some questions about 802.11

One:
As far as i know, signal in 5GHz decreases faster than in 2.4GHz,why?

Two:
802.11a, 802.11b ,802.11g's data rate decreases as the distance increase ,why? there exits some relationship?

Three:
throughput and data rate,their relationship?

Four:
What is 54g , and 802.11g?

Path loss depends by frequency. Higher the frequency, higher the loss.
http://www.rfcafe.com/references/ele.../path_loss.htm
Here you can find the relation for WLAN between Signal field and Data rate
http://www.iitk.ac.in/mladgp/Thesis_...tion_Draft.pdf

The relationship between data rate and throughput depends on the overhead (preamble, error coding etc. ) of your protocol.