## Chemistry: An Atoms-Focused Approach

(a) sodium sulfide: Na$_2$S (b) strontium chloride: SrCl$_2$ (c) aluminum oxide: Al$_2$O$_3$ (d) aluminum chloride: AlCl$_3$
(a) sodium sulfide: Na$_2$S charge on sodium ion = 1+ charge on sulfur ion = 2- The charges must be balanced in a compound. 2 sodium ions = 2(1+) = 2+ charge 1 sulfide ions = 1(2-) = 2- charge (2+) + (2-) = 0 The charges balance out, so you need two atoms of sodium for every one atom of sulfur to make a compound. (b) strontium chloride: SrCl$_2$ charge on strontium ion = 2+ charge on chloride ion = 1- The charges must be balanced in a compound. 1 strontium ion = 1(2+) = 2+ charge 2 chloride ions = 2(1-) = 2- charge (2+) + (2-) = 0 The charges balance out, so you need one atom of strontium for every two atoms of chlorine to make a compound. (c) aluminum oxide: Al$_2$O$_3$ charge on aluminum ion = 3+ charge on oxide ion = 2- The charges must be balanced in a compound. 2 aluminum ions = 2(3+) = 6+ charge 3 oxide ions = 3(2-) = 6- charge (6+) + (6-) = 0 The charges balance out, so you need two atoms of aluminum for every three atoms of oxygen 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.