What holds two ions together in an ionic bond ?

Almost always it's a case of opposites attract. In general metals give away electrons to the non metals on the right side of the periodic table. The metals have a + charge, the non metals have a - charge. Instant attraction. The attraction is pretty powerful: the ionic bond is among  the strongest in chemistry.

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.

a=c and then d= c because of the band so you get ac/dc and you add them together you get sound waves