All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If your mother is an atheist and does not believe in Allaah, the Only True God, then there is no doubt that she is a non-Muslim and she should be treated as a non-Muslim, but you are obliged to be kind and dutiful to her in what is permissible. Allaah urged us to be kind and dutiful to our parents even if they are non-Muslims, especially the mother. Allaah Says (which means): {And We have enjoined upon man [care] for his parents. His mother carried him, [increasing her] in weakness upon weakness, and his weaning is in two years. Be grateful to Me and to your parents; to Me is the [final] destination.}[Quran 31:14]. So you are obliged to be kind and dutiful to your mother and you are not permitted to cut relations with her under any circumstances. The fact that she has mistreated you and abused you does not make it permissible for you to cut relations with her. Therefore, we advise you to keep good relations with her and be kind to her, and endeavour to forget what she has done to you. If you are really sincere in doing so, Allaah will enable you in your endeavour. Nonetheless, you should be careful not be influenced by her in your belief, and you should never obey her in things which bring the wrath of Allaah.
You have to know that among the best acts of kindness to her is to be keen to guide her to Islam by supplicating Allaah as much as possible to guide her to the truth and to accept her repentance. You should choose the best and appropriate times in advising her and admonishing her. There is no harm in seeking the help of righteous and pious people in doing so. It is confirmed that the Prophet said: "I swear by Allaah! If Allaah guides one single man (to Islam) by your efforts, that will be better for you than possessing an abundant number of red camels (a very expensive breed of camels)." [Al-Bukhari and Muslim]
For more details please refer to Fatwa 90264.
Allaah Knows best.