I know that it's hard to hear this, but it sounds as though your father has been abusing your mother for a long time. Throwing things at her and slapping is physical abuse. And those are only the things that you saw (or remember seeing).

I'm sure that it's a tough thing to face, but it's better now than never! Please check out the following link for a full description, as well as what to do when abuse is discovered.
Domestic Violence and Abuse: Signs of Abuse and Abusive Relationships

Please get some help for your mom. Gentle hugs sent your way.