In chapter 24, Aunt Alexandra is hosting one of her missionary tea parties and the women are criticizing Tom Robinson's wife, Helen, for not being Christian enough for them. Among other things. Mrs. Merriweather even criticizes Atticus by saying that "some people" think they are helping the black community, but in reality, they are only stirring things up. Just as everyone is biting their lips, Atticus comes home early from work. He tells the ladies to continue with their meeting and heads straight for the kitchen to talk to Calpurnia. Aunt Alexandra, Scout and Miss Maudie wind up in the kitchen to see why Atticus is home so early. The reason he's home is because he wants to get Calpurnia to accompany him to the Robinson home to break the sad news to Helen that Tom was killed in prison. 

Atticus reports that it was exercise time at the prison when Tom "just broke into a blind raving charge at the fence" (235). The prison guards said that if he would have had the full use of his other arm, he would have made it safe and sound--that's how fast he was going. Alexandra asks if the guards gave Tom any warning shots, which they did, but then they shot to kill. Atticus is very disappointed because he truly believed that he could have gotten Tom back home with his family after he made a strong appeal to a higher court. Unfortunately, all hopes for a winning appeal were dashed when Tom took matters into his own hands and lost.

I think you are referring to the Missionary Tea that Alexandra hosts at the Finch home during chapter 24 . He tells Alexandra that Tom has been shot 17 times and that he has to go and see the family. He also gives this news in the presence of Cal, Miss Maudie, and Scout. The significance of the news being brought to the women during this meeting couldn't be more ironic. The women had spent the whole meeting talking about those poor souls that J. Grimes Everett works with while hypocritically criticizing their own local African Americans for their poor attitudes when one of their own was wrongly convicted.

Atticus reports that Tom was shot "trying to escape" by the guards. He needed Cal so that Helen could have someone to comfort her.