An alpha particle is a charged helium atom released from a larger atom undergoing radioactive decay. Often accompanying them during decay are “beta” and “gamma” particles. 

The atomic number of a given element is the total number of protons in its nucleus. Hydrogen has one such proton; its atomic number is one. Helium has two protons for an atomic number of two; oxygen has eight protons and an atomic number of eight. Uranium contains 92 protons and, at 92, has the highest atomic number of the common natural elements; atoms above 92 occur almost always in laboratories. 

The atomic weight of a given element is the total number of protons and neutrons in its nucleus. Some elements have multiple atomic weights due to varying numbers of neutrons in their nuclei; these are the isotopes of a given element.