## Chemistry: An Atoms-Focused Approach

(a) potassium bromide: KBr (b) calcium hydride: CaH$_2$ (c) lithium nitride: Li$_3$N (d) aluminum chloride: AlCl$_3$
(a) potassium bromide: KBr charge on potassium ion = 1+ charge on sulfur ion = 1- The charges must be balanced in a compound. 1 potassium ion = 1(1+) = 1+ charge 1 bromide ion = 1(1-) = 1- charge (1+) + (1-) = 0 The charges balance out, so you need one atom of potassium for every one atom of bromine to make a compound. (b) calcium hydride: CaH$_2$ charge on calcium ion = 2+ charge on hydride ion = 1- The charges must be balanced in a compound. 1 calcium ion = 1(2+) = 2+ charge 2 hydride ions = 2(1-) = 2- charge (2+) + (2-) = 0 The charges balance out, so you need one atom of calcium for every two atoms of hydrogen to make a compound. (c) lithium nitride: Li$_3$N charge on lithium ion = 1+ charge on nitride ion = 3- The charges must be balanced in a compound. 3 lithium ions = 3(1+) = 3+ charge 1 nitride ion = 1(3-) = 3- charge (3+) + (3-) = 0 The charges balance out, so you need three atoms of lithium for every one atom of nitrogen to make a compound. (d) aluminum chloride: AlCl$_3$ charge on aluminum ion = 3+ charge on chloride ion = 1- The charges must be balanced in a compound. 1 aluminum ion = 1(3+) = 3+ charge 3 chloride ions = 3(1-) = 3- charge (3+) + (3-) = 0 The charges balance out, so you need one atom of aluminum for every three atoms of chlorine to make a compound.