a. T o draw the lewis structure of this molecule, we have to know the valence electrons of each of the element that make part of it. Bromine has 7 valence electrons and so does fluorine.
They are bonded by covalent bonds, then, the lewis structure is:
In this structure, the dots represent the valence electrons and the line the bonds, there are 5 fluorine atoms and 1 bromine atom.
b. Carbonate has the following formula CO₃²-. Carbon has 4 valence electrons and oxygen has 6, nevertheless, in this structure, 2 of the oxygens gained 1 electron each, which makes the structure to have a negative charge, the oxygen that has not an extra electron has a double bond with the carbon. The lewis structure is the following:
Remember to draw the negative charges to the oxygens that gained an electron.