Any element in group 18 has eight valence electrons (except for helium, which has a total of just two electrons). Examples include neon (Ne), argon (Ar), and krypton (Kr). Oxygen, like all the other elements in group 16, has six valence electrons.
The molar mass (atomic weight ) of sodium is 23.0 grams/mole and the molar mass of sodium azide, NaN3 , is the mass of sodium, 23.0 gram/mole added to the molar mass of three atoms of nitrogen (14.0 x 3 = 42 gram/mole) which equals 65.0 grams/mole. The percentage of sodium is 23.0 /65.0 x 100 % = 35 %
[Ar] 3d10 4s2 4p5 is the electron configuration of bromine
So the equation is balanced, meaning they have the smallest amounts of each element in the reactants to create the products.
So, 2 moles of H2S (the coefficient) contributes to 2 moles Ag2S, which is why the ratio is 2:2.
I hope that made sense.
