lesson 3: making compounds. aim: how can we know how elements form compounds, and in what...
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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 & 3Oxidation States
Polyatomic Ions
• Polyatomic ion: group of atoms bonded together, with an overall charge; listed on Table E
• OH— SO42— PO4
3— Cr2O72—
• 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 Ratios in CompoundsCompounds
• 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—
Cl—Cl—
Mg+2
The Criss-Cross Method
• Write the charge of one ion as the subscript of the other (without the sign, # only)
• MgCl2 = 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
Mg2+ & Cl—
MgCl2
Ca2+ & S2—
Ca2S2 CaS=
• Write the charge of one ion as the subscript of the other (without the sign, # only)
• Reduce subscripts if you can
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