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? I would like to yell at the students "Listen to me!" but obviously that would be a bad idea, and would destroy my teaching evaluation ratings. Don't worry, I am just joking about yelling at the students.

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.