The number and type of particles it emits gives rise to the type of radiation being emitted. In the case of alpha decay, a helium nucleus (two protons, and two neutrons) are emitted. This is the lowest energy radiation type, but it is none the less dangerous to human health.
This abbreviation is usually found on maps, on expense reports, and it is very common to see miles abbreviated to mi. just about any time they are referenced. You might abbreviate the word miles to mi. on a road sign or a map with directions to a destination. It is also common to see such abbreviations in headlines or newspaper titles where space is a concern. Outside of maps, business mileage reports, etc. the word is not abbreviated in general prose.
In standard formal English, they are spelled out. If you spell out the number, spell out the unit of measurement. There is no need to add an s to an abbreviation to show a plural. This is sometimes done in advertising (3 lbs. for a dollar), but it is not necessary. English Unit Abbreviations. The single hatch mark ' can stand for foot or a geographical minute (a minute of longitude or latitude).