the simple pattern: alpha equals helium nucleus with two protons and two neutrons, beta is an electron or positron, and gamma is high energy electromagnetic radiation. This pattern makes it easier to connect each type of radiation with its properties and behaviour.

Final Answer:
An alpha particle is best described as A helium nucleus with two protons and two neutrons, which carries a charge of plus two and has relatively high mass.