Chemistry Writing Formulas

CHEMICAL FORMULAS AND CHEMICAL COMPOUNDS

Types of Bonds IONIC COVALENT Bond Formation e- are transferred from metal to nonmetal e- are shared between two nonmetals Type of Structure crystal lattice true molecules Physical State solid liquid or gas Melting Point high low Solubility in Water yes usually not Electrical Conductivity yes (solution or liquid) no Other Properties

Ionic Bonding - Crystal Lattice Types of Bonds Ionic Bonding - Crystal Lattice The crystals are made up of alternating positive and negative ions.

Covalent Bonding - True Molecules Types of Bonds Covalent Bonding - True Molecules Diatomic Molecule

Types of Bonds METALLIC e- are delocalized among metal atoms Bond Formation e- are delocalized among metal atoms Type of Structure “electron sea” Physical State solid Melting Point very high Solubility in Water no yes (any form) Electrical Conductivity Other Properties malleable, ductile, lustrous

Metallic Bonding - “Electron Sea” Types of Bonds Metallic Bonding - “Electron Sea” The valence electrons move freely around the atoms

Predicting Ionic Charges Group 1: Lose 1 electron to form 1+ ions H+ Li+ Na+ K+

Predicting Ionic Charges Group 2: Loses 2 electrons to form 2+ ions Be2+ Mg2+ Ca2+ Sr2+ Ba2+

Predicting Ionic Charges Loses 3 electrons to form 3+ ions Group 13: B3+ Al3+ Ga3+

Predicting Ionic Charges Lose 4 electrons or gain 4 electrons Group 14: C, Si and Ge, +4 and -4; Sn and Pb +4

Predicting Ionic Charges Nitride Gains 3 electrons to form 3- ions Group 15: P3- Phosphide As3- Arsenide

Predicting Ionic Charges Oxide Gains 2 electrons to form 2- ions Group 16: S2- Sulfide Se2- Selenide

Predicting Ionic Charges F1- Fluoride Br1- Bromide Group 17: Gains 1 electron to form 1- ions Cl1- Chloride I1- Iodide

Predicting Ionic Charges Stable Noble gases do not form ions! Group 18:

Vocabulary Chemical Bond electrical attraction between nuclei and valence e- of neighboring atoms that binds the atoms together bonds form in order to… decrease energy increase stability

BINARY IONIC COMPOUNDS BINARY - TWO ELEMENTS: A METAL AND A NONMETAL MOST ARE SALTS OTHER COMPOUNDS ARE OXIDES

Writing Chemical Formulas We use the “criss cross” method Translate the names of the elements to symbols. Use the periodic table to find the charges. Find the charge on the positive ion, and write it above the symbol. Write the charges small! Find the charge on the negative ion, and write it above the symbol. Write the charges small!!

The numbers, not the charges, determine the subscripts. If the charges do not have the same number, then you need subscripts.

Writing Chemical Formulas Example: potassium bromide Potassium is K, bromine is Br K+1 Br-1 KBr Since -1 and +1 = 0, the charges cancel each other out, and subscripts are not needed.

Writing Chemical Formulas Example: magnesium chloride Mg+2 Cl-1 MgCl2 The 2 from magnesium’s charge will become a subscript. We do not use a 1 for a subscript. We need 2 chlorines for every one magnesium

Writing Chemical Formulas Example: lithium sulfide Li+1 S-2 Li2S The 2 from sulfur’s charge will become a subscript. We do not use a 1 for a subscript. We need two lithium atoms for every one sulfur atom

Writing Chemical Formulas Example: aluminum oxide Al+3 O-2 Al2O3 This time, the numbers are “criss crossed” in both directions. Each element will get a subscript.

Check your subscripts! Compounds do not have a charge. They are zero. So, the charges on the ions must equal zero. Check to be sure! Al2O3 (2 Al ions) (+3) = +6 (3 O ions) (-2) = -6 +6 and -6 = 0

Writing Formulas You try these: magnesium bromide strontium oxide aluminum sulfide zinc iodide Answers MgBr2 SrO Al2S3 ZnI2

POLYATOMIC COMPOUNDS Contain three or more atoms Use the list on your blue sheet.

Writing Formulas Calcium carbonate Ca+2CO3-2 CaCO3 Sodium sulfate Na+1SO4-2 The 1 from Na is not used as a subscript. Na2SO4

Always check the charges and write them above the symbols Always check the charges and write them above the symbols! After you are comfortable with the charges, you may leave them off. If you have more than one polyatomic ion in a compound, you must enclose the ion in parenthesis, then use a subscript. No parenthesis if there is not a subscript!!!

MORE FORMULAS Magnesium nitrate Mg+2 NO3-1 Mg(NO3)2 You must put parenthesis around the NO3 ion because you have two of them. Do not use parenthesis around single atoms

More polyatomic compounds Barium phosphate Ba+2 PO4-3 Ba3(PO4)2 Ammonium carbonate (NH4)+1 CO3-2 +1 -2 (NH4)2 CO3

Writing formulas Calcium carbonate Sodium sulfate

MORE FORMULAS Sr+2OH-1 Sr(OH)2 Strontium hydroxide This compound is tricky because you may forget to add parenthesis around the hydroxide ion. Sr+2OH-1 Sr(OH)2

REMEMBER TO: WRITE THE CORRECT SYMBOL WRITE THE CORRECT CHARGE ABOVE THE ION BRING DOWN THE NUMBER FOR THE SUBSCRIPT IF NEEDED. REWRITE THE FORMULA WITHOUT THE CHARGES.