two protons joined to two neutrons

An alpha particle is two protons joined to two neutrons.

a highly charged Helium nucleus

An Alpha particle is essentially a highly charged Helium nucleus, and a highly charged Helium nucleus doesn't travel very far.

a type of ionizing radiation ejected by the nuclei of some unstable atoms

Alpha particles (symbol α ) are a type of ionizing radiation ejected by the nuclei of some unstable atoms.

a helium nucleus without any electrons

An alpha particle is a helium nucleus without any electrons.

a type of ionizing radiation

Alpha particles are a type of ionizing radiation.

the same as the nucleus of a helium atom

As stated above, an alpha particle is the nucleus of a Helium atom, i.e., two protons and two neutrons.

The nucleus of a helium-4 atom

The nucleus of a helium-4 atom is known as an alpha-particle.

low-penetrating types of radiation

Alpha particles are low-penetrating types of radiation, and generally will not penetrate your skin and probably in all likelyhood, won’t penetrate your clothing.

a form of ionizing radiation that is given off by radioactive materials such as uranium and radon

Alpha particles are a form of ionizing radiation that is given off by radioactive materials such as uranium and radon.

helium nuclei ( 2 protons bound together with 2 neutrons) ejected at high velocity from the nuclei of certain decaying atoms

Alpha particles are helium nuclei (2 protons bound together with 2 neutrons) ejected at high velocity from the nuclei of certain decaying atoms.

a helium nucleus - one of the products of a nuclear fission

The alpha particle is a helium nucleus - one of the products of a nuclear fission.

energetic helium nuclei that are given off by decaying radioactive materials such as radon gas and uranium

Alpha particles are energetic helium nuclei that are given off by decaying radioactive materials such as radon gas and uranium.

a heat source in radioisotope thermoelectric generators, which are used to power some spacecraft

alpha particles is a heat source in radioisotope thermoelectric generators, which are used to power some spacecraft.

a particular type of ionizing radiation that can be shielded by material as thin as a piece of paper

Alpha particles are a particular type of ionizing radiation that can be shielded by material as thin as a piece of paper.

a type of ionizing radiation that is exerted from radioactive materials

An alpha particle is a type of ionizing radiation that is exerted from radioactive materials.

a helium nucleus i.e. a helium atom that has lost i.e. a helium atom's two orbital electrons

The alpha particle is a helium nucleus i.e. a helium atom that has lost i.e. a helium atom's two orbital electrons.