teaching - How to quiet noisy undergrads students during lecture

I am teaching a class of about 100 undergraduates. The class meets for a three-hour block every week in a lecture hall which seats about 120. We have about 2-3 breaks every lecture, to give students time to buy food, go to the restroom, and also to work on some in-class problems which they have to submit after the lecture.

The problem that I am facing is that many of the students like to chit-chat even when I am talking. I find that this is distracting for the students who want to concentrate on what I am saying.

What are some techniques which I can apply to nicely tell the students to keep quiet?

Clarification: When it's time to work on the in-class problems, the students get 20 minutes to do so, and are free to talk as much as they like. The problem with the students being noisy is that they are noisy during the lecture when there are supposed to be quiet.

Answer

Call them out when you notice it. Stop what you're doing - single the chit-chatters out, and ask them if they have anything they'd like to say. If they have questions they should ask them so that the entire class can hear - that way everyone can learn. Otherwise, make it clear that you won't tolerate chit-chat and, if they persist, simply kick them out of the class. Make it clear that they are welcome to return to the next lecture on the condition that they keep quiet during the lecture. Be sure to be polite and cordial, of course, but don't waver on being firm.

It is not acceptable for a few noisy students to compromise the learning environment for everyone else. Just get them out of there. You probably won't have to do this more than once or twice before the rest figure out that you're serious. As a lecturer, you're the captain of the ship. Don't be afraid to be the captain - keeping order is your job.

Also keep in mind that it is not your job to make sure that they pay attention - that's their job. "Listen to me" should not be your responsibility, it should be theirs. Your responsibility is primarily "Don't disrupt my lecture! Others are trying to listen to me". If they would like to quietly not pay attention, that's fine. They're paying for a seat in the lecture hall - what they do with it is up to them, so long as it does not affect the learning environment for everyone else (who are also paying for their seat in that lecture hall!).