Hello Vijay Kumar Verma,

RCC slab is supported on two ends. The maximum tensile stress is in the middle of the slab whereas maximum compressive stress is at both ends. We place MS steel in RCC slab for taking tensile and compressive stress. For making RCC slab safe from tensile stress we place maximum steel in the middle of RCC slab. For making RCC slab safe from compressive stress we place MS steel in form of crank provided in steel bar and extra steel bars at both ends of the slab.
The spacing of main bar for 12 MM and 10 MM dia bars should be 6 inches center to center. For 8 MM dia bar it should be 4 inch center to center. For distribution bars the spacing for 10 MM dia should be 9 inches to 12 inches and for 8 MM dia bar 7 inches to 9 inches. The spacing depends on the span and thickness of slab. 
These main steel bars provided in the RCC slab placed according to the arrangement that one bar laid straight and the other bar has a crank on both ends.
For further detail on steel bars, click at the link given below. 
For more detail you may take help from the article M S steel in raw material link given below.

Steel Bars

Thanks