Section 2.3—Chemical Formulas We need to be able to read the formulas for chemicals in the antacids! Objective: Explain and use nomenclature rules of writing ionic and covalent chemical formulas
Dec 16, 2015
Section 2.3—Chemical Formulas
We need to be able to read the formulas for chemicals in the antacids!
Objective: Explain and use nomenclature rules of writing ionic and covalent chemical formulas
Recall these Definitions:
Binary Ionic Compound- compound containing two elements—one metal and one non-metal – bonded through an ionic bond.
+Cation
+Cation
-Anion
-Anion Ionic Compound
Ionic bond- bond formed by attraction between + and - ions
Ionic Charges
Think about it: If ionic bonds are the attractive force between a cation (+ charge) and an anion (- charge) AND that an ionic compound contains a metal and a nonmetal, it must also be true that the metals must be charged and the nonmetals must be charged!
HOW CAN WE DETERMINE THAT CHARGE??
Charges for these elements will be GIVEN by a roman numeral following the cation’s nameex. iron (III) oxide: Fe is +3
Variable Positive Charges
Writing Formulas for Binary Ionic Compounds
To write these formulas:Write the symbol & charge of the first
element (the metal, cation)Write the symbol & charge of the second
element (the non-metal, anion)Add more of the cations and/or anions in
order to have a neutral compoundUse subscripts to show how many of each
type of ion is there.
Example #1
Sodium chloride
Cation
Anion
Na+1
Cl-1
NaCl
Na+1 Cl-1
+1 + -1 = 0
The compound is neutral…no subscripts are needed.
Example #2
Calcium bromide
Cation
Anion
Ca+2
Br-1
CaBr2
Ca+2 Br-1
+2 + -1 = +1
Ca+2 Br-1 Br-1
The subscript “2” is used to show that 2 anions are needed.
+2 + -1 + -1 = 0
Let’s Practice
Example:Write the following chemical formulas
Cesium chloride
Potassium oxide
Aluminum sulfide
Calcium bromide
Let’s Practice
CsCl
K2O
Al2S3
CaBr2
Example:Write the following chemical formulas
Cesium chloride
Potassium oxide
Aluminum sulfide
Calcium bromide
Shortcut
Criss-cross the NUMBER of the charge to obtain the subscripts. Reduce if necessary.
Example1: Al+3 S-2 Al2S3
Example 2: Ca+2 O-2 Ca2O2 CaO
Recall these Definitions
Polyatomic Ion- a group of atoms that are bonded together and have an overall charge
+Cation
+Cation
Polyatomic Ionic Compound- compound containing at least one polyatomic ion
-Polyatomic
Anion
Polyatomic IonicCompound
To write these formulas:Write the symbol & charge of the cation &
anion; if either is a polyatomic ion, look up its charge.
Add additional cations or anions in order to have a neutral compound (OR use the criss-cross shortcut.)
Use subscripts to show the number of ionsWhen using subscripts with a
polyatomic ion, you MUST put the polyatomic ion in parenthesis.
Identifying & Naming Polyatomic Ionic
Example #3
Sodium carbonate
Cation
PolyatomicAnion
Na+1
CO3-2
Na2CO3
Na+ CO32-
+1 + -2 = -1
Na+ Na+ CO32-
The subscript “2” is used to show that 2 cations are needed.
+1 + 1 + -2 = 0
Example #4
Magnesium nitrate
Cation
PolyatomicAnion
Mg+2
NO3-1
Mg(NO3)2Parenthesis are used to show 2 anion groups are needed.
Mg+2 NO3-
+2 + -1 = +1
Mg+2 NO3- NO3
-
The subscript “2” is used to show that 2 anions are needed.
+2 + -1 + -1 = 0
Let’s Practice
Example:Write the following chemical formulas
Sodium nitrate
Calcium chlorate
Aluminum sulfite
Calcium hydroxide
Ammonium Phosphate
Let’s Practice
NaNO3
Ca(ClO3)2
Al2(SO3)3
Ca(OH)2
(NH4)3PO4
Example:Write the following chemical formulas
Sodium nitrate
Calcium chlorate
Aluminum sulfite
Calcium hydroxide
Ammonium Phosphate
Definition
Binary Covalent Compound compound made from two non-metals that share electrons
Nonmetal
Nonmetal
Nonmetal
Nonmetal Covalent compound
Covalent bond atoms share electrons
How we named them:Prefixes were used to indicate the number of
atoms of each elementExample: N5O pentanitrogen monoxide
Identifying & Naming Binary Covalent
Writing Formulas
To write these formulas:Write the symbols of the first and second
elementTranslate the covalent prefixes (assume the
first element is “1” if there’s no prefix) into subscripts to show number of atoms.
Atoms DO NOT form charges when bonding covalently…you DO NOT need to worry about charges with this type!
Let’s Practice
Example:Write the following chemical formulas
Carbon monoxide
Nitrogen dioxide
Diphosphorus pentaoxide
Let’s Practice
CO
NO2
P2O5
Example:Write the following chemical formulas
Carbon monoxide
Nitrogen dioxide
Diphosphorus pentaoxide
Diatomic Elements
•Some elements are so chemically reactive that they cannot and do not exist in nature as single atoms.•If they do not find an atom of another element to bond to, they will bond to an atom of their same kind.•The formula for these such elements ALWAYS has a “2” subscript.
•These elements are: hydrogen, H2 chlorine, Cl2 nitrogen, N2 bromine, Br2
oxygen, O2 iodine, I2 fluorine, F2