Lesson 3: Making Compounds

AIM: How can we know how elements form compounds, and in what proportions? Ions can be positive or negative, from unequal # of protons and electrons Ionic charge (how much charge the ion has) is indicated by a superscript Superscript is a (+) or (-), often with a number, written to the top right of a chemical symbol Al +3, Na +, Mg +2, Cl —, P -3, S -2 Page 2 superscript

Ion Review Plus sign = positive charge from losing electrons (Al +3, Na +, Mg +2 ) Minus sign = negative charge from gaining electrons (Cl —, P -3, S -2 ) For many elements, possible charges of its ions are equal to the oxidation states listed on Periodic Table Pages 2 & 3 Oxidation States

Polyatomic Ions Polyatomic ion: group of atoms bonded together, with an overall charge; listed on Table E OH — SO 4 2— PO 4 3— Cr 2 O 7 2— Sometimes written in parentheses NH4 + or (NH4 + ) “Polyatomic” = more than one atom

Compounds are electrically neutral, from equal numbers of negative and positive parts One way to form a compound: By attraction of oppositely charged ions Monatomic or polyatomic ions attract each other in a ratio that produces a neutral compound Compounds

Ratios in Compounds 1:1 ratios Na + & Cl - yields NaCl Mg +2 & S -2 yields MgS Not 1:1 ratios Mg +2 & Cl — Na + Cl — Mg +2 Cl — Mg +2

The Criss-Cross Method Write the charge of one ion as the subscript of the other (without the sign, # only) MgCl 2 = 1 Mg (with a + 2 charge) & 2 Cl (each with a – 1 charge) (1 x + 2) + (2 x – 1) = 0, neutral charge Reduce subscripts if you can Mg 2+ & Cl — MgCl 2 Ca 2+ & S 2— Ca 2 S 2 CaS =

Write the charge of one ion as the subscript of the other (without the sign, # only) Reduce subscripts if you can