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