Which is proper AM or am?

Answer 1

The proper form depends on what you are abbreviating:

The form is a.m. or A.M. is an abbreviation for ante meridiem (before midday); used to designate the first twelve hours of a day (10 a.m.).

The form Am (no periods) is the chemical symbol for the chemical element americium.

The form AM (no periods) is an acronym for amplitude modulation, a term used in radio broadcasting (AM and FM).

Note: Today it is becoming more common to use AM as the abbreviation for time of day; I have taken to using that form myself. This is most likely due to the fact that so many people communicate by email or text, and students are often required to type assignments. It is just easier to type, and, for aging eyes, easier to read. But, whichever you use in your everyday life, you should know what is considered the "correct form".