This depends on the specific rules you are using. In most cases they are left out, but you may have to follow a particular style book.
PM or p.m. same fro AM or a.m
If they're both on the same date, then they mark a period of 21/2 hours.
It's neither, 12 noon and 12 midnight are just that although there are a lot of illiterates in positions where they should know better!)
It was 3 am when Frank got home.
Habit, and convention. We all have to speak the same language if we're going to communicate. Personally, I prefer the 24 hour clock; so 1 PM is 13:00, and 9 PM is 21:00. Of course, I did spend 21 years in the Navy, and we used the 24-hour clock exclusively. But for talking to civilians in the US, you have to use the 12 hour clock with AM and PM.
PM or p.m. same fro AM or a.m
PM
PM or p.m. same fro AM or a.m
3 PM until 6 PM
The end of a sentence always needs a period.
7 PM to 4 AM GMT is the same period as 2 PM to 11 PM EST.
If they're both on the same date, then they mark a period of 21/2 hours.
Ca As you go down a period, the atomic radius decreases
AM is in the morning until 11:59 and PM is in the evening until 11:59
The covalent atomic radius of carbon is 70 pm; oxygen has 60 pm and lithium 140 pm.
buon giorno or buona sera
Look for the highest numbered atom in the group: radium. Also barium has an empirical radius similar to the radius of radium: 215 pm.