There are eleven obligatory acts for prayers:
951. Some of the obligatory acts of prayers are elemental (Rukn). Hence, a person who does not offer them, whether intentionally or by mistake, his prayers become void. Some other obligatory acts of prayers are not elemental. Therefore, if they are omitted by mistake, the prayers does not become void.
The elementals of Namaz are five:
Any addition made to these elemental (Rukni) acts, intentionally, will render the prayers void. If the addition is done by mistake, the prayers does not become void except when a Ruku' is added, or more than two Sajdah are offered in one Rak'at.