Alpha decay

An alpha particle consists of 2 protons and 2 neutrons in a cluster  

They tend to be ejected by nuclei which are unstable because they are too heavy or have too many protons.

The alpha particle is ejected with kinetic energy of about 1 to 10 MeV. This depends on the isotope. A particular isotope will emit particles which all have the same energy.
In air it will lose this energy quickly by ionising air molecules ( several thousand typically). It will then pick up a few electrons from somewhere and become helium.

Here's a summary of the properties of alpha particles ( 1u = 1 atomic mass unit, about the mass of a proton)

so what happens to the parent nucleus when an alpha is emitted?

Alpha particles are very dense, positive, energetic particles. They have about the same kinetic energy as beta particle but because their mass is so much bigger they travel a lot slower.

Because they are traveling slower they will spend more time in the vicinity of the molecules they pass and so there is a greater chance that an inisation will take place. This is why they have a shorter range in air than beta particles. Alphas and betas will ionise about the same number of particles but alphas do it over a much smaller distance and so lose their energy quicker.