In the Lewis structure of SiBr4, silicon (Si) is the central atom surrounded by four bromine (Br) atoms. Silicon is in Group 14 of the periodic table and has four valence electrons. When it forms bonds with the four bromine atoms, each bond involves the sharing of one electron from silicon and one electron from bromine. This results in four Si-Br single bonds.
Now, considering bromine: each Br atom is in Group 17 and has seven valence electrons. When it forms a single bond with silicon, it shares one of its seven electrons. This leaves each bromine with three lone pairs of electrons because a total of four electrons (one used in bonding) are still available to each Br atom.
As for silicon, once it forms these four bonds, it has no remaining electrons to form lone pairs. All four of its valence electrons are involved in bonding. Therefore, in the Lewis structure of SiBr4, silicon has four bonding pairs and no lone pairs, while each bromine has one bonding pair and three lone pairs. This distribution reflects the octet rule, where each atom seeks to complete its outer shell of electrons.