Writing formulas and naming compounds

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Transcript Writing formulas and naming compounds

 What
is a chemical formula?
 It
indicates the relative number of atoms of
each kind in an ionic compound.

Ex Al2O3 has 2 atoms of Al and 3 atoms of O.
 It
indicates the number of atoms of each
element contained in a single molecule of a
molecular compound.
Ex. C8H18 has 8 atoms of carbon and 18 atoms of
H in a molecule of octane.
 There
are different rules for naming and
writing formulas for ionic and covalent
compounds.
 The compound is ionic if it is composed of a
cation and an anion, or a metal and a
nonmetal. The elements will be located on
opposite sides of the periodic table. They
have a high electronegativity difference.
 The compound is covalent (or molecular) if
both elements are nonmetals.
Binary
because composed of only 2
elements.
Ionic
because composed of a metal
and a nonmetal.
Example
aluminum oxide.
1). Write the symbol and charge for the cation.
2). Write the symbol and charge for the anion.
3). Criss cross the oxidation numbers and drop
the charges. The oxidation number will
become the subscript for the other element.
4). Reduce the subscripts if possible. If they
are the same, they cancel completely.
5). If the subscript is 1, do not write anything.
Write the formula for calcium iodide.
 Write

symbols and charges.
Ca+2
I-1
 Criss-cross

Ca1I2
 Cancel

oxidation numbers, drop charges.
and take away ones.
Answer: CaI2
 Question:
Write the formula for Magnesium
Oxide.
 1)

Write the symbol and charge.
Mg+2
O-2
 2)
Criss cross oxidation number and drop
charges.

Mg2O2
 3).
Cancel and don’t write 1’s.

Answer: MgO

Complete page 4 and the bottom of page 5 of
formula writing packet.
 If
a compound consists of a metal and a
nonmetal, we use the name of the first
element (always the cation) followed by the
second element (always the anion). For the
anion only, drop the last syllable and add
–ide. Subscripts are not included in the
name.
Examples: CaO: calcium oxide
LiCl: lithium chloride
K2S: potassium sulfide
 P.
223 sample problem A
 P. 223 practice #1 a-e, and #2 a-f
 Complete packet page 1 and 8.
 Some
elements, such as iron, form 2 or more
cations with different charges. To distinguish
the ions formed by such elements, scientists
use the Stock system of nomenclature. This
system uses a Roman numeral to indicate the
ion’s charge. The numeral is enclosed in
parentheses and placed immediately after
the metal name.
 Fe+2 is iron (II) and Fe+3 is iron (III)
 Names of metals that commonly form only
one cation, do not include a Roman numeral.
 CuCl2
 Determine
the charges of each element, by
writing ions side by side.
 Backwards criss-cross to determine charge of
cation.
 Name by writing name of cation , roman
numeral, name of anion with ide instead of
last syllable.
 Cu
Cl-1
 Cu+2 Cl-1
 Copper(II) chloride
 Example:
Write the formula of lead(IV)
oxide.
 Write symbols with charge.

Pb+4 and O-2
 Criss
cross and drop signs.
 Pb2O4
 Reduce
 PbO2
and remove 1’s.
 Page
225 #1(a-f) and #2 (a-d)
 Complete packet page 3 and 5(top)
A covalently bonded group of atoms that has a
positive or negative charge and act as a unit
is a polyatomic ion.
The prefix poly means “many”.
Most simple polyatomic ions are anions.
Ammonium (NH4) is the only polyatomic
cation.
Sometimes there are parentheses in the
formula, so you can tell its polyatomic.
 If
it contains a polyatomic ion, it is
ionic, so it follows the rules for naming
ionic compounds.
 If anion is polyatomic, do not change
the ending to ide. Keep it the same as
is listed on the chart. Name cation
then name anion.
 If the cation is polyatomic and the
anion is binary, name is just like
before. The ide ending means the
anion is not polyatomic.
 Al2(SO4)3
 Al+3
and SO4-2
 Aluminum sulfate
 Fe(OH)2
 Fe+2
and OH-1
 Iron(II)hydroxide
1. Write the symbols for the monatomic and
polyatomic ions in the compound.
 2. Look up the oxidation numbers of the ions
involved.

If a single atom, use the periodic table.
 If roman numeral (variable oxidation number) or
polyatomic, use the chart.

3. Criss cross the oxidation numbers, and drop
the charges.
 4. Put polyatomic ions in parentheses if they
have a subscript.
 5. Reduce (simplify) and erase ones.
 6. DO NOT ADD< SUBTRACT, MULTIPLY, OR DIVIDE.
Just criss cross and reduce if necessary!

 Write
the formula for Lead (II) hydroxide.
 1. Write the symbol and oxidation number.

Pb+2
 2.

Use parentheses if needed.
Pb1(OH)2
 4.

Criss cross charges and drop signs.
Pb1 OH2
 3.

OH-1
Reduce and drop 1’s.
Pb(OH)2
 Write
the formula for ammonium sulfate.
 1. Write the symbol and oxidation number.

NH4+1
SO4-2
 2.
Criss cross charges and drop signs. Use
parentheses if needed.

(NH4)2 (SO4)1
 3.

Reduce and drop 1’s.
(NH4)2 (SO4)
 Molecular/covalent
compounds are made of 2
nonmetals.
 Generally, the most metallic element is written
first. These elements are farther to the left in the
periodic table. If both elements are in the same
group, the more metallic element is closer to the
bottom of the group.
 The name of the second element is changed to
-ide.
 Because molecular formulas can combine in many
combinations, prefixes are used to indicate the
subscript. Do not criss cross and do not use
oxidation numbers.
 Don’t use mono on first element.
 1-
mono
 2- di
 3- tri
 4- tetra
 5- penta
 6- hexa
 7- hepta
 8- octa
 9- nona
 10- deca
 P2O5


Prefix of 1st element, name of 1st element
Prefix of 2nd element, name of 2nd element,
change ending to ide.

Diphosphorus pentaoxide
 CO


Carbon monoxide
(Don’t write mono on first element.)
 Write
the symbols for the elements in the
order that they appear in the name.
 The prefixes indicate the number of atoms of
each element in the molecule.
 The prefixes appear as subscripts in the
formulas.
 If there is no prefix, there is only one atom.
(1st element only)
 Remember, do not criss cross.
 Example tetraphosphorus dichloride

P4Cl2
***1ST PUT I (ionic) OR C (covalent) BESIDE THE
QUESTION
FRONT LEFT COLUMN:
1—20, 25, 26
FRONT RIGHT COLUMN:
1—18, 23, 25, 26
 HCl


This is binary, H and Cl. Follow rule #1.
Hydrochloric acid
 H2SO4-
This is polyatomic. Hydrogen and
sulfate (SO4)-2. Must find polyatomic anion
and name. Ending is -ate so change to -ic.

Sulfuric acid (notice no hydro, hydro is only for
binary acids).
 HNO2-
Polyatomic, H and nitrite. -ite
becomes –ous. Nitrous acid.
 Cation
is always H+1. From the name of the
acid, you can figure out the anion.
 Write symbols and charges for cation and
anion.
 Criss cross oxidation numbers and drop signs.
 Reduce and drop 1’s.
 Example




Phosphoric acid
H+1 and phosphoric so phosphate, so
H+1 and (PO4)-3
H3PO4