CHEMICAL FORMULAS AND CHEMICAL COMPOUNDS Objectives 1 Explain

CHEMICAL FORMULAS AND CHEMICAL COMPOUNDS

Objectives: 1. Explain the significance of a chemical formula. 2. Determine the formula of an ionic compound formed between two given ions. 3. Name an ionic compound given its formula. 4. Using prefixes, name a binary molecular compound from its formula. 5. Write the formula of a binary molecular compound given its name.

Chemical names and Formulas There are millions of natural and synthetic chemical compounds Calcium carbonate – limestone Sodium chloride – table salt Dihydrogen monoxide – water �These are their chemical and common names �Chemical names help to describe the atomic makeup of the compounds

Significance of a Chemical Formula Chemical formula Indicates the relative number of atoms of each kind in a chemical compound. Molecular formula Indicates the relative number of atoms of each kind in a molecule. (Covalently bonded) C 8 H 18 Subscript indicates there are 8 atoms of carbon in a molecule of octane Subscript indicates there are 18 atoms of hydrogen in a molecule of octane

Chemical formula for ionic compound Ionic compound consists of lattice of positive and negative ions held together by mutual attraction. Chemical formula represented by one formula unit �Simplest ratio of the compounds positive and negative ions �Aluminum sulfate below consists of aluminum cations and sulfate ions Al 2(SO 4)3 Subscript 2 refers to 2 Aluminum atoms Note: when you only have one of an atom, no subscript is used Subscript 4 refers to 4 oxygen atoms in the sulfate ion Subscript 3 refers to everything inside the parentheses giving 3 sulfate ions, with a total of 3 sulfur atoms and 12 oxygen atoms Note: parentheses are used to identify polyatomic ion as one unit

Monatomic ions Ions formed from a single atom Examples �Na+1 �Mg 2+ �S 2�N-3 �Cl 1 - lose one electron gain two electrons Not all main-group elements readily form ions Examples �Carbon & Silicon form covalent bonds d-block elements form variable charges examples �Copper, can be Cu+1 or Cu+2 �Iron, can be Fe+2 or Fe+3 �Lead, can be. Pb+2, Pb+3, or Pb+4

Naming Monatomic ions Positive ions Name of element �Ex: K+ Potassium Mg+2 Al+3 Sr+2 Magnesium __________ Negative ions Base of element + -ide ending �Ex: F-1 Fluoride N-3 O-2 Br-1 Nitride _______

Binary Ionic Compounds composed of two different elements Total # of positive charges must be equal to total # of negative charges Writing formulas, Ex: Aluminum oxide 1. Write the symbols for ions (Cation first) Al+3 O-2 2. Cross over the charges as subscripts +3 -2 Al 2 O 3 3. Check to make sure total charges are equal, divide by largest number, to give smallest whole-number ratio Al 2 O 3 2 x (+3) = +6 3 x (-2) = -6

Naming binary ionic compounds Nomenclature Naming system Name Al 2 O 3 Name cation first : full name of cation Aluminum Name Anion last : base of anion + -ide oxide Al 2 O 3 aluminum oxide

Practice Naming and Writing Formulas Name 1. Ag. Cl silver chloride 2. Zn. O zinc oxide 3. Sr. F 2 strontium fluoride Write the formulas for 1. Zinc iodide Zn. I 2 2. Zinc sulfide Zn. S 3. Aluminum sulfide Al 2 S 3

Stock System of Nomenclature Some cations may have two or more different charges Use stock system of naming (usually with d-block elements) Roman numeral represents charge in parentheses Fe+2 Iron(II) Fe+3 Iron(III) Some cations that commonly form only one cation Do not use roman numerals ( main group elements) No Anions form more than one charge +2 -1 Cu. Cl 2 copper(II) chloride

Practice stock system Write formula and give name for compound formed by ions Cr+3 and F-1 Cr. F 3 chromium(III) fluoride Write formulas and give name for the following ionic compounds: Cu+2 and Br-1 Cu. Br 2 copper(II) bromide Fe+2 and O-2 Fe. O iron(II) oxide Fe+3 and O-2 Fe 2 O 3 iron(III) oxide

Compounds containing Polyatomic Ions All but NH 4+, ammonium ion, are negatively charged Most are oxyanions �Examples �NO 3 -1 nitrate NO 2 -1 nitrite Most common anion has –ate ending Anion with one less oxygen has –ite ending �Anion with two less oxygen has hypo prefix and –ite ending �Anion with one extra oxygen has per prefix and –ate ending Cl. O 3 -1 chlorate Cl. O 2 -1 chlorite Cl. O-1 hypochlorite Cl. O 4 -1 perchlorate

Naming Compounds with Polyatomic Ions Same as naming for ionic compounds except Name polyatomic ion as one unit �Example: �Ag. NO 3 silver nitrate Use parentheses if more than one polyatomic ion �Example �Al 2(SO 4)3 aluminum sulfate Show 2 Al+3 ions and 3 SO 4 -2 ions

Writing Formulas and Naming Compounds with Polyatomic Ions Write the formula for these: 1. 2. 3. 4. 5. tin(IV) sulfate Sn(SO 4)2 calcium chloride Ca. Cl 2 lithium nitrate Li. NO 3 calcium nitrite Ca(NO 2)2 KCl. O 4 potassium perchlorate Write the names for these: 1. 2. 3. 4. 5. Ag 2 O Ca(OH)2 NH 4 OH Fe. Cr. O 4 KCl. O silver oxide calcium hydroxide ammonium hydroxide iron(II) chromate potassium hypochlorite

Naming Binary Molecular Compounds May use stock system to name these New system – must understand oxidation numbers Prefix system Old system – must know numerical prefixes 1. 2. 3. 4. 5. monoditritetrapenta- 6. hexa 7. hepta 8. octa 9. nona 10. deca-

Rules for prefix system of Nomenclature 1. less-electronegative element is given • First element only gets a first Ex: PO prefix if it has more than one 4 10 2. Second element is named by combining a. Prefix indicating number of atoms b. Root of name of second element c. -ide ending Ex: tetra phosphorus decoxide 3. The o or a at the end of a prefix is usually dropped when the word following the prefix begins with another vowel

The 6 binary compounds of Nitrogen and Oxygen N 2 O NO NO 2 N 2 O 3 N 2 O 4 N 2 O 5 dinitrogen monoxide nitrogen dioxide dinitrogen trioxide dinitrogen tetroxide dinitrogen pentoxide • Name the following molecular compounds • Write formulas for the following molecular compounds 1. SO 3 sulfur trioxide 1. carbon tetraiodide CI 4 2. ICl 3 iodine trichloride 2. phosphorus trichloride PCl 3 3. PBr 5 Phosphorus pentabromide 3. oxygen difluoride OF 2