Materials

[mimitabby]

well, i'll start. (and you forgot Titanium)

Steel (atomic weight of IRON= 56, steel is like 98% Iron) has been used to make bikes for 100+ years. It is flexible and sturdy.
It absorbs shock well and is considered more flexible than Aluminum.

Aluminum(atomic weight 26) is low cost and very lightweight. It is more rigid than steel, but weighs less. People in search of a good ride with lots of bucks will avoid aluminum, but i've had 2 aluminum bikes and they were pretty nice. So it's a darn good choice for someone without
a lot of funds but who wants a light bike. andyou can get carbon forks on it too.

Titanium is lighter than steel (atomic weight 48) but heavier than aluminum. It's VERY expensive. It's supposed to be a great ride, but it seems to me that unless you are a fanatic (those are the people who buy these bikes)
the 15% lighter than steel doesn't make it worth it.

Carbon fiber is what my company is making their new airplanes out of. I figure if they are making airplanes out of it; it should be safe enough for bicyclists too.
It's a man made composite material, sort of like plastic or resin. It is super light; sorry; no atomic weight; it's not an element, it's a compound.
All i can say is go pick a carbon bike up in the store. They are like riding on air (i am told) but they are expensive.

Most good bikes now have carbon fiber forks, because it is superior to even
steel for absorbing road bounce.

Last night i asked the bike guy what were the high points of carbon forks versus steel. He told me that carbon will give you the more comfortable ride, but if you want fenders, lights, etc attached to your fork, you are better off
with steel. I hope that helps you
mimi