J OURNAL 24 Why is it important for chemist to know chemical names and formulas? What is the difference between an Ionic and Covalent bond?

T ODAY WE WILL DETERMINE THE FORMULA OF AN IONIC COMPOUND FORMED BETWEEN TWO GIVEN IONS

S IGNIFICANCE OF A C HEMICAL F ORMULA Chemical formula- indicates the relative number of atoms of each kind of chemical compounds. Example C 8 H 18 Subscript- indicates that there are 8 carbon atoms in a molecule of octane Indicates that there are 19 hydrogen atoms in a molecule of octane Unlike a molecular compound, an ionic compound consists of lattice of positive and negative ions held together by mutual attraction.

Al 2 (SO 4 ) 3 Note how the parenthesis is used. They surround the polyatomic anion to identify it as a unit. The subscript 3 refers to the entire unit. When there is no subscript next to an atom’s symbol, the value of the subscript is one. A C HEMICAL F ORMULA

M ONATOMIC I ONS Ions formed from a single atoms are known as monatomic ions. By gaining or losing electrons, many elements form ions with noble-gas configurations. Examples Group 1 metals lose one electron to give 1+ cations Na+ Group 2 metals lose two electrons to give 2+ cations Mg 2+ Nonmetals of groups gain electrons N3- S2-

N AMING M ONATOMIC I ONS Monatomic cations are identified by the elements name Monatomic anions Ending of the name is dropped -ide is added to the root name Examples: chloride, sulfide, fluoride Elements with multiple ions Examples: Iron (II), Iron (III), Cobalt (II), Cobalt (III) Copper (I) Copper (II)

B INARY I ONIC C OMPOUNDS Binary compounds- compounds composed of 2 elements The total number of positive charges must equal the number of negative charges Examples: Mg and Br, As an aid to determining subscripts in formulas for ionic compounds, the positive and negative charges can cross – over. Example: Al (3+) and O (2-)  Al2O3

N AMING B INARY I ONIC C OMPOUNDS The name of the cation is given first followed by the anion. Examples Al2O3  Aluminum Oxide CaBr2 BaO

Y OUR T URN T O P RACTICE

C AN YOU WRITE THE F ORMULA WHEN GIVEN THE NAME ? Barium oxide Potassium chloride Strontium bromide Lithium sulfide

Y OUR T URN TO P RACTICE

S TOCK S YSTEM O F N OMENCLATURE Some elements have 2 or more cations (with different charges) Examples: Iron, Copper Use Roman numeral to indicate the ion’s charge The numeral is in parentheses and placed right after the metal name Fe 2+ - Iron (II) Metals that form only one cation do not include a roman numeral

P RACTICE Write the formula and give the name for the compound formed by the ions Cr 3+ and F - Write the symbols for the ions side by side (cation first) Cross over charges to give subscripts Answer: Chromium (III) Flouride; the formula is CrF 3

P RACTICE Write the formula and give the name for the compound formed by the ions Cu 2+ and Br - Write the symbols for the ions side by side (cation first) Cross over charges to give subscripts Answer: Copper (II) Bromide; CuBr 2

P RACTICE Write the formula and give the name for the compound formed by the ions Fe 2+ and O 2- Write the symbols for the ions side by side (cation first) Cross over charges to give subscripts Answer: Iron (II) oxide; FeO

P RACTICE Write the formula and give the name for the compound formed by the ions Pb 2+ and Cl - Write the symbols for the ions side by side (cation first) Cross over charges to give subscripts Answer: Lead (II) chloride; PbCl 2

P RACTICE Write the formula and give the name for the compound formed by the ions Hg 2+ and S 2- Write the symbols for the ions side by side (cation first) Cross over charges to give subscripts Answer: Mercury (II) sulfide; HgS

P RACTICE Write the formula and give the name for the compound formed by the ions Sn 2+ and F - Write the symbols for the ions side by side (cation first) Cross over charges to give subscripts Answer: Tin (II) fluoride; SnF 2

P RACTICE Write the formula and give the name for the compound formed by the ions Fe 3+ and O 2- Write the symbols for the ions side by side (cation first) Cross over charges to give subscripts Answer: Iron (III) oxide; Fe 2 O 3