In a chemical equation what does the downward pointing arrow signify ?​

An arrow pointing down (as in PbI2?) indicates that the product will spontaneously precipitate from the solution. ("Precipitate," in this sense, means the formation of a solid from the combination of two aqueous solutions.)

atomic number of carbon is 6

electronic configuration of carbon is 1s²2s²2p²

orbital diagram of c is as shown here in the image.

a) the particles of the chair are strongly held together.